Interim Procedure to Measure the

Thermal Performance of Window

Systems

M. E. McCabe

W. P. Goss

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

September 1986

issued June 1987

D ror\o rci/H fQf

QC artment of Energy

100 le Power Administration

. U56 : 3621

i Oregon 97^08

1987

C.2

Research Information Center

iNational Bureau of Standards

Gaithersburg, Maryland 20899

INTERIM PROCEDURE TO MEASURE THE

THERMAL PERFORMANCE OF WINDOW

SYSTEMS

NBSIR 87-3569 MZSc

CiCiOO

(j c*7-

{ 91

M. E. McCabe

W. P. Goss

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Gaithersburg, MD 20899

September 1986

Issued June 1987

Prepared for: U.S. Department of Energy

Bonneville Power Administration

P.O. Box 3621

Portland, Oregon 97208

o

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

FORWARD

In 1984, the National Bureau of Standards (NBS) began work on preparing an interim test procedure for the Bonneville Power Administration (BPA) for

measuring the thermal performance of a class of products known as movable

insulations. Those products are used primarily to reduce winter time heat

loss through windows. At that time, BPA planned to use the test procedure

for developing specifications and certifying compliance with the BPA

Residential Wea ther iza t ion Program which included credits for movable

insulation systems for windows. Due to the limited funding available for

preparing a draft test method, no testing was to be performed, however the

test procedure was to be based on a technical evaluation of test methods

described in the literature, visits to testing laboratories and consultation

with researchers, testing lab operators and members of standards committees

working in similar areas. A letter report describing the rationale upon

which the interim test procedure was to be prepared along with an outline

for a future program which could increase the scope into a more general

procedure that addresses improved accuracy and extended environmental

conditions. Dr. Mark McKinstry was named as the BPA Contracting Officer's

Technical Representative.

In July 1984, Dr. McKinstry advised NBS that the movable insulation products

would not be included in the BPA weatherization program as originally

planned, and that a more pressing need was for a recommended thermal test

procedure for glazing which could be used in the Model Conservation Standard

proposed for adoption by the Northwest Power Planning Council. The proposed

standard described maximum allowable U-values for windows, which were to be

determined by testing according to either the ASTM C236, ASTM C976 or AAMA

1503.1 Standards. In response to this need, NBS prepared and submitted a revised work statement to BPA.

The revised work statement included the preparation of a report containing

the previously proposed technical reviews along with a Draft Standard Test

Method describing facilities used for determining window U-values and a Draft Standard Practice describing the calibration procedures, the specific

test conditions, the specimen mounting provisions and calculation

procedures. Because of the discrepancies between the data obtained using

the different test methods and the controversy existing in the testing

industry, NBS proposed that the two draft documents be reviewed and

ballotted by members of a joint C-16/E-06 task group within ASTM, before

making the recommendations to BPA. Progress within the ASTM task group was

not as rapid as originally anticipated due to the slowness in obtaining

technical data, and the deliberate pace of a consensus standards

organization in considering the controversy between the ASTM and AAMA test

methods, therefore resulting in the delayed release of this report to BPA.

in
TABLE OF CONTENTS

Page

FORWARD iii

LIST OF TABLES viii

LIST OF FIGURES viii

1 . INTRODUCTION 1

2. U-VALUE DATA FOR WINDOW SYSTEMS 3

2.1 HEAT TRANSFER THEORY 3

2.1.1 SURFACE HEAT TRANSFER 3

2.1.2 AIR SPACE CONDUCTANCE 6

2.1.3 SOLAR RADIATION AND/OR AIR LEAKAGE 8

2.2 HISTORICAL REVIEW 10

2.3 ASHRAE HANDBOOK U-VALUE DATA 11

2.4 CONCERNS OVER CURRENT U-VALUE DATA BASE 13

3. TEST METHODS FOR MEASURING FENESTRATION U-VALUE 16

3.1

UNITED STATES PRACTICES (HOT BOXES) 16

3.1.1 ASTM C296 GUARDED HOT BOX 16

3.1.2 ASTM C976 CALIBRATED HOT BOX 17

3.1.3 AAMA 1503 .6 GUARDED HOT BOX 18

3.1.4 SOURCES OF ERROR IN HOT BOXES 19

3. 1.4.1 TEMPERATURE MEASUREMENT 19

3. 1.4.

ELECTRICAL ENERGY 20

3. 1.4.

AIR MOTION 20

3. 1.4.

HEAT TRANSFER RATE THROUGH TEST WINDOW .. 20

3. 1.4.

SIMULATION OF FORCED CONVECTION ON

EXTERIOR SURFACES 21

IV
Page

3.1

.4.6 AIR LEAKAGE 22

3.1.5 FUTURE HOT BOX ACTIVITIES 23

3.2 INTERNATIONAL PRACTICE (HOT BOXES) 24

3.2.1 NORWEGIAN PRACTICE 24

3.2.2 SWEDISH PRACTICE 25

3.2.3 BELGIAN PRACTICE 25

3.2.4 CANADIAN PRACTICE 26

3.3 FIELD VISITS TO LABORATORIES 26

3.4 U-VALUE MEASUREMENT CONFERENCES 28

3.4.1 WORKSHOP ON LABORATORY MEASUREMENTS OF " U-VALUE

OF WINDOWS 28

3.4.2 LOW E ROUNDTABLE 28

4. THERMAL PERFORMANCE TESTING OF MOVABLE INSULATION SYSTEMS FOR

FENESTRATION 35

4.1 FEDERAL PROGRAMS TO DEVELOP MOVABLE INSULATION 35

4.2 HISTORICAL THERMAL TESTING OF ROLLER SHADES 37

4.3 FIELD TEST METHODS 3 8

4.3.1 IN-SITU TESTING IN BUILDINGS 38

4.3.2 NIGHTTIME MEASUREMENTS IN TEST CELLS 39

4.3.3 NIGHTTIME MEASUREMENTS IN CALORIMETERS 39

4.4 LABORATORY TEST METHODS 40

4.4.1 SINGLE CHAMBER WITH HEAT FLUX TRANSDUCER 40

4.4.2 GUARDED HOT PLATE 41

4.4.3 HOT BOXES 42

4.5 PROBLEMS USING HEAT FLUX TRANSDUCERS 44

v
Lagg

4.6 TESTING STANDARDS FOR MOVABLE INSULATION SYSTEMS 45

4.6.1 CANADA HOUSING AND MORTGAGE STANDARD 45

4.6.2 WINDOW ENERGY SYSTEMS STANDARD 1584 45

5. TEST STANDARDS UNDER DEVELOPMENT 52

5.1 RATIONALE FOR CHANGES TO ASTM TEST METHODS 52

5.2 RATIONALE FOR CHANGES TO AAMA TEST METHOD 53

5.3 NEW ASTM TEST STANDARDS FOR WINDOWS 53

6. RESEARCH PROGRAM FOR DEVELOPING WINDOW TEST STANDARDS 55

6.1 PHASE 1 RESEARCH 55

6.1.1 TEST FACILITY DESCRIPTION 55

6.1.2 HEAT FLUX TRANSDUCER 55

6.1.3 TEMPERATURE MEASUREMENTS 55

6.1.4 TEST SPECIMENS 56

6.1.5 TEST CONDITIONS 56

6.2 PHASE 2 RESEARCH 56

6.2.1 RESEARCH LABORATORY TESTING AND ANALYSIS 57

6.2.2 COMMERCIAL LABORATORY HOT BOX TESTING 57

6.2.3 FIELD TESTING 58

7. CONCLUSIONS AND RECOMMENDATIONS 59

8. REFERENCES 61

APPENDIX A-l NORWEGIAN STANDARD NBI-13 8, 1982 "SEALED GLAZING

UNIT: THERMAL INSULATION AND SURFACE TEMPERATURE" .. A-l -1

APPENDIX A-2 SWEDISH STANDARD SS 81 81 29, "WINDOWS - THERMAL

RESISTANCE TEST" A-2-1

vi
Page

APPENDIX A-3 CHAPTER 6 OF BELGIUM STANDARD NBN B62-002,

"DETERMINATION OF THE COEFFICIENT K OF WINDOWS, NEW

STANDARDIZATION METHOD" A-3-1

APPENDIX B "STANDARD PRACTICE FOR DETERMING THE THERMAL

PERFORMANCE OF WINDOW AND DOOR SYSTEMS B-l

vn
LIST OF TABLES

Page

1. WORKSHOP ON LABORATORY MEASUREMENTS OF M U-VALUES " OF WINDOWS .. 29

LIST OF FIGURES

1. IDEALIZED DRAWING OF DOUBLE-GLAZED WINDOW 14

2. U-VALUES FOR SOLAR IRRADIATED GLAZINGS 15

3. ASTM C236 GUARDED HOT BOX 31

4. ASTM C976 CALIBRATED HOT BOX 32

5. AAMA 1503 .6 HOT BOX 33

6. NRC CANADA WIND SIMULATION MACHINE 34

7 . NBS PASSIVE SOLAR CALORIMETER 46

8. LBL MOWITT CALORIMETER 47

9. WINDOW TREAMENT TEST FACILITY USING A HEAT FLUX TRANSDUCER ... 48

10. WINDOW TREATMENT TEST FACILITY USING A GUARDED HOT PLATE 49

11. WINDOW TREATMENT TEST FACILITY USING LABORATORY ROOMS 50

12. GUARDED HOT BOX AT TECHNICAL UNIVERSITY OF DENMARK 51

viii
1 . INTRODUCTION

Transmission heat losses through fenestration system such as window and

glass doors represent one of the major energy demands for the heating of

residential and smaller size commercial buildings [Kusuda and Collins,

19783. Prior to the "energy crisis" of the 1970's, the availability of low

cost energy led to the widespread use of single glazed windows in most parts

of the United States, with double glazed windows being used mainly to

increase thermal comfort in the colder regions. Recent research has shown

the considerable positive benefits that windows can have on building

performance considering passive solar heating and daylighting benefits,

provided the thermal transmission heat losses are within reasonable limits.

A range of new window designs and products have appeared on the market or are in advanced stages of development which address the problem of reducing

the thermal transmittance (or U-value) of windows [Selkowitz, 1979].

Multiple-glazed windows with sealed double or triple insulating glazing

units or single glazed windows with storm sash are available for both new

and retrofit buildings. Window assemblies are constructed from a variety of

sash and frame materials, including wood, plastic, aluminum and various

composites. Recent advances in materials sciences have led to significant

improvements in thermal performance of sealed glazing units by introduction

of "heat mirror" plastic films [Lampert, 1981] and low-emittance coatings

[Grange and Owens, 1984], with the promise of additional improvements

through the commercialization of e lec t rochromic coatings, gas fills,

evacuated glazings and silica-gel windows [Arasteh et al., 1985]. As a retrofit option for existing buildings, a class of products known as movable

insulations, including a wide variety of shutters, shades and curtains

[Gibson, 1980; Shurcliff, 1980; Langdon, 1980] are currently available.

These products are frequently installed in tandem with existing single

glazed windows and usually require manual operation by the building occupant

to effect improved thermal performance.

In principle, the thermal performance of a fenestration can be predicted

using heat transfer theory and correlations of experimental data involving

convective and radiative heat flow, when the air space dimensions and

radiative properties of the window are known and the adjacent air space's

temperature and flow field are specified [Rubin, 1985; Siegel and Howell,

1981]. The ASHRAE Handbook published by the American Society of Heating,

Refrigerating and Air Conditioning Engineers [ASHRAE 1985, p. 27.10]

provides thermal transmittance data for a number of widely used

fenestrations based on a specific set of design conditions and recommends

their use for determining peak energy loads in sizing heating and cooling

equipment. However, because the specific design conditions are more severe

than those to which the fenestration are exposed to on a continual basis,

ASHRAE does not recommend the design data to be used for analysis of annual

energy usage.

Sash and frame members present considerable complications in predicting

thermal performance of windows due to their multi-dimensional heat transfer

characteristics and the uncertainties in thermal contact resistance at the

1
moving joints. The ASHRAE Handbook provides a set of adjustment factors to

account for frame and sash effects. These factors vary over a wide range of

values, do not account for differences in glass to frame area ratio, and are

not available for composite frame materials; therefore the handbook data are

not particularly useful for comparing alternative products. The ASHRAE

Handbook recommends the use of manufacturer's test results to determine the

thermal performance for each specific product.

The purpose of this report is to review the current sources of information

on U-values and to describe the state of thermal test methods used for

windows in order to provide the Bonneville Power Administration with some

general guidelines in the application of thermal test data for use in the

Model Conservation Standards (MCS) by the Northwest Power Planning Council.

At present, considerable controversy exists in the window industry regarding

the thermal testing of windows, therefore no consensus-based standards are

available.

After the topics of window heat transfer theory and sources of U-value data

for windows are reviewed, the currently available methods for measuring

thermal performance of windows and movable insulations will be described and

new standards, currently under development, will be presented. Background

information on thermal test methods for windows that was obtained by visits

to different commercial testing laboratories and research laboratories will

also be described along with planned activities related to development of

window test standards.

2
2 . U-VALUE DATA FOR WINDOW SYSTEMS

The rate of heat transfer through an idealized, double-glazed window shown

in Figure 1 is proportional to the ins ide-to-outside air temperature

difference. The heat transfer process involves the combined effects of

conduction, convection and radiation heat transfer. Heat transfer theory

relates the steady rate of heat flow per unit area of window, to the air

temperature difference, thereby defining the U-value, or thermal

transmittance as:

U

Q

A(T . . -T

1 ,air o ,air

( 1 )

9 9 where A = heat flow area of glazing, m^ ( ft^) , Q

= steady rate of heat transfer from heated space to cold space, W

( Btu/h)

,

T. . = temperature of warm space air, C (F)

,

T *

. = temperature of cold space air, C (F)

.

o,air

The U-value can be computed from a knowledge of

involved or it can be determined from test data when

are measured.

the individual factors

Q s A, T. . i ,air and T

o ,air

It should be noted that in an actual window system, there are usually

parallel heat flow paths through the central glazing unit, the edge spacers

(not shown), and through the window sash and frame members that support the

glazing unit. Thus, the heat transfer process to be described applies only

to the center portion of the glazing unit and in determining the overall

performance of a window unit, additional consideration must be given to heat

transfer via the frame and edges.

2.1 HEAT TRANSFER THEORY

Theoretical and experimental work on dynamic heat conduction in opaque

building components such as walls and roofs have been used to compute

cooling loads for air-conditioned buildings [Stephenson and Mitalas , 1967;

Kusuda, 1969]. Less attention has been devoted to surface heat transfer in

building components, especially fenestration, which have intrinsically

greater sensitivity to surface heat transfer due to their greater thermal

conductance and negligible thermal mass. Moreover, studies have shown the

difficulty of establishing relationships between surface heat transfer

coefficients and the environmental factors on which they depend [ito et al.,

1972].

2.1.1 Surface Heat Transfer

Consider a situation where a double glazed window separates a heated space

from the colder outdoors. Air may flow past the outdoor surface at

considerable velocity due to the influence of wind; and indoor air may

circulate by bouyant forces in which air that is cooled by contact with the

cooler glass surface descends and heated air rises. The former is called

3
forced convection and the latter is called free, or natural convection and

is dependent on the surface geometry, orientation and the difference in

temperature between the surface and the air. In both types of convection,

the transfer of heat occurs in a thin layer of air known as a boundary

layer, which contacts the surface and has little motion. The thickness of

the boundary layer, which is not readily determined, has a profound effect

on the flow of heat by convection.

A second heat transfer mechanism is by radiation heat exchange between the

surface and its surroundings, which takes place simultaneously with, and

independant of, convection heat transfer. Often, the total effect is

measured experimentally, and no attempt is made to separate one mode of heat

transfer from the other.

In performing heat transfer calculations, surface heat transfer is usually

treated approximately, by combining the convective and radiative components

into a single, constant surface heat transfer coefficient. Sometimes two

values are used; one for winter and one for summer design calculations

[ASHRAE 1985, p. 27.10]. Consider the steady rate of heat transfer, Q,

which occurs between the room air^ and the window surface shown in Figure 1.

This is normally written as:

Q

= h • A(T . . - T,

1 i,air A

( 2 ) where T^ = window indoor surface temperature, °C (F)

h^ = combined surface heat transfer coefficient, W/m2 *K (Btu/h*ft *F)

The combined surface coefficient consists of two independent components

given by:

hi = ri + hc 1 (3)

where radiative heat transfer coefficient,

convective heat transfer coefficient

W/m2# K ( Btu/ h * ft 2 * F ) ,

, W/m2 *K ( B tu/h * f

2 *F)

Although Eq . 2 i

Radiative heat

fourth power of

therefore, not

surfaces are at

the convective

roughness, local

and is therefore

s simple, it does not accurately represent physical reality,

transfer, in fact, depends on the difference between the

the absolute temperature of the radiating surfaces, and is

nearly proportional to T. . -Tj^ and often, the radiating

a temperature different tlia>if

1the air temperature. Moreover,

coefficient, h

c ^ , is a function of temperature, surface

air velocity, the direction of heat flow and other factors,

, not constant.

In considering the transmission

surface coefficients are required

For most building applications,

falls into the heat transfer regime

of heat through fenestration,

for the interior and exterior

the inside surface convective

of free convection. For many

d if feren

surfaces

component

years ,

1 - Assuming the room surfaces are at the same temperature as room air.

4
ASHRAE used a combined inside surface coefficient of 8.3 w/m^*K (1.46

Btu/h'ft^*F) for design purposes. This value has also been widely used in

the European community, however the German speaking countries use the

slightly lower value of 8.0 w/m^'K (1.41 Btu/h'ft^'F) [Jonsson, 1985].

The average inside surface free convection heat transfer coefficient, h . is

however, a weak function of the temperature difference between the inside

surface and the inside air provided air motion is not induced mechanically

by fans. This relationship is given by:

h . ci

= C- (T. . 3- l.air (4)

where a constant depending on the inside air thermophysical properties,

m = 1/3 or 1/4, depending upon whether the flow is laminar or turbulent

•

To some extent h^ varies with window size due to boundary layer thickness

variations in the vertical direction. Shorter windows would tend to have

larger average convection coefficients than taller windows.

Different methods of calculating the natural convection heat transfer

coefficient, h are documented by ASHRAE. For isolated vertical plates, a convection coefficient of 3.24 w/m^’K (0.57 Btu/h*ft^*F) is obtained by

subtracting the radiative component from the total surface heat transfer

coefficient [ASHRAE 1985, p. 23.3] based on a surface temperature of 21 C

(70 F) and a surface to air temperature difference of 5.6 C (10 F) . In the

same reference, equations are given for temperature-dependent, freeconvection laminar and turbulent flow [ASHRAE 1985 , p. 3.13]. In yet

another chapter of the same reference, combined radiative and convective

coefficients for the interior surface of windows are given for laminar freeconvection with different values of surface emittance, surface temperature,

and surface to air temperature difference [ASHRAE 1985, p. 27.14].

Convective coefficients obtained by these three methods can easily differ by

a factor of two. Recent research indicates that the convective correlations

obtained for isolated vertical surfaces may not be representative of walls

or windows in room enclosures, because of the frictional effects of the

other room surfaces [Bauman et al., 1983]. That research developed new

correlations which compared favorably with the ASHRAE turbulent heat

transfer correlations, although the experimental observations indicated a laminar flow regime.

The radiative component of surface heat transfer for the interior of a glazing surface depends primarily on the relative amounts of glazed and

unglazed surfaces seen by the particular surface and on the temperature

difference between them. For normal situations in which a window views

only the non-glazed surfaces of a heated room that are at approximately room

air temperature, the radiation coefficient is of the same order of magnitude

as the convection coefficient. If, however, the window views mainly other

windows such as in an atrium or sunspace, the radiative component of heat

transfer is essentially zero, because the temperature of all the window

surfaces are approximately the same. Similarly, the radiant component will

be substantially greater than the convective component whenever the window

5
receives radiant energy from surfaces that are much warmer than the room

air. This can occur when a window views a radiant heater, an operating

fireplace, or a solar storage wall.

The combined heat transfer coefficient for the outside surface, h

Q is a function of the air temperature, the effective radiating temperature of the

outdoor surfaces and the wind speed. The wind speed and orientation of the

building surface relative to the wind direction are important factors in

determination of the outside heat transfer coefficient. Wind speed can vary

from less than 0.25 m/ s (50 ft/min) for calm weather, free convection

conditions, to over 11.2 m/ s (25 mph) for stormy weather conditions. ASHRAE

has used two values; a winter design value of 34.1 W/m^’K (6.0 Btu/h’ft^’F)

corresponding to a wind speed of 6.7 m/ s (15 mph), and a summer design value

of 22.7 w/m^ *K (4.0 Btu/hr * f t^ *F) corresponding to a wind speed of 3.3 m/

(7.5 mph). In contrast, the European community uses design outside surface

coefficient varying from 18 w/m^'K (3.17 Btu/h*ft^*F) in the United Kingdom

[CIBS, 1980] to 23 w/m^'K (4.1 Btu/h*ft^*F) in the German speaking nations

[ Jonsson , 1 985 ]

The simplified representation of surface heat transfer described above,

however, is often adequate in many applications, such as heating or cooling

load calculations, where the design values represent extreme heat loss

conditions and when low-thermal-conductance components (e.g., insulated

walls) are considered. In that case, the conductive thermal resistance of

the wall itself is large compared with the surface resistances, and even

large variations of surface resistance have little affect on the overall

heat transfer. However, a more detailed analysis of surface heat transfer

is required to improve accuracy when long-term or seasonal energy

calculations are performed. In that case, it is incorrect to assume values

for the heat transfer coefficients corresponding to extreme weather

conditions (e.g., winter design values). This is particularly important

when window heat losses are analyzed.

2.1.2 Air Space Conductance

The rate of heat transfer between the inside and outside surfaces of a glazing system in Figure 1 is proportional to the glazing conductance, or Cvalue, given by:

Q

C = -_2 (5)

A(TrT

2 )

where T^ = average temperature of the window interior surface C (F),

= average temperature of the window exterior surface C (F)

,

and

C = thermal conductance, w/m^*K (Btu/hr * ft *F)

The air space thermal conductance, or C-value is an intrinsic property of

the fenestration system, while the transmittance or U-value is a function of

the C-value and the combined inside and outside surface coefficients that

depend on the environmental conditions. The relationship between these two

characteristics is given by:

6
u = ( 6 ) h i

The C-value is useful for comparing insulating properties of alternative

glazing systems having prescribed temperatures of the bounding surfaces,

while the U-value is useful for computing heat flow when the adjoining space

temperatures and air motion is known.

The geometry of many insulating glass units in windows is such that the

spacing between the lites of glass is small compared to the height and width

of the air space, therefore edge effects can often be ignored and a onedimensional heat transfer model used to evaluate thermal performance. Often

the thickness of the glazing material is small enough to ignore any

temperature gradients. With those assumptions, the C-value of an insulating

glazing unit depends only on the radiative properties of the glazing

surfaces, the glazing geometry and orientation, and the transport properties

of the gas in the space.

The rate of radiative transfer between the glass surfaces in Figure 1 is

given by

L 4

Aq(T^-t

2 ) Q_ j = r7 T7 r (7) raa l/e^ + l/£ 2 ”

Stefan-Boltzmann constant = 5.6697 x 10"® w/m^'K^ (1.73 x 10~®

Btu/h *f t^ # R^)

,

total hemispherical emittance of the indoor and outdoor glass

surfaces adjoining the air space, respectively.

Equation 7 is strictly valid for spaces containing a nonabsorbing gas (such

as dry air) that does not participate in the radiation exchange by absorbing

or emitting radiation, and for surfaces (such as uncoated plate glass) that

are both opaque to radiation in the infrared region and whose emittance does

not vary appreciably with wavelength. The development of new highperformance windows having non-air gas fills, partially transparent plastic

films and/or low emittance coatings might violate any of those assumptions,

therefore evaluation of radiative transfer in those products requires

additional consideration.

where a -

e

1 ,2 =

The convective rate of heat transfer across the air space in Figure 1 is

given by

Qconv

= AMT

1 - V (8)

where h

Q = convective coefficient of heat transfer between two isothermal

parallel surfaces separated by an air space, W/m^’K

(Btu/h *f t^ *F)

.

7
Measurements of air space convective heat transfer were made by NBS

[Robinson and Powell, 1954] that have been widely accepted by ASHRAE [1985,

p. 23.4] for computing convective heat flow across air spaces. Empirical

heat transfer coefficients for air have been generalized for fluids by

others [Caton, 1974; Raithby et al., 1977] using dimensionless correlations

involving the Nusselt, Raleigh and Prandtl numbers given by:

Nu = hL/k

Ra = gftATL^/va

Pr = v/a

where L

k

g

8

AT

v

a

air space width,

fluid thermal conductivity,

gravitational constant,

volumetric expansion coefficient,

temperature difference between glazings,

kinematic viscosity,

thermal diffusivity.

(9)

( 10 )

( 11 )

Recent research [El Sherbiney et al., 1982; Jonsson, 1985] has shown that

the convective heat transfer coefficient in a vertical air space is also

sensitive to the height-to-wid th ratio of the air space, which implies that

the air space convective conductance measured for a particular window height

and space width might not be valid for windows having height-to-wid th ratios

that are significantly different from that measured.

It is convenient to combine the radiative and convective components of heat

transfer across an air space into a single value of conductance for use in

equation (5). Tabulated values of conductance for vertical air spaces are

given by ASHRAE [1985, p. 23.4] as a function of the effective emittance of

the confining surfaces and the spacing, the mean temperature, and the

temperature difference of the air space.

In applications involving more than a single air space, such as triple or quadruple-glazed windows, or double-glazed windows with interior plastic

films, the glazing layer conductive resistance is usually ignored and the

individual air space conductances are combined into an overall conductance

in a manner similar to combining series resistances in an electrical

circuit. This method is adequate for most windows, however care must be

exercised whenever any surface in a glazing unit is partially transparent in

the infrared spectral region, such as with certain plastic films. In that

case the non-contiguous glazing surfaces are radiatively coupled via the

transparent intermediate layer, and the radiative and convective components

cannot be combined in the simple manner discussed.

2.1.3 Solar Radiation and/or Air Leakage

The heat transfer process in a window system is modified whenever short-wave

radiant solar energy strikes the glazing system and/or whenever air leakage

occurs which might be due to pressure or temperature difference. In both

8
the solar and air leakage processes, the surface temperature of the various

glazing elements may be affected by the process and in turn these modified

surface temperatures influence the free convective and the longwave

radiative heat transfer coefficients.

Air leakage usually occurs only at the moving joints of operable windows and

at the opening in the building wall where the window frame is placed,

therefore the heat transfer effect due to air leakage is assumed to be

independent of the heat transfer effects due to solar irradiation and window

heat transmission. In this report, air leakage considerations on window

heat transfer will not be discussed further, although there are potentially

significant effects that air leakage has on measured U-values

.

To deal with solar irradiated windows, ASHRAE [1985, Chapter 27] uses a simple procedure for predicting the instantaneous rate of net heat transfer

through building window systems. That procedure, while originally intended

for determining summer design-day heat gains to permit sizing of cooling

systems, is also used to characterize fenestration systems used in heating

applications. The transfer of heat through a solar irradiated window into a building space can be considered to consist of the following three

processes

:

1. Direct transmission of radiant solar energy,

2. Convective and radiative transfer of radiant solar energy absorbed

by the glazing,

3. Convective and radiative transfer of heat due to outdoor-to-indoor

temperature difference.

In the ASHRAE procedure, the two solar-driven processes are combined into a single term, and the net rate of heat transfer into the conditioned space

per unit area of glazing is given by:

qNET = F I T

+ U

day(To,air " T i,aiP (9)

where F = solar heat gain coefficient, dimensionless

= total solar irradiance, w/m^ (Btu/h'ft^),

= overall daytime coefficient of heat transfer, w/m

7 (Btu/hr-ft*-F)

.

It is common practice to reference the solar heat gain coefficient F, for a particular glazing or shading product to a reference glazing consisting of a single sheet of double-strength clear glass. The ratio F/F^^, i s called

Shading Coefficient. Manufacturers of glazing products characterize their

products by providing Shading Coefficient data at a selected value of solar

irradiance, ambient temperature, and an external heat transfer coefficient

corresponding to the ASHRAE summer design condition.

Daytime U-values are in general, different from nighttime U-values due to

the affect that solar absorption has on increasing the surface temperature

of each glazing layer. The increased temperature of each glazing layer in

turn affects the radiative and convective heat transfer coefficient and air

space conductances, depending on the total number of glazing layers, the

9
solar absorption coefficient for each layer and the solar radiation

intensity. Figure 2 depicts the variation in U-value for single and triple

glazings as a function of solar irradiance, for a range of free and forced

convection surface coefficients. The U-values are normalized to the winter

design conditions and the results display considerable variation between

free and forced convection, and winter and summer conditions. The tripleglazed window also shows considerable sensitivity to solar irradiance.

2.2 HISTORICAL REVIEW

As previously mentioned, the determination of the thermal transmittance of

windows was necessitated by the desire for heating and ventilating engineers

to be able to predict the heating loads for buildings so that they could

have a basis for sizing heating systems. ASHRAE and its predecessor, the

American Society for Heating and Ventilating Engineers (ASHVE) , has been the

primary data source for window U-values through their research projects

carried out by the ASHVE Research Center in the 1930's and 1940's, by

sponsoring research in several universities, and by publication of results

in the ASHRAE/ASHVE Transactions, Journals and Handbooks.

The earliest known research on window U-values is attributed to E. Peclet, a French experimenter who measured thermal conductivity for a wide range of

materials, including glass, in about 1860. Pdclet was apparently among the

first to recognize the importance of the surface heat transfer coefficients

in relation to the overall transmittance of a given system as given by

Equation 6, and he computed U-values for single and double glazed windows

based on analytical relationships for the surface coefficients. Pdclet's Uvalues varied with window height and also with the radiative heat flux on

the inside surface, depending on whether the window viewed unexposed

interior surfaces assumed to be at the average air temperature, or the

window viewed exposed window surfaces assumed to be at the same temperature

as the window. In that case, no radiative component exists on the inside

surface and the resulting U-values are approximately 40 percent lower than

with unexposed walls. Reviews of P£c let's data appear in very early ASHVE

Transactions [Carpenter, 1907] and tend to confirm his experimental values

for thermal conductivity and his calculated U-values for walls and windows.

ASHVE research projects in the early 1930's focused on measuring combined

convective and radiative heat transfer coefficients with respect to the

effects of wind on the outdoor surface. ASHVE sponsored research at the

University of Minnesota by Rowley and coworkers [1930a, 1930b and 1932]

measured surface heat transfer coefficients for air flow over a small heated

plate located in a confined duct space with flow parallel to the plate, and

in an air stream at the discharge of a duct with flow impinging at an angle

to the plate, varying between parallel and perpendicular. Laboratory

experiments conducted at the ASHVE Laboratory in Pittsburg by Houghton and

McDermott [1931] with a small heated plate confirmed the parallel-flow

surface coefficients of Rowley. However in field experiments using large,

unheated surfaces exposed to parallel winds [Blackshaw and Houghton, 1932],

a comparison of velocity gradients between the large unheated panels and the

smaller heated panels suggested that the surface heat transfer coefficients

observed for the small panels in a confined duct may be somewhat greater

10
than that occurring with larger surfaces in free stream conditions that are

representative of windows installed in buildings. Rowley's original data

for parallel flow surface heat transfer coefficients appear in Chapter 23 of

the current ASHRAE Handbook of Fundamentals as the basis of design heat

transfer coefficients for a variety of building materials, including

windows

•

In the 1940's, Parmelee and coworkers at the ASHVE Laboratory in Cleveland

published several papers on surface heat transfer coefficients of relevance

for windows. In one paper, experimental studies of turbulent, forcedconvection heat transfer for parallel flow were conducted in a wellcharacterized wind tunnel [Parmelee and Huebscher, 1947a] , and generalized

relationships were developed using non-dimensional heat transfer and flow

parameters. These results and the previously mentioned ASHVE research

results were compared and recommendations were made to use the new data,

which was about 20 percent lower than the previous results. Field studies

of surface heat transfer were also conducted for glazing surfaces in which

the significance of the radiative component for both the indoor and outdoor

surfaces was mentioned [Parmelee and Aubele, 1949, 1952]. It was concluded

that significant variation in the overall heat transfer coefficient for

windows would be expected in actual field installations due to the

variability of the radiative effects.

Houghten and coworkers [1941] attempted to compare U-values for 50 windows

of various descriptions, including size, number of glazings, air spacing and

frame types, which were reported between 1916 and 1932 for a variety of test

apparatus and conditions of air temperature and velocity. They concluded

that the data were inadequate and inconclusive for practical usage and

recommended development of a rigid set of test specifications based on the

guarded hot box method. Elements to be addressed included test specimen

description preparation and placement, heating and cooling equipment

description, and specification for measurement of temperature, air velocity

and heat flow.

Subsequently, the ASHVE laboratory published an exhaustive compilation of

all window heat transfer data to date, comparing both published and

unpublished data from a variety of sources extending back 40 years

[Parmelee, 1947b]. This work, which is known as ASHVE Research Bulletin

Number 1 , extended prior ASHVE work by comparing window U-values and surface

heat transfer coefficients, examining alternative hot box test methods and

commenting on the potential sources of error in hot box tests. They

concluded that it was extremely important to characterize the convective and

radiative environments to which a test window in a hot box is exposed, if

reasonable comparison between measurement and heat transfer theory is to be

obtained. Although many papers on window U-values have subsequently

appeared in the ASHRAE Transactions, the data and conclusions in ASHVE

Research Bulletin Number 1 appear to be still valid, although perhaps

forgotten by many contemporary researchers.

2 .3 ASHRAE HANDBOOK U-VALUE DATA

U-values for single, double and triple-glazed windows and skylights were

11
published in the ASHVE Guide since the early 1920 's, apparently based on 6.7

m/s (15 mph) outdoor conditions and still indoor conditions. Carr and

coworkers [1938], commented that no distinction was made in the 1937 Guide

regarding window size, method of installation, description of muntins, and

glass spacing for multiple-glazed windows, and no means were given to

distinguish between different outdoor velocities and temperatures.

Subsequently, Parmelee and Aubele [1950], described the basis of a revised

table of U-values which appeared in various editions of the ASHVE Guide and

the ASHRAE Handbook between 1950 and 1963. The U-value table consisted of

four sections; Section A for flat vertical glass with one to three glazings

and variable spacings. Section B for horizontal glazing (skylights with heat

flow up). Section C for hollow glass block, and Section D providing

multiplication factors that modify that data in parts A and B to account for

the effects of sash and frame heat conduction. A range of adjustment

factors were given depending on the percent glass, sash material (wood or metal) and number of glass layers (single, double, or single with storm

sash). According to Parmelee and Aubele [1950], these factors were derived

from the data reported in ASHVE Research Bulletin Number 1 [Parmelee, 1947b]

by comparing U-values for unframed glazing panels with U-values for windows

based on comparisons between tests performed in the same hot box under

identical conditions. The U-value data in parts A and B were calculated at

winter design conditions of 6.7 mj s (15 mph), and -18 C (0 F) outdoor

temperature, and a separate table was provided to enable conversion of the

U-value data between 6.7 m/ s (15 mph) conditions and other wind speeds

between 0 and 13.3 m/ s (30 mph). The origins of that table are unclear,

however it appears to be based on calculations involving surface

heat transfer coefficients measured under variable outdoor conditions and

described by Parmelee and Huebscher [1947a].

The current window system U-values are given in Table 13, Chapter 27 of the

1985 ASHRAE Handbook of Fundamentals [ASHRAE, 1985], which is revised every

four years. These design values are used extensively by architects and

engineers designing building heating, ventilating, and air-conditioning

systems and is the basis for the window thermal analysis in many building

energy analysis computer programs. In 1965, the U-value table was changed

to its present format containing three parts; part A for vertical panels,

part B for horizontal panels, and part C for frame adjustment factors. The

table was also expanded to include U-values at s ummer design conditions. In

1977, U-values for window systems with interior shades appeared in part A

for both winter and summer conditions, despite the warning given in the 1972

Handbook not to consider the presence of shades when computing design

heating loads. In the 1981 Handbook, part A was further expanded to include

glazing U-values with various combinations of storm sash including either

acrylic or glass sheets added to the inside, and glass sheets added to the

outside. U-values are provided for all combinations of storm sash for both

winter and summer conditions, with and without interior shades. At the same

time, the part C adjustment factors were expanded to include storm and sash

added to multiple-glazed windows, however the factors previously given for

different glass fractions were combined into a single range of factors

without distinction for size. In the 1985 Handbook, air space conductance

data was provided to show the effects of various gas-fill compositions for

12
different glass spacings, and a figure showing the variation in U-value with

outdoor temperature was provided to show the effect of various glass

spacings, wind speeds, low emittance coatings and glazing layers for winter

and summer design conditions.

2.4 CONCERNS OVER CURRENT U-VALUE DATA BASE

In recent years, questions have been raised regarding both the accuracy and

the adequacy of the U-value data appearing in the ASHRAE Handbook. This may

be partially due to the deletion of much of the rich anecdotal,

bibliographical and technical information on the sources of the U-value data

provided in earlier versions of the Handbook. Unexplained revisions to the

winter design U-values for double and triple glass from the 1955 values

appeared in the 1965 and again in the 1977 editions. In addition, the table

of U-values has grown significantly in recent years with the addition of

summer design conditions, interior shades and interior and exterior storm

sash combinations. Meanwhile, newer types of windows are not included;

including glazing units with low-emittance coatings and plastic films, and

framing materials comprised of composites of wood, metal and plastic. The

effects of window size are also not included in the U-value table. Another

concern is that ASHRAE Handbook data have been used in various ways by

window and window system manufacturers to arrive at U-values used in

advertising claims, instead of using test data.

The net result of this situation is that the advertised U-values for windows

and window systems are not generally comparable since they do not have a standardized method, either experimental or analytical, for being

determined. This is especially true for window treatments where a wide

variety of shutters, shades, and curtains, are applied to various types of

windows and the resulting range of the advertised U-values can differ by

more than 100 percent for the same product. This tends to confuse consumers

who are interested in conserving energy in their buildings and would like to

have some reliable standardized information on which to base their window

system purchase decisions.

There are two general ways that the consumer is provided window system

energy conservation information. The first is the manufacturers advertised

U-value which is provided for a single set of environmental conditions that

are sometimes not specified. The second is the information provided by

building energy analysis computer programs which calculate the thermal

performance of window systems installed in various types of buildings

(residential, commercial, etc.) and then determine the overall heating and

cooling season energy and cost savings. Both of these consumer information

sources require accurate (and comparable) window system U-value information

to be useful. The first requires a single U-value at a standard

environmental condition while the second requires a range of U-values for

the environmental conditions to which the window system might be exposed.

All of this indicates that there is an urgent need to have a standard

procedure for determining the U-value of windows and window systems. The

next section reviews the current test methods used in the United States,

Canada and in Europe for measuring the U-values of windows. Section 4 will

13
present information on test methods used for measuring thermal performance

of movable insulation systems used in conjunction with windows.

To.air^

Outdoors

^i.air

Indoors

Figure 1. Idealized Drawing of a Double-Glazed Window

14
U/Uref

SOLAR IRRADIANCE (Btu/hr-ft 2

) 0 50 100 150 200 250

Figure 2. U-Values for Solar Irradiated Glazings

15
3 . TEST METHODS FOR MEASURING FENESTRATION U-VALUE

The purpose of this section is to provide an overview of existing standard

and non-standard laboratory and field test methods and to discuss their use

in measuring U-values for windows. Section 3.1 describes in some detail the

hot box test methods most frequently used in the United States for measuring

window and skylight U-values under laboratory conditions. Section 3.2

briefly describes the laboratory hot box test methods used in several other

nations. Section 3.3 describes site visits to a number of testing

laboratories conducted as part of this program and Section 3.4 briefly

outlines discussions of U-value test methods at two recent national

conferences

3 .1 UNITED STATES PRACTICES (HOT BOXES)

There is no single United States standard test method for measuring window

U-value that has been accepted. The majority of the manufacturers of

windows and window treatment products use hot boxes to test windows. The

reasons why this situation exists are outside the scope of this report,

however, a discussion of the laboratory test methods currently used, in the

order of their existence, will be presented.

Numerous technical papers and reports deal with experimental measurements of

U-value of glazings and various types of windows. As previously mentioned,

descriptions of the early apparatus and test procedures are summarized in

ASHVE Research Bulletin No. 1 [Parmelee, 1947b]. With a few exceptions, the

testing apparatus has been some form of hot box, which still forms the basis

for the current laboratory practice. In essence, a hot box is an enclosure

made in whole or in part with the material to be tested. One side of the

test specimen is heated, usually by exposure to warm air in the adjacent

enclosure, and the other side is cooled by exposure to conditioned air

either at room temperature or to a refrigerated space. The air on either

side of the test specimen may be allowed to circulate by natural thermal

action or by forced action using fans. The U-value is computed using

Equation 1 from the measured heat source dissipation rate minus any heat

flow through surfaces other than that of the test specimen, divided by the

product of the projected area of the test specimen in the heat flow

direction and the air-to-air temperature difference. The major difference

in classification of hot boxes today is based on whether the metering

chamber is of the calibrated design or the guarded design.

3.1.1 ASTM C236 Guarded Hot Box

The ANSI/ASTM C236 Standard Test Method for Thermal Conductance and

Transmittance of Built-up Sections by Means of the Guarded Hot Box [1985a],

which was first approved as an ASTM consensus standard in 1949, has been

edited, revised, and reapproved several times. The current edition was

issued in 1966 and was reapproved in 1980. It is presently undergoing study

for revision by ASTM Committee C16.30.

The ASTM C236 test apparatus shown in Figure 3 consists of a cold box, which

simulates winter environmental conditions, a metering box, which simulates

16
the inside conditions of a building, and a guard box, which surrounds the

metering box on five sides and is controlled to maintain the same

temperature as the metering box. The electrical energy input to the

resistance heaters in the metering box is assumed to be equal to the rate of

heat transfer through the test specimen that is placed between the cold box

and the guard/metering boxes. In using this method for window testing, the

window is normally installed in a mask wall constructed from a material of

known thermal conductivity. Low thermal conductivity homogeneous walls are

preferable, however nonhomogeneous mask walls constructed of wood studs and

filled with an insulation product are also used. The metering box covers

the window and a portion of the adjoining mask wall, and the net heat

transfer rate through the window is calculated by subtracting the heat

transfer rate through the mask wall from the total metering box power input.

3 .1 .2 ASTM C976 Calibrated Hot 3ox

The ASTM C976 Standard Test Method for Thermal Performance of Building

Assemblies by Means of a Calibrated Hot Box [1985b] was approved as an ASTM

standard in 1982. There have been calibrated hot boxes in operation since

the early part of this century, however it took approximately six years for

the present standard to be drafted, revised, and approved. Because it is of

more recent vintage, the calibrated hot box standard reflects more of the

current developments in hot box operation, instrumentation, and equipment

that gradually evolved based on years of hot box operational experience. An

updated C236 test method would probably contain a lot of these newer

developments, and some individuals have been suggesting that the ASTM C236

and C976 test methods be combined in a manner similar to that being

currently described in an International Standard Organization (ISO) Guarded

and Calibrated Hot Box Test Procedure.

The primary difference between the calibrated hot box and the guarded hot

box is the absence of a guard box in the calibrated hot box method. Figure

4 shows a schematic drawing of a calibrated hot box. The absence of the

guard box requires that the metering chamber heat transfer rate be

accurately known by calibration, which requires determination of the

flanking heat transfer rate that occurs through the test specimen frame at

the juncture of the metering and cold boxes. The method to accurately

perform this calibration procedure is still undergoing development. The

advantage of the calibrated hot box is that it can measure the U-value of

larger test specimens for a given cold box size since there is no guard box

to reduce the specimen heat transfer area. Several calibrated hot box

facilities have been constructed or are undergoing modification so that

dynamic measurements can also be made for thermally massive building

elements such as masonry walls. In these facilities, both the cold sides

and hot sides are capable of undergoing variations in temperature,

barometric pressure and relative humidity so that the nonsteady effects of

heat transfer, air leakage, and moisture transfer can be studied

simultaneously. If solar simulation were to be added to the environmental

side, then the overall dynamic thermal performance of windows and window

systems can be determined in the laboratory. This capability is not yet

available, however the activities in dynamic testing should be followed and

evaluated from an overall thermal performance viewpoint for window systems.

17
3 .1 .3 AAMA 1503 . 6 Guarded Hot Box

The AAMA 1503.6 Voluntary Standards and Tests of Thermal Performance of

Residential Insulating Window and Sliding Glass Doors [1980] was developed

by the American Architectural Manufacturers Association as a method to test

aluminum framed windows, doors, and curtain walls for overall thermal

transmittance (U-value) and also to test multi-paned window systems for

resistance to water vapor condensation. Although the AAMA method is

essentially a calibrated hot box test method, the original version of this

standard was developed from the ASTM guarded hot box method, however more

recent versions of the AAMA standard deleted most of the references to ASTM

C236. This standard test method differs from the ASTM C236 and C976 hot box

test methods in several ways including wind simulation, temperature

measurement and calibration. Figure 5 shows a schematic drawing of an AAMA

hot box of the calibrated configuration. At least one AAMA hot box has been

constructed that is of the guarded type.

As shown in Figure 5, the primary difference between the AAMA and ASTM

methods is the direction and magnitude of the simulated outside wind

velocity, which is directed normal to the window surface at a speed of 6.7

m/ s (15 mph) in the AAMA method. The fans used in the AAMA hot boxes have

much larger dimensions and flow capacities than those used in the ASTM hot

boxes to achieve as uniform coverage as possible. In order to minimize the

possibility of leakage due to air impingement in the AAMA test, the metering

side box (warm side) is pressurized to 25.0 pascal (0.10 inches of water)

above the ambient pressure to effect a balance with the dynamic pressure on

the cold side due to the impinging wind. It should be noted that even

though the wind is delivered perpendicular to the window, at the window

surface the air in the boundary layer will be tangential due to the turning

of the flow caused by the solid window surface.

Another difference between the AAMA and the ASTM test methods in the

measurement of the U-value is the number and location of the temperature

sensors. In the AAMA method, a single air temperature sensor (usually a radiation shielded thermocouple) is located on the cold side and three air

temperature sensors are located on the warm side. Both the ASTM C236 and

ASTM C976 test methods specify many more temperature sensors on both the

test specimen surfaces and in the air stream adjacent to these surfaces,

thus allowing both area weighted thermal transmittance (U-value) and thermal

conductance (C-value) to be determined.

A third difference between the AAMA and ASTM test standards is the method of

determining the rate of heat transfer through the mask wall in which the

test window is installed and the method of measuring the surface heat

transfer coefficients during calibration. In the AAMA test method, the

surface heat transfer coefficients at the window surfaces are estimated by a calculation procedure using a calibration panel consisting of a double pane

window with a 51 mm (2 in) air space. A value of thermal conductance (Cvalue) for the calibration panel is prescribed, although the rationale for

the selected value is not discussed in the AAMA 1503 .6 standard. The

prescribed C-value of the calibration panel is subtracted from the measured

thermal transmittance (U-value) to determine the inside and outside surface

18
heat transfer coefficients. When the prescribed exterior and interior

surface coefficients are obtained with the calibration panel installed, the

hot boxes are considered to be calibrated to the AAMA 1503.6 standard. This

calibration procedure differs from those in other laboratories in which the

temperature difference across a known thermal conductance replacement

material in the window space is measured along with the air and surface

temperatures on each surface. The known-conductance material is usually

characterized by testing in an ASTM G177 guarded hot plate or some other

absolute thermal conductance device.

3.1.4 Sources of Error in Hot Boxes

As previously mentioned, early hot box investigators concluded that in order

to reduce potential sources of error in hot box tests of windows, it was

extremely important both to miniminize sensor inaccuracies and to

characterize the convective and radiative environments to which a test

window is exposed. For most hot box test methods, the accuracy of the basic

instrumentation such as temperature sensors (usually thermocouples),

electrical energy sensors (watt-hour meters or voltage/current meters), and

velocity and pressure sensors affects the computed test results in a similar

manner.

3. 1.4.1 Temperature Measurement

Most temperatures are measured by thermocouples and it is important that the

calibration of the thermocouple wire be known since the emf-temperature

relationship varies in commercial thermocouple wire. A possible source of

error is introduced when the lead wires are connected in junction boxes to

other lead wires extending to the readout device. It is possible for heat

conduction to set up thermoelectric voltages at junctions formed between the

thermocouple leads and connectors. These voltages tend to cancel but can be

a possible source of error. It is good practice to make the extensions of

the thermocouples continuous from the thermocouple junction to the switching

device or to the meter, both of which should be kept at uniform

temperatures. The use of platinum resistance thermometers has some

advantages over thermocouples due to improved accuracy, sensitivity and

stability, however at greater sensor cost and with the possibility of error

resulting from mechanical strain resulting from the handling or supporting

of lead wire.

Unshielded thermocouples that measure air temperature also respond to the

radiant effects of their surroundings. The relation between the true air

temperature and the apparent air temperature depends on the mean radiant

temperature of the surroundings and the convective and radiative surface

coefficients at the junction of the sensor. Reducing the emittance of the

thermocouple junctions by polishing, increasing the air motion or shielding

the junction by multiple-concentric shields will reduce this source of

error

.

The measurement of glass surface temperature is also difficult and subject

to considerable error and uncertainty. In many cases the test panels are

not homogeneous, but the glass is broken up by muntins or other structural

19
elements of windows. Also, variation in the air temperature from top to

bottom affects the surface temperature, A reliable average temperature can

be obtained by weighting many temperature measurements.

Surface temperatures are usually obtained by fastening both the thermocouple

junction and lead wire to the surface with epoxy or transparent tape. Fine

gauge (28 gauge or less) thermocouple lead wire is preferred for obtaining

surface temperatures of glass.

3 .1.4.2 Electrical Energy

Although it is generally considered that the electrical measurement of the

heat source energy input to any test box is probably the most accurate

single measurement made, two factors are important. One factor is the range

of the readout instrument. For example, an instrument may claim to-be

accurate within 1/4 of 1 percent. This means 1/4 of 1 percent of the full

scale reading of the instrument. Therefore, if an energy input of 20 W

(watts) were read on a watt meter which read 100 W at full scale, the

percentage of error would be 1-1/4 of 1 percent. The second factor is the

waveform of the electrical energy source. Modern temperature controllers

usually function by chopping the AC waveform. Watt-hour transducer that are

calibrated using sinusoidal waveforms may have significant errors introduced

due to distorted waveforms caused by the controllers, therefore calibration

of all transducers over the range of operating power is essential for

accurate measurement of all electrical sources, including power for heaters

and operating fans.

3.1 .4.3 Air Motion

Another significant factor affecting the results of tests is the matter of

air motion in the test boxes. Measurement of the air velocity at a specified distance from the test surface only partially defines the

character of the convection heat transfer. The usual method is to use a hot

wire anemometer which has the disadvantage of being subject to radiation

effects and of being non-directional. Only when the air stream is uniformly

distributed and moves along the test surface in a definite path (a condition

which rarely occurs) does the velocity measurement take on significance in

comparing different sets of data.

3.1 .4.4 Heat Transfer Rate Through Test Window

Measurement of the rate of heat transfer through a test window as in

equation 1 to calculate the U-value, is usually accomplished by subtracting

heat losses from the metering box surfaces and the mask wall from the

measured electrical energy input. In general, those heat transfer rates are

minimized by designing high thermal resistance paths, and/or by guarding and

then measuring the resulting heat transfer rates by calibration.

For guarded hot boxes specifically, there is a need to measure the metering

box heat transfer to the guard box, which although relatively small for

conventional high-U-value windows, may be more significant for the lower Uvalue window systems being marketed or under development. In addition, the

20
guard box must be larger than the window or other fenestration device to be

tested. For some guarded hot boxes currently in use, this limits the size

of the window that may be tested and eliminates the possibility that larger

fenestration products such as glass doors can be studied. Another possible

source of error specific to guarded hot boxes is the contact between the

guard box and the test wall (or mask) in which the test window is mounted.

Until recently, the thermal transmittance of the nonhomogeneous mask walls

used by laboratories with AAMA 1503 .6 hot boxes were not calibrated

experimentally, but were estimated using standard ASHRAE calculation

methods. Since their construction was not specified in the AAMA 1503.6

standard, the heat transfer rate through these mask walls was not well

known. For higher U-value windows this may not be a major problem, but for

lower U-value window systems, the uncertainty in the mask wall heat transfer

rate may cause a significant error in the window system U-value. More

recently, an AAMA laboratory constructed improved test walls (wooden stud

wall with space completely filled with extruded polystyrene) which should

lend to better quantification of the wall heat transfer rate. If these

walls were to be calibrated experimentally by filling the window space with

a known high R-value insulation product, they would be quite well

characterized from a thermal view-point.

3 .1.4.5 Simulation of Forced Convection on Exterior Surfaces

A standard practice in existence for many years has been to rate the thermal

performance of windows and other building envelope components based on

forced-convection conditions on the exterior and free-convec tion condition

on the interior surfaces. For sizing heating equipment, ASHRAE winter

design U-values are based on a 6.7 m/s (15 mph) wind, which provides as combined convective and radiative surface coefficient of 34.1 W/m^*K (6.0

Btu/h*ft^ *F) . This coefficient is traceable to the original work of Rowley

[1930b], for parallel flow over a small heated plate, which was previously

described in section 2.2.

The simulation of the outside wind condition is one of the primary

differences in the test methods used to measure the thermal performance of

window and window systems and is a matter of considerable controversy. For

the ASTM Guarded Hot Box test method, the external air motion is induced by

a fan past the window surface in an upward direction and at a constant

speed. Wind speed can vary from natural convection conditions to forced

convection conditions depending upon the size and speed of the fan. The

direction of the simulated wind is tangential or parallel to the plane of

the window. In the ASTM Calibrated Hot Box test method, the simulated wind

direction is parallel and may vary from vertical to horizontal. Conclusions

cannot be made regarding the "best" speed or direction to simulate outside

wind conditions because the actual wind speed and direction are quite

variable, depending upon the local weather conditions, the location and

surroundings of the building, and the particular exposure of the building

surface on which the window is located.

Most often, convective heat transfer research data are obtained from smallscale measurements conducted in carefully controlled wind tunnels with the

21
flow fields fully characterized with respect to velocity and temperature

profiles and turbulence intensity [Gray, 1985]. In contrast, thermal

performance measurements in hot box test facilities with simulated wind are

often conducted on a full-scale test specimen and U-value is measured

without any real attempt to either characterize the flow field or measure

the convective heat transfer rate on the exterior of the test specimen.

The ASTM C236 and C976 test methods currently permit the testing of windows

at nearly zero external wind speed and adjusting the results to the ASHRAE

winter design wind speed. This adjustment technique is controversial

because of the uncertainty of the distribution of surface coefficients with

wind speed and direction. The testing of windows at 6.7 m/ s (15 mph) is

also controversial because field measurements have shown that in many

locations, wintertime climatic conditions produce local wind speeds that are

very low, therefore a mixed convection regime occurs in which both free and

forced convection mechanisms are comparable in magnitude. Under conditions

such as those, radiative heat transfer may contribute significantly to the

heat transfer rate from a fenestration system.

AAMA Standard 1503.6 prescribes a method for adjusting the air distribution

on the cold side of a window to achieve the ASHRAE winter design average

surface heat transfer coefficient. The AAMA procedures uses a calibration

panel, which consists of an insulating window with a 51 mm (2.0 in.) air

space with a number of thermocouples mounted to the warm and cold side glass

surfaces, that is installed in the mask wall in place of a test window. The

air flow is adjusted until the temperature difference between the warm and

cold surfaces is equal to a prescribed fraction of the temperature

difference between the cold surface and the cold room air. The technical

basis for this method of determining the surface heat transfer coefficients

is not well substantiated.

3. 1.4. 6 Air Leakage

Utilizing fans to provide air motion on either side of a test window

increases the possibility of air leakage through the window seals and

between the window frame and opening in the mask wall. If the direction of

air leakage is from the cold side box into the warm side (metering) box,

additional heater energy will be required to heat the cold air and the

calculated U-value will be in error. Several mechanisms have been

hypothesized for air leakage, depending on the test method.

It has been recognized by several operators of calibrated hot boxes that if

a test specimen has any potential sites for air leakage, such as the sliding

joint of a window or the unfaced surface of a fibrous insulation board

,

that breathing may occur. Breathing is a process whereby the cold air and

warm air on either side of a test specimen is periodically exchanged. It is

believed, but not proven, that this breathing is due to the unsteady nature

of the circulating air fans. All fans produce pressure and flow pulsations

due to the finite blades "chopping” the air flow. Thus, larger fans have a greater potential for producing air leakage through the window air leakage

area. This mass transfer tends to increase the heat transfer rate, and

therefore, the overall thermal transmittance. However it has not been

22
established for window testing using the different types of hot boxes. It

may be necessary to install infiltration measuring equipment in some of

these boxes to ascertain the magnitude of this air leakage for different

types of windows.

A greater potential for air leakage between the cold and hot boxes is

believed to exist with the AAMA method than the ASTM method due to the

perpendicular wind impingement on the cold side of the test window.

Although the warm side chamber is pressurized to maintain a static pressure

equal to the dynamic pressure in the discharge plenum of the cold side fan,

non-uniform pressure distribution within the cold chamber, particularly at

the window surface, may result in local pressure differences that contribute

to air leakage. Thermal contraction of window seals due to cold side

temperatures and mechanical contraction of window surfaces and seals due to

pressure forces may also contribute to air leakage.

3 .1 .5 Future Hot Box Activities

The guarded hot box and the calibrated hot box have been the most widely

used test method for measuring the U-value of window systems. Under

laboratory conditions, the calibrated hot box has been increasing in

popularity as indicated by the number of facilities being built in recent

years. With improved calibration procedures and operational experience,

calibrated hot boxes may soon attain the maturity of guarded hot boxes . They may be utilized more frequently for research and development work,

while the guarded hot box facilities may be used for rapid, repetitive

testing applications.

Previous discussion of the AAMA test method should not be construed as criticism of either the AAMA 1503 .6 standard or the AAMA hot boxes.

Comments were offered to assist the evolution of the AAMA hot boxes as they

are starting to reach the maturity of the C236 hot boxes. The ultimate goal

should be the development of a standard thermal test method for window

systems in which all of the existing hot box test facilities can be modified

and/or calibrated to allow them to meet such a standard. None of the boxes

should be required to undergo substantial physical modifications, however

the test results for the same window or window system should be comparable.

The errors associated with the calibrated hot box instrumentation are

similar to those for the guarded hot box. The potential calibration errors

are greater, however, with the evolution of more accurate calibration

procedures they should be minimized. The errors associated with

instrumentation in the AAMA 1503.6 standard, are similar to those discussed

for the ASTM C236 hot boxes. The potential errors in calibration have been

discussed previously and possible methods to reduce these have been

presented. Since the AAMA standard appears to be evolving towards the ASTM

C236/C976 test methods, it would be expected that the overall errors will be

similar. Since none of the laboratories with an AAMA hot box decided to

participate in the recently completed ASTM round-robin test of rigid

insulation board, a direct comparison between AAMA and ASTM hot boxes is not

available. That is why there is a real need to have an interlaboratory

comparison (not a full round-robin) with selected hot boxes (AAMA and ASTM)

.

23
Chapter 6 of this report outlines an interlaboratory comparison program that

is needed in the near future.

3 .2 INTERNATIONAL PRACTICE (HOT BOXES)

Similar to the United States, there is no unique international method for

testing windows although individual countries have adopted national

standards. The International Standards Organization (ISO), which has been

developing an international standard test method for guarded and calibrated

hot boxes, recently proposed the addition of an appendix in which the

application of this standard to the measurement of window U-values is

addressed. Since 1983, the International Energy Agency ( IEA) has been

working on a research project "Fenestration and Windows," designed to

standardize the evaluation of thermal and solar properties of windows and to

clarify the interaction between windows and building energy consumption [Van

Dijk and Knorr, 19853.

Other European nations and Canada have also been active in research directed

toward understanding the basic physics of window heat transfer, in addition

to developing standard test methods.

The Scandanavian countries, in particular Norway, Sweden and Denmark, all

have national standards for measuring U-values of window systems. In the

opinion of the authors, the coordinated national efforts that are funded by

the public sector in the European community have resulted in testing

facilities for windows and standards that are in an advanced state of

maturity. Other countries, especially Canada, Belgium, Switzerland and

Germany also have significant national programs aimed at the development of

test methods for windows.

In this section, window test methods used in Norway, Sweden, Belgium and

Canada are outlined. NBS obtained copies of several national standards for

testing windows and for calculating thermal performance. Translations of

these standards are provided in Appendix A.

3.2.1 Norwegian Practice

Norwegian building code requires that the U-value of windows for design

purposes be less than prescribed values. The U-values are based on testing

that is performed at the Norwegian Building Research Institute, located in

Trondheim, Norway. In addition, research programs have been conducted at

that institute to measure the thermal performance of sealed glazing units

with special low-emi t t anc e coatings and with gas fills [Breder and

Heiersted, 1984].

Appendix A-l provides a translation of Norwegian Standard NBI-138, 1982 for

thermal measurements of sealed glazing units. The standard defines the test

method, sample size, temperature sensor placement, and by reference to other

standards, the measurement accuracy and calibration procedure. The test

results consist of the measured surface-to-surface conductance (C-value),

and the U-value is calculated based on addition of prescribed surface

24
resistances corresponding to 8.3 w/m^*K (1.45 Btu/h*ft^*F) inside and 20.0

w/m *K (3.52 Btu/h*ft *F) outside surface coefficients.

In the draft standard for window U-values, the measurements are made with a parallel wind on the outside and still air conditions on the inside of the

test specimen [Heiersted, 1983]. The prescribed exterior surface heat

transfer coefficient is obtained by adjusting the air flow rate, using a calibrated heat flux transducer to measure the surface coefficient. The

heat flux transducer is a laminated assembly, consisting of two 4 mm (0.16

in) glass face sheets and a 10 mm (.39 in) extruded polystyrene core, that

is the same overall dimensions as the test window.

3.2.2 Swedish Practice

Swedish building codes, which are similar to those in Norway, also require

testing of windows in hot boxes under standard conditions. Although Sweden

is not a participant in the IEA fenestration research previously mentioned,

Sweden recently assumed the secretariat position for preparing the new ISO

window U-value test standard.

Appendix A-2 provides a translation of Swedish Standard SS 81 81 29,

"Thermal Resistance Test of Windows." The standard defines the test

facility, instrumentation, specimen preparation and test procedure. The

test results include a composite C-value, based on area weighting of

conductances for the glazing unit, sash, and frame members. The Cvalue is adjusted to a U-value by the addition of standard film resistances,

given by the Swedish Building Code.

Researchers at several Swedish universities have made significant

contributions to the knowledge of window heat transfer and materials

technology. Of particular importance is a treatise "Heat Transfer Through

Windows During the Hours of Darkness with the Effect of Infiltration

Ignored," by Jonsson [1985] at the Lund Institute of Technology. English

language translations of this and other important work are available from

the Swedish Council of Building Research, in Stockholm^-. This report

provides a very detailed descriptions of the test method, calibration

procedures, test results and a very comprehensive theoretical analysis of

all modes of heat transfer in windows, including convection in the air space

and conduction through the sash and frame windows.

3.2.3 Belgian Practice

The Building Research Institute in Brussels, Belgium developed a new

standard for window U-values based on improved methods for calculating heat

transfer through the component parts. The improved calculation method,

which is based on substantial laboratory testing and theoretical analyses , is presented in an English language translation in Appendix A-3 . This

document will be incorporated into a revised version of Belgian Standard NBN

1 - Address is Sankt Goransgatan 66, S-112 33, Stockholm, Sweden.

25
B62-002. It is believed that the new ISO standard for windows will be based

largely on the Belgian standard.

The basic approach in NBN B62-002 is that the central glazing unit U-value

and C-value are calculated based on theoretical considerations, involving

the transport properties of the gas fill, radiative properties of the

glazing surfaces and the geometrical description of the unit [NBN B62-004,

1986 3 .

The overall U-value for a window is determined by a calculation involving

the area weighted U-value of the central glazing unit, the spacer between

glazings, the frame, and any opaque panels that may be present. Frame and

spacer heat transfer coefficients are provided in the standard for a wide

range of materials and configurations, and simplified formulae are presented

that permit calculation of the two-dimensional heat conduction effects' of

glass, spacers and frames. In addition, a proprietary computer program,

developed by Standaert in his doctoral studies at the Catholic University of

Leuven, Belgium is available to perform the two-dimensional heat transfer

analysis of window, spacers and frames [Standaert, 1984].

3.2.4 Canadian Practice

The National Research Council (NRC) in Ottawa, Canada decided to build a new

guarded hot box for window testing rather than use the existing guarded hot

box previously used for measuring the thermal performance of building wall

assemblies (Bowen, 1985). Their new window hot box differs from a typical

guarded box by the presence of an isothermal baffle on the warm side and a wind machine on the cold side. The wind machine provides a uniform velocity

perpendicular to the window surface using a variable speed fan which directs

air through a number of parallel, rotating tubes shown in Figure 6. The

isothermal room side baffle enables accurate characterization of the radiant

environment and determinant on an effective temperature of the room side

surface. By having an accurately-known baffle temperature, the mean window

surface temperature is calculated from an energy balance on the room side.

This allows determination of both the air-to-air U-value, and the surfaceto-surface C-value for a test window.

The NRC is an active participant in the ASTM C16/E06 joint task group

activities described in Section 5 of this report. The ASTM draft standard

practice in Appendix B-3 of this report is based on the NRC window hot box

calculation procedure.

3 .3 FIELD VISITS TO LABORATORIES

Field trips have been made by the authors of this report to several

laboratories which have guarded hot boxes that are operated in accordance

with ASTM C236. Laboratories visited in the past several years include

Dynatech in Cambridge, Massachusetts, W. R. Grace in Cambridge,

Massachusetts, Jim Walter Research in Tampa, Florida, Dynatherm in Blue

Lake, Minnesota, Dow Chemical Research Center in Granville, Ohio, and Butler

Manufacturing in Kansas City, Missouri. Other guarded hot boxes are located

at Wiss, Janey and Fisher in Skokie, Illinois. The guarded hot boxes at W.

26
R. Grace, Jim Walter Research, and Dow Chemical were manufactured by Wiss

,

Janey and Fisher. Most of the above boxes have been used for in-house

product testing of a wide range of building assemblies such as insulated

studded ceilings and roofs, insulated metal building roofs, walls, and

windows and doors installed in walls. They have been used to develop and/or

verify some of the building assembly transmission coefficients (U-values)

given in Tables 4A to 4L , Chapter 23 of the 1985 ASHRAE Handbook of

Fundamentals. In addition, most of the laboratories listed above have

participated in the recent ASTM round-robin for guarded and calibrated hot

boxes so that data on the precision and accuracy for these boxes can be

developed in the near future.

Based on visits to these laboratories, the authors have concluded that

guarded hot boxes are mature thermal test facilities and are operated by

experienced personnel. Through participation on ASTM Committee C16, Thermal

Insulation and Vapor Retarders, these laboratories have developed and

improved the understanding and use of guarded hot boxes to this mature

state

.

The authors also visited several of the laboratories with ASTM C976

Calibrated Hot Boxes. The laboratories visited are Owens-Corning Fiberglass

Technical Center in Granville, Ohio which has two (vertical and horizontal

test specimens) Calibrated Hot Box (CHB) test facilities, Jim Walter

Research in Tampa, Florida (its Wiss, Janey and Fisher hot box can operate

in either the guarded or calibrated mode) , the University of Massachusetts

at Amherst, Massachusetts which is based upon the Owens-Corning vertical hot

box design, Portland Cement Association Construction Technology Laboratory

in Skokie, Illinois, Manville Service Corporation in Denver, Colorado, and

the CHB at the National Bureau of Standards in Gaithersburg, Maryland.

The Calibrated Hot Box of the Construction Technology Laboratory, Portland

Cement Association has had the capability of operating a dynamic cycle on

the environmental side (usually the cold box) for several years and has

produced a significant quantity of dynamic cycle data. The Owens Corning

Fiberglass horizontal Calibrated Hot Box, the Manville Service Corporation

Calibrated Hot Box, and the National Bureau of Standards Calibrated Hot Box

all are also capable of operating a dynamic cycle and should start producing

dynamic cycle thermal performance data in the near future.

Visits have also been made to two of the laboratories with hot boxes

conforming to the AAMA 1503.6 window thermal performance tests. These are

Architectural Testing, Inc. of York, Pennsylvania and National Certified

Testing Laboratory also of York, Pennsylvania. The third laboratory is

Electrical Testing Laboratory (now ETL) of Cortland, New York. The new hot

box at Architectural Testing appears to be setting the evolutionary trend

for the current AAMA standard now under revision. It has state-of-the-art

instrumentation and controls and is fully computerized. It is a true

guarded hot box and with minor revisions would probably be able to meet the

current ASTM C236 standard test method. The converse may also be true,

where the current ASTM C236 and C976 hot boxes with minor revisions might

ultimately meet the revised AAMA standards, if some accommodation can be

made for the differences in the outside wind speed and direction. It is

27
anticipated that the proposed revisions to ASTM C236 and C976 and the draft

standard practice presented in Appendix B of this report will be a first

step in this accommodation.

3 .4 U-VALUE MEASUREMENT CONFERENCES

In 1985, two important conferences were conducted that are relevant to the

discussion of window systems thermal test methods.

3.4.1 Workshop on Laboratory Measurements of ”U-Values” of Windows

This workshop was held at NBS in Gaithersburg, MD on February 26-27 , 1985

and jointly sponsored by BTECC, ASHRAE , ASTM and DOE. The stated objectives

of the meeting were to bring together researchers, manufac tuers , architects

and others interested in measurement and specification of fenestration

system U-values, to permit an exchange of ideas on testing methods, to

identify future research efforts and to develop a consensus standard test

method

The agenda consisted of invited papers, short contributions (brief comments)

and panel discussions with speakers and participants drawn from a wide range

of interests. Published conference proceedings, including peer-reviewed

papers and transcriptions of the panel discussions were to be provided by

BTECC. Approximately 80 persons participated in the workshop. Table 1 presents the meeting agenda.

Although the final proceedings of this conference have not yet been

published, the authors concluded that a large majority of the workshop

participants agreed that a standard test method for window U-values was

urgently needed. A wide range of debate over the specific details of the

proposed test method was a divisive factor in the panel discussions.

3.4.2 Low E Roundtable

The U.S. Department of Energy, Lawrence Berkeley Laboratory and the National

Fenestration Council (NFC) jointly sponsored this conference, which was held

in Las Vegas, NV, August 28-29, 1985. The objective of this meeting was to

review the terminology, heat transfer technology, durability, and energy

savings potential of windows with low-emit tance coatings. The agenda

included overview papers on the various issues and a panel discussion on the

need for development of standards.

The conference was attended largely by glass industry representatives. No

formal proceedings of the papers presented at the conference was planned,

however notes taken by one of the authors (McCabe) lead to the conclusions

that the "window industry” strongly desired the development of standards

that would uniformly address the performance attributes and design

requirements of all fenestration products. The NFC assumed the

responsibility of preparing an industry standard for measurement of infrared

emittance of the coatings used in the manufacture of ”low-E” windows.

28
TABLE 1 Workshop on Laboratory Measurements of

the "U-Values" of Windows

Laboratory Meaaureaenta of the "0-Value*" of Windows

at the

National Bureau of Standards, Gaithersburg, HD

February 26-27 1985

Sponsored by i Cosponsors i General Chairman : Local Chairman <

Building Thermal Envelope Coordinating Council

ASHRAE, ASTM, Do£ s MBS

Tye, Dynatech E/D Cosrpany , Cambridge, MA

M.C. McCabe, Center Building Technology, BBS

PEOGEAM

Tuesday, February 26, 1985

08:30 Registration - Administration Building, Ground Floor

09:15 Welcome and Opening Remarks

Dr. J. Gross, Deputy Director - Genter of Building Technology, NBS

Mr. J. Roehm, Roeha Associates - BTECC

Mr. J. Boulin, Conservation and Solar Division, DoE

Mr. R.P. Tye, Chairman - ASTM and ASHRAE

Mr. M.C. McCabe, Local Chairman - Local Arrangements

09:30 - 10:45

10:45

Session I Chairman:

S. Treado

R. Tye

J. Gurniak

R. Berg

F. Pratt

History Current Pse of Tests and Py>«ry Audience

Mr. Jean Boulin, DoE

: National Bureau of Standards "History of ASHRAE

Activities in Measuring Performance of Windows"

: Dynatech R/D Company "ASTM Test Methods

Development for Windows"

: American Architectural Manufacturers Association

"AAMA History and Current Use of Laboratory Tests"

: Veterans Administration "Test Considerations for U Values of Windows"

: Bonneville Power Authority "Windows Thermal

Performance in BPA Conservation Programs"

Coffee Break

11:00 - 12:45 (continued)

i National Research Council Canada "Thermal Testing

of Windows at NRC

J. Anderson: Albany International "Laboratory and In-Use Comparison of Insulation Effectiveness of Window

Treatments”

D. Pellish : Department of Energy "Department of Energy Programs

Relating to Window Performance"

W. Haynes : New York Landmarks Conservancy "NYC Audiences and

Needs in Landmark Buildings"

Session I

R. Bowen

12:45 - 13:30 Lunch - National Bureau of Standards Dining Area

13:30 - 15:00 Session II Chairman:

W. Coss

R. Kinney

Methods Apparatus and Instrumentation

Dr. George E. Courville - Oak Ridge National Laboratory

: University of Massachusetts "Test Methods and Instrumentation for the Measurement of U Values

of Windows" *

i Synertech Corporation "Cost Effective Method for

Determining the Effective R-values of Insulating

Shutters and Similar Window Treatments"

29
TABLE 1 (continued)

F. Romesburg t Dow Chemical, USA "Use of a Heat Flow Meter

Unit to Measure Apparent Thermal Conductivity

of Window Composites

R. Dixon : University of FLorida ''Fenestration Confleur. jn

and Guarded Hot Box Adaptation for Determination

of Their 0 Values"

H. Taylor i Architectural Testing Laboratories "Instrumentatlon for Testing in Accordance with the AAMA

Standard"

15:20 Coffee Break

15s 20 - 16:10

16:00 - 17:00

Panel Discussion on Session I - Convenor - Jesn Boulin

Panel Discussion on Session II - Convenor - George Courville

Wednesday, February 17, 1985

09:00 - 10:30

10:30 - 10:45

10:45 - 12:45

12:45 - 13:30

Session III Chairman:

A. Van Dijk

M. McCabe

S. Fullarton

G. Page

Comparability and Relationship of Results

Dr. Heinz Trechsel - Trechsel Associates

: Institute of Applied Phvsics, Delft "Preliminary

Comparison between Measured and Calculated

0 Values of Windows"

: National Bureau of Standards "Comparison

between Laboratory and Field Measured C Values"

: Oniversitv of Wisconsin "Testing Window

Treatments In-Place"

: United Technologies "Considerations Relating to Thermal Performance of Window Frames" Coffee Break

Session IV Chairman

J. Klems

A. Van Dijk

T. Cardenas

W. Putnam

Other Areas and Future Research

Mr. D. Pellish, Department of Energy

: Lawrence Berkeley Laboratory "Towards Accurate Predictions of Comparative Fenestration Performance'

: Institute of Applied Physics Delft, "Hot Box Measurements in a Proposed Test Facility In The

Netherlands"

: Steven Winter Associates "An Assessment of Standards that Measure Thermal Performance of Windows and Their Relevance to The Future of ASHRAF. Standard 90" .•

: DSET Laboratories "A Large Multi-Purpose Solar

Illuminated Eight Foot Integrating Sphere for

Optical Properties Measurements"

Lunch - National Bureau of Standards Dining Area

13:30 - 14:10

14:10 - 14:45

14:45 - 15:00

15:00

Panel Discussion on Session III - Convenor

Panel Discussion on Session IV - Convenor

Concluding Remarks J.M. Roehm / R.P. Tye CLOSE

H. Trechsel

D. Pellish

All sessions will be held in Lecture Room B, Administration Building

30
Elevation View

Figure 3 ® ASTM C236 Guarded Hot Box

31
MIXING

CHAMBER

ORIFICE

EVAPORATOR

TEST

SECTION

INSULATION

BLOWER. CONTROL ^ HEATER CHAMBER

DISTRIBUTION

HEADER

GASKETS <T

TRANSPORT

FRAMES Njg

DISTRIBUTION

HEADER

ORIFICE

HEADER

CLIMATIC CHAMBER SPECIMEN METERING BOX

Elevation View

Figure 4 ASTM C976 Calibrated Hot Box
%

NOTE: At dimensions shown in miilimetsrs (inches) ELEVATION VIEW

Figure 5. AAMA 1503.6 Hot Box

33
SURROUND

MASK

SPECIMEN

50-mm

OIAMETER

TUBES

Figure 6. NFC Canada Wind Simulation Machine

34
4 . THERMAL PERFORMANCE TESTING OF MOVABLE INSULATION SYSTEMS FOR

FENESTRATION

The development of movable insulation systems for windows was stimulated

largely by the needs of builders and owners of passive solar residential

buildings to reduce the nightime heat loss through relatively large areas of

south-facing glazing, and thereby provide increased thermal performance and

comfort for the occupants [Balcomb, 1982], The generic term, movable

insulation, refers to materials and components that are installed with

conventional windows, glass doors and skylights that have at least two

operating modes to modify and control the transfer of radiant solar energy

and conductive heat loss (or gain) . Movable insulation is also known as night insulation, or as insulating window treatments. It can be installed

on the inside or outside of fenestration or between glazing layers, and may

be manually operated by the building occupants, or automatically operated by

motorized or by non-electrical devices.

Since a movable insulation system is always installed with a conventional

fenestration, the term "ef fee t iveness" is often used as a characteristic of

thermal performance. Effectiveness is defined as the ratio of the

difference in U-value between the window system with and without the

insulation deployed, to U-value of the basic (undeployed) system.

Effectiveness is, therefore, the fraction of energy saved relative to a particular fenestration system. Obviously, effectiveness of a movable

insulation will vary; depending on the particular reference fenestration

selected, the outdoor and indoor air temperatures and surface conductances,

and possibly with system size, and method of installation. Therefore,

development of a standard method for measuring thermal performance, requires

that all of the factors that can possibly cause variation in test results

must be defined. This section will discuss some of the test methods and

apparatus for meauring thermal performance of movable insulations that have

appeared in the literature.

4.1 FEDERAL PROGRAMS TO DEVELOP MOVABLE INSULATION

As part of a National Program for Solar Application, the Department of

Energy initiated a program for the development and commercialization of

components that would improve the performance of passive solar buildings

[U.S. Department of Energy, 1979]. As part of the commercialization

efforts, thermal performance testing was required, however the details of

how this was to be done were left to the discretion of the grantee. Of the

26 grants made, six were for companies developing movable insulations,

primarily insulating curtains or shutters . As described in the Department

of Energy final report [Mueller Associates, 1984], thermal test results for

those movable insulation systems actually tested were often far less the

expectations of the grantee. In general, insufficient detail was provided

in the grantee reports or in the final report to evaluate the adequacy of

the thermal test methods used for measuring component performance, however

it is instructive to review the attempts to comply with the program testing

requirements

.

35
One of the primary concerns in the design of movable insulation was the

sealing of joints located on the perimeter between moving and stationary

components. A loss of thermal performance is anticipated due to the

potential for free convection exchange between the cold air space adjacent

to the glazing and insulation, and the room air. A related concern is the

possibility of condensation of moisture from room air on cold surfaces and

the deterioration in thermal performance due to change in surface properties

and insulating properties of the components. Therefore a significant design

effort was undertaken by private sector participants in the DOE program to

develop effective perimeter seals and a number of different test methods

evolved to evaluate thermal performance of the seals in the various

proprietary movable insulation systems.

One participant in the DOE Passive Components Program, the ABRI Company,

developed an inflatable insulation window curtain [Mueller Associates, 1984,

p. 80-85]. ABRI reported of new testing approaches that perhaps typify«the

development of movable insulation products [ABRI Inc., 1981]. While

recognizing the value of testing in an ASTM hot box in a commercial

laboratory, ABRI judged that a less costly and time consuming test method

was needed to permit optimization of various design parameters and to permit

experimentation with design changes in an incremental manner.

The ABRI test facility consisted of two small boxes constructed of extruded

polystyrene insulation joined by a wall containing a single glazed window

and the test specimen. One box was heated to constant temperature using a shielded heating element and circulating fan. The other box was cooled by

circulating chilled water through a liquid-to-air heat exchanger. An array

of thermocouples monitored temperatures in the facility. Measurement of

heat flow through the test specimen and window was based on the assumption

that the hot box heat source dissipated a known and constant power and that

heat loss through the hot box walls could be calculated knowing the air

temperatures on both sides of the box and thermal conductivity of the walls.

Knowing the hot and cold side temperatures and heat transfer rate, the Uvalue of the test specimen could be then calculated. The reported time

period for temperature stabilization of this facility was one hour and data

were taken over three one-half hour periods. It is not clear whether the

power input to the heating element was independant ly measured , or whether

the U-value calculations were based on the nominal power rating of the

heater. It is also not clear whether the test facility produced any useful

thermal test data, since the final report indicated that only estimated

values for the product were developed.

In another DOE commercialization project, researchers at the Thermal

Technology Corp (TTC) received a grant to develop a movable insulation

product consisting of a roller shade containing four reflective layers with

provisions for inflating and deflating the air space between layers. A

prototype shade was tested under both laboratory and field conditions

[Steele et al., 1982] with substantial disagreement in measured thermal

performance. In one field test, four heat flux sensors were taped to the

outer layer of a prototype shade facing a single-glazed patio door in a residence and used to measure the heat flow rate. The prototype test shade

consisted of the four reflective fabric layers stretched over individual

36
wooden frames that maintained 18 mm (3/4 in) air space dimensions and sealed

all the edges s thus providing an idealized test configuration for comparison

with actual shade performance. The thermal resistance of the glass/shade

combination indicated by the heat flux sensors exceeded the maximum possible

theoretical resistance calculated using ASHRAE Handbook data. A second

field test using a larger-sized unit with actual roller shade hardware was

performed, resulting in R-values derived from heat flux sensor readings that

were about 35 percent below that of the ideal shade configuration.

Subsequent laboratory testing of production shades in a commercial hot box

indicated far lower thermal resistance than that from the field tests.

Although a number of explanations for the anomolous test results were

hypothesized based primarily on the size differences and edge sealing of the

units tested, the heat flux sensors used in the field testing were also

suspect. The potential sources of error from using heat flux sensors as the

primary measurement sensors are discussed in Section 4.5. An important

conclusion stated in the TTC report, ".... to be drawn from this testing is

that sizes of the curtains, the method of test, and the effectiveness of

layer separations and edge seals can allow the product to demonstrate Rvalues ranging from 2.97 to 11 .008”

.

4.2 HISTORICAL THERMAL TESTING OF ROLLER SHADES

Roller shades are probably the oldest example of movable insulations for

fenestration systems, even though their primary function has often been to

reduce solar gain and to provide a measure of privacy for occupants. It has

long been recognized that the presence of opaque shade fabrics that are

suspended vertically a short distance from the interior of cold glazing

surfaces, reduced the radiative heat loss from the warmer room surfaces,

however the magnitude of the reduction in heat loss was difficult to

estimate. ASHVE Research Bulletin No. 1 [Parmelee, 1947b] reported

published U-values for windows with and without roller shades based on hot

box measurements made as early as 1919. Those measurements showed

significant reduction in heat loss relative to an unshaded window, depending

on the particular method of mounting. The ASHRAE Research Laboratory

[Ozisik and Schutrum, 1959] examined the effects of different shade

materials, shade mounting positions and edge sealing techniques using a field test method involving the ASHRAE Solar Calorimeter. Winter night ime

U-value data were obtained and adjusted for standard outside and inside

surface conductances. The test results indicated a significant reduction in

U-value for roller shades having a low emittance shade material (aluminumfoil) in conjunction with sealed edges that was not evident in the same

material without the sealed edges.

More recently, a variety of field and laboratory test methods have been

reported for measuring the thermal performance of movable insulation

systems, varying in sophistication from complexity equal to hot box methods

to extreme simplicity. The following sections describe some of these test

methods

.

37
4.3 . FIELD TEST METHODS

Field test methods using actual outdoor conditions can often be performed at

a lesser cost than laboratory hot box test methods and with greater realism,

however generalization of test results may be difficult, due to measurement

uncertainties in climatic variables such as nocturnal radiation and wind

speed, and in unsteady heat transfer conditions. Moreover, variation in

these climatic factors tends to make the measured thermal performance

somewhat time-of-year and site-specific, thereby requiring longer testing

periods and necessitating the use of well-characterized control specimens

for comparison purposes . Three approaches have been taken for field

measurement of movable insulation, depending on the type of test facility.

These include the following: 1) in-situ testing in buildings, 2) test

cells, and 3) field calorimeters.

4.3.1 In-Situ Testing in Building s Perhaps the least costly test method for field evaluation of movable

insulation thermal performance was proposed by Shurcliff [1980]. In this

method, three conventional liquid-in-glass thermometers are installed; one indoors, one outdoors and one in the air-space between the movable

insulation and the window. By measuring the temperature difference between

the air space thermometer and the outdoor thermometer, with and without the

movable insulation in place, the fractional reduction in nighttime heat loss

is determined. The indoor and outdoor thermometer readings are used to

adjust these results to standard indoor and outdoor conditions of 21°C

(70°F) and -1 °C (30°F), respectively.

Aside from the questionable accuracy and difficulties in temperature

measurement with unshielded, liquid-in-glass thermometers, inferring Uvalues of movable insulations from measurements of air space temperatures

has probably the lowest accuracy of all the proposed methods. Fowlkes

[1980] reported significantly higher U-values for movable insulations using

air space temperatures and Shurcliff 's method than expected based on the

thermal properties of the materials comprising the insulation system.

In another in-situ test method, thermal performance of insulating window

shades were compared with conventional draperies by installing the shades

over north-facing, single-glazed windows in identical rooms in a commercial

inn, and monitoring each room's energy consumption over a three month

heating period [Grasso and Anderson, 1986]. The measured reduction in room

energy consumption was 47% compared to a 49% reduction based on calculated

U-values for the insulating shades and conventional draperies , however no attempt was made to monitor either the frequency of opening and closing of

the window treatments or the setpoints of the room thermostats, which were

controlled at the discretion of the room occupants.

Another in-situ measurement technique is described by McCabe and Hill

[1987], which involves the use of portable calorimeters to measure

comparative thermal performance of identical north-facing windows with

different types of thermal coatings. This technique would also be

applicable for in-situ testing of movable insulation provided some means of

38
deploying and retracting the interior mounted insulation without removing

the metering boxes, is available. Alternatively, testing two identical

side-by-side windows; one with a deployed insulation and one with a retracted insulation would provide useful test results for measuring the

effectiveness of these products as actually installed in buildings.

4.3.2 Nig,btt_ime_Measurements in Test Cells

Researchers at the Los Alamos National Laboratory established a methodology

for comparative field testing of a range of passive solar systems, including

movable insulations, using room-size test cells [Balcomb et al., 1978; Hyde,

1980]. A number of different systems, including solar storage walls with

different movable insulations were installed in identical, south-facing test

cells and exposed to prevailing winter climatic conditions in the high

desert of New Mexico. Performance evaluation consisted of comparing

auxiliary heater energy requirements and air temperature variation between

the different test cells, however, no attempt was made to evaluate nighttime

U-values for the commercial movable insulation products tested.

Lexen and Muldary [1985] describe an arrangement of three side-by-side,

room-size test cells designed for field comparison testing of prototypes and

new window products including shades, blinds, films, shutters and movable

insulations. One of the test cells is designated as a reference and

contains a heavily insulated "plug” of known thermal conductivity, instead

of a window. The second cell has a standard double-glazed window, and the

third cell has the standard window plus the window accessory being

evaluated. For purposes of nighttime U-value testing, the test cells face

north to avoid daytime solar gains, interior temperatures are held constant

at 22 C (72 F) , and heater power is monitored for each test cell. By using

the three-cell arrangement described, both absolute U-values and

differential U-values are obtained with sufficient accuracy to meet the

stated objectives of the test facility.

4.3.3 Nighttime Measurements in Calorimeters

It was apparent that field testing either in actual rooms or in test cells

had shortcomings of one kind or another that limited the generality and the

usefulness of the test results. It was reasoned that the thermal

environments in the test cells and actual buildings were both uncontrolled

and largely unmeasured. There was also the distinct possibility for

uncontrolled air leakage in a test component or room, and that heat storage

or heat release by room surfaces could contribute significantly to

uncertainties in test results. In consideration of those problems, NBS

under sponsorship of the Passive Solar Division of the U.S. Department of

Energy, constructed a calorimetric type of test facility that was used to

test a range of passive solar components, including movable insulation

systems [McCabe et al., 1982; McCabe et al., 1984]. In the NBS facility

shown in Figure 7, the temperature of interior surfaces and air was closely

controlled and the mounting interface between the test specimen and

calorimeter was carefully designed to minimize edge heat loss. The design

objectives for the NBS calorimeter were to eliminate, or to at least

minimize the previously mentioned shortcomings of the test cells and rooms.

39
A single-glazed window with interior bifold insulating shutters was tested

in the NBS facility in Gaithersburg, Maryland during the winter of 1982-

1983. The same shutter/window combination had previously been tested in two

different laboratory hot box facilities [McCabe et al., 1986], and although

the field measurements were generally in the range of the laboratory

measurements, significant scatter in the U-value data occurred. It was

concluded that a major problem in field testing in the NBS calorimeter was

the imprecise measurement of the outdoor conditions, including air

temperature, radiative heat loss and wind. Furthermore, it was evident that

data collection needed to be taken over a much longer time than the one-week

test period actually chosen, in order to ensure a statistically valid sample

of climatic conditions representative of winter conditions.

An advanced, outdoor-calorimetric test facility, which uses large-area heat

flux meters instead of water-cooled walls, was constructed by the Lawrence

Berkeley Laboratories under sponsorship of the Building Systems Division of

the U.S. Department of Energy, which is known by the acronym MoWITT [Klems

,

1982]. The planned use of the MoWITT includes the testing of windows and

movable insulation systems for windows. This test facility, which is shown

in Figure 8, is designed to make highly accurate performance measurements

under field conditions, and utilizes two air-guarded, metering chambers

mounted on a truck chassis for mobility. The initial testing phase of this

facility was recently completed [Klems and Keller, 1987]. Current plans are

to conduct field measurements with MoWITT and compare these with laboratory

measurements for three sealed insulating glass units made at the University

of Massachusetts as part of an overall U-value measurements program

sponsored by the Department of Energy [Goss and McCabe, 1985].

4.4 LABORATORY TEST METHODS

A variety of laboratory test methods have been reported for measuring Uvalues for movable insulation systems. In addition to the conventional hot

box test methods previously described, a number of alternative test methods

have been reported.

4.4.1 s iqg_i^._Chambs.t- vith. He, at TxaB3d.vc.,e.rs New Shelter, a consumer-oriented magazine focusing on energy issues in

housing, tested eleven different commercial products, including roller

shades, curtains, interior shutters and interior storm windows [Rawlings

1980]. The test facility consisted of a single-glazed window installed in a wall located between a room heated to "regular" temperatures and a room

maintained at least 17°C (30 F) colder, with a small fan directing cold air

at the cold side of the window "to simulate a breeze". Test specimens were

mounted to the warm side of the window and a commercial heat flux transducer

was taped to the warm side window surface. Details regarding the method of

attachment of the test specimens, were not provided. The test results

reported were "percent heat saved", which varied between 46 and 89% for the

products tested. This index was computed by comparing the heat flux

transducer output voltage reading for the movable insulation products in the

deployed and non-deployed positions and adjusting the results in some

unspecified manner to a 6.7 m/s (15 mph) wind.

40
Fabric and textile researchers at several universities utilized similar

techniques to investigate the thermal performance of various drapery

systems. Horridge and coworkers [1983] at Texas Tech University, report on

a test method in which a small refrigerated box maintained between -13 and7 C (5 and 20 F) was positioned on the exterior side of a single-glazed

window mounted in the wall of an environmental test chamber. A single heat

flux transducer was attached at midheight to the interior surface of the

glass, as shown in Figure 9, and various insulating treatments were mounted

to a wooden frame on the interior side of the window. The window treatment

was exposed to environmental chamber air maintained at 21 C (70 F) and 60%

relative humidity. R-values varying between 1.50 and 2.15 were reported for

the various systems (window plus treatment) tested, however the basis of

these reported R-values was not stated.

Epps and coworkers [1984] at the University of Georgia, report on measured

U-values of various textile fabrics using a similar apparatus, however, with

three heat flux transducers tightly clamped to the glazing surface and a cold chamber cooled by an air conditioning unit with controlled convective

heat transfer on the exterior side of the reference window. The warm side

was maintained at 21 C (70 F) and at two levels of relative humidity (45 and

6 8%). U-values were reported for the window-fabric system at the two

relative humidity levels, based on the measured output of the heat flux

transducer and the air-to-air temperature difference.

Despite the widespread use of heat flux transducers in measurement of the

thermal performance of windows with movable insulation, several problems are

evident. These problems, which will be discussed in some detail in Section

4.5, tend to limit the utility of the test method, because of the

possibility that the test results may not be reproducible by others using

the same technique, but with different sensors.

4.4.2 Guarded Hot Plate

A popular method for measuring the thermal properties of textile fabrics

based on principles of the guarded hot plate has been adapted for the

testing of windows with movable insulations [Anderson, 1982]. A similar

apparatus located in the merchandise testing center of a major retailer was

used by researchers at Cornell Univerity to measure the effectiveness of

roller shades [Grasso and Buchanan, 1979], and insulating shades, draperies

and blinds [Cukierski and Buchanan, 1979]. Figure 10 shows a schematic

drawing of the guarded window test apparatus described by Anderson [1982] , which consists of three heater panels installed in a wooden framed

calorimeter box. The primary or test heater is installed as a vertical

plate facing the exterior surface of the test window/ movable insulation.

Two guard heaters; a vertical panel heater located behind the test heater

and a narrow perimeter heater, limit the rear and edge to flow horizontally

through the test window. The window treatment test specimen is located

outboard of the test window and is exposed to the laboratory environment.

Anderson's comparison of percent reduction of heat loss (effectiveness) made

for five different shade fabrics using an AAMA hot box and the Guarded Hot

Plate apparatus showed excellant agreement.

41
The primary advantage of this test method over a hot box test method in

testing of insulating window treatments is the substantially lower initial

costs of the facility, the smaller laboratory floor space occupied and the

rapid achievement of steady-state conditions [Anderson, 1982]. This permits

quantitative measurements of window treatment products to be rapidly made at

low cost. One possible disadvantage of this test method is that the

direction of heat flow is reversed from the normal inside-to-outs ide , winter

time direction. Atypical air space and surface temperatures that occur as a result of the reverse heat flow direction may either suppress or enhance

convection loops that mask performance attributes of the window treatment in

an uncertain manner. Dix and Lavan [1974], at the Illinois Institute of

Technology investigated the thermal performance of window coverings with a two room test module, in which one room represented the outdoor environment,

and the other the inside of a house. The wall dividing the two rooms had a double glazed window over which the test shades were installed. Summer

conditions were simulated with a bank of lamps and winter conditions were

simulated with a refrigeration unit.

Tomany [1981], concluded that the Dix and Lavan method was best for

measuring the effectiveness of movable insulations in both summer and winter

conditions and constructed a similar facility, however with improved

measurement techniques and controls. Figure 11 shows a schematic drawing of

the facility described by Tomany. An energy balance on the room chamber,

which is maintained at constant temperature by means of the air circulation

system, is performed by measuring circulating system air flow rate and

temperature change, and by measuring heat loss through the walls, floor and

ceiling with a differential thermopile. The heat loss through the

window/movab le insulation is the difference between the energy supplied by

the air system and the heat loss through the room surfaces.

4.4.3 Hot Boxes

The previously described hot box method for laboratory testing of windows

have also been applied for measuring thermal effectiveness of movable

insulation. The primary advantage of the hot box method over all other

methods discussed is the ability to accurately control the thermal

environment on each side of the test specimen. This suggests that testing

of a movable insulation and fenestration system in each of its operating

modes by a single testing laboratory should be the most accurate test method

available

Miller and Carey [1982] describe the hot box testing performed on two types

of insulating shutters installed on the interior of a double glazed window

and on a sliding glass door. Their test facility can operate in either the

guarded mode [ASTM C236] or the calibrated mode [ASTM C976], to accomodate

different test sample sizes. In the guarded mode that was used for testing

the double-glazed window and wall combination, the percent reduction in heat

flow for the two types of shutters varied from 38 to 47 percent. In the

calibrated mode that was used for testing the double-glazed sliding door,

the percent reduction in heat flow for the sliding type shutter tested was

70 percent. Unfortunately, in testing the window in the guarded mode the

guard box covered a substantial portion of the mask wall, therefore, the

42
effectiveness of the movable insulation and window could not be compared to

the test results for the sliding glass door system.

The Thermal Insulation Laboratory of the Technical University of Denmark

reported on the design and thermal performance testing of insulating window

shutters proposed for use in new low energy housing and in retrofit housing

[Byberg et al., 1983]. The test facility consisted of a guarded hot box

shown in Figure 12 with specially constructed 0.35 m (13.8 in) thick mask

wall representative of construction in new buildings. The wall has an

opening in which a double glazed window and the test shutters are located.

Small fans in the cold box and metering box circulate air parallel to the

window and shutter surfaces, respectively, with flow velocities that result

in free convection surface coefficients. The test results with and without

the insulating shutters in place were used to estimate annual energy savings

in typical Danish residential buildings. The significance of this report is

that it adequately describes the test specimen and test facility and

provides an excellent model by which the development, testing and marketing

of movable insulation products could follow.

Because of the differences in the various hot box methods used for testing

windows, NBS participated in a testing program to compare U-values of

movable insulations and windows using alternative test methods [McCabe et

al., 1986]. Four representative components, including a multiple-glazed

window and three single-glazed windows with movable insulation systems, were

purchased and tested at two commercial testing laboratories, using the ASTM

C236 and the AAMA procedures for a range of simulated outdoor conditions.

A number of conclusions from this study were presented. At the zero wind

speed test condition, the two laboratories using different apparatus and

test methods but testing the same component, provided U-value measurements

that compared favorably. Agreement was closest (within 10%) when the test

component was either non-operable (i.e., fixed glazing) or tightly-sealed,

with somewhat less favorable agreement (between 10% and 16%) for operable

components that do not seal consistently. At simulated wind conditions of

6.7 m/ s (15 mph) , there were no favorable comparisons between the test

results with different simulated wind directions. These test results also

suggested that the U-value measured at one test condition should not be

adjusted to different wind conditions. The significant increase in U-value

experienced in both test laboratories at the higher wind conditions for two

test components suggested the possibility of additional heat transfer due to

leakage of cold air into the warm box.

Other conclusions in that report were that component manufacturer's thermal

performance claims, even when based on test data, differed significantly

from the test results. Estimates of energy savings based on comparing

product performance data for one set of conditions with single-glazing

performance at the 6.7 m/ s (15 mph) winter conditions could be very

misleading. Significant variations in U-value were observed for the two

components tested with variable wind speed and ambient temperature

conditions. These variations, which could not always be explained by heat

transfer theory, might be the result of subtleties in testing, such as what

43
might be caused by reversing the test specimen in the hot box during

simulated summer conditions.

In Section 6 of this report, several of the recommendations made in the NBS

study are presented.

4.5 PROBLEMS USING HEAT FLUX TRANSDUCERS

As previously mentioned. Heat Flux Transducers (HFTs) have been used in a number of different testing applications with varying degrees of success.

This section discusses some of the concerns for their use in testing movable

insulation.

A HFT is typically a thin wafer of material with a known, stable thermal

resistance. The temperature difference across the wafer is measured with a series arrangement of thermocouple junctions across the wafer in a thermopile arrangement. The thermopile multiplies the small electrical

signal produced by each pair of thermocouple junctions to give the average

heat flux across the wafer. The device can be affixed to a wall and

connected to an appropriate readout device to measure the heat flow.

The use of a HFT in testing movable insulation systems is complicated by

several factors. First by adding a heat flux transducer to a relatively

thin and low thermal resistance surface such as a glass window pane, the

local thermal resistance and heat flux is changed relative to an undisturbed

glass surface and the measured heat flux is not representative. Second, the

local changes in thermal resistance result in multi-dimensional heat flow

paths, therefore, the heat flux sensor design and calibration, which is

based on the principal of one-dimensional heat flow, is violated. Third,

the local convection heat transfer coefficient at the sensor are affected by

the discontinuity resulting from attaching the sensor to a large flat

vertical surface. These considerations result in errors in measured heat

flux, which can probably be corrected by proper calibration techniques and

by very careful evaluation of the particular application, otherwise errors

of 100% are possible [Courville et al., 1983].

Another area of concern in use of commercial HFT's has been the inaccuracy

in calibration data provided by the sensor manufacturer. Researchers at the

Massachusetts Insitute of Technology (MIT) report on large percent

differences (as much as 50%) between one particular manufacturer's

calibration data and calibration performed in a special, high accuracy, MIT

facility [Bligh and Apthorp 1983]. One of the reasons proposed for these

discrepancies is that the manufacturers calibrations are conducted at flux

levels that were two orders of magnitude greater than that of the proposed

application. Moreover, individual HFTs designed to the same specifications

had calibration factors that varied by 100%. It was concluded that all

commercial HFTs "should be recalibrated for the flux level at which they

will be used, because the manufacturer's quoted values may be effectively

useless .”

44
4.6 TESTING STANDARDS FOR MOVABLE INSULATION SYSTEMS

In spite of the fact that consensus standards were not available for windows

and there was no technical basis on which to develop a standard for movable

insulation, several standards were prepared to address the thermal

performance requirements of movable insulation.

4.6.1 Canada Housing and Mortgage Standard

A standard for movable insulation devices was prepared by the Canada

Mortgage and Housing Corporation [1979] for the administration of a grant

program for existing residential buildings and for a finance program for new

construction in Canada. The standard applied to insulating shutters and

blinds located on the interior or exterior of glazing or to any insulating

devices inserted between glazings, however the standard specifically

excluded storm windows. Although absolute values of thermal performance

were not specified, the added R-value relative to a reference window of the

movable insulation was required to be at least 0.35 m^'K/W (1.99

ft^ *hr *F/Btu) . The Standard prescribed the reference window size and type,

test chamber size, test temperatures, and an exterior surface coefficient of

22.7 W/m^«K (4.0 Btu/hr*ffF) and required the use of an ASTM C236 guarded

hot box apparatus . 4.6.2 Window Energy Systems Standard 1584

In 1984, the Window Energy Systems (WES) Division of the International

Fabrics Association adopted a test method for measuring U-value of movable

insulations and insulating window treatments [Window Energy Systems, 1984].

The membership of the Window Energy Systems Division includes manufacturers,

distributors, suppliers, dealers and retailers of window treatment products,

including awnings, blinds, draperies, films, interior shutters, screens and

shades. The WES Standard was patterned essentially after the AAMA Standard

1503 [AAMA, 1980] with several modifications introduced to address the

specific issues relative to testing of these products. These include

provision for maintaining the warm chamber air at relative humidity levels

less than 30%, the requirement for a base window unit consisting of a single

layer of glass mounted in a insulated wooden test buck with bottom extension

panel to simulate a carpeted floor, and the format for presenting test

results. In addition, detailed installation specifications, mounting

procedures and clearance dimensions were provided to suit specific types of

insulating treatments and movable insulations. Although a "round robin"

comparison test of the several movable insulation systems has been mentioned

in WES correspondence, the results of this comparison have not been made

public

.

45
Figure 7. NBS Passive Solar Calorimeter

46
Figure 8* LBL MoWXTT Calorimeter

47
Figure 9. Window Treatment Test Facilty

Using a Heat Flux Transducer

48
Edge guard

heater

Glass test

surface

Front test

heater

Rear guard

heater Urethane foam

insulation

Figure 10. Window Treatment Test Facility

Using a Guarded Hot Plate

49
Figure 11. Window Treat Test Facility

Using Laboratory Rooms

50
Stud walls with mineral wool insulation

Figure 12. Guarded Hot Box at Technical

University of Denmark

51

i\

r\"rt

7n

\jwn
5. TEST STANDARDS UNDER DEVELOPMENT

Although both the ASTM C236 and C976 test standards and AAMA Standard 1503

have been used extensively for thermal testing of window systems

,

considerable controversy exists with regard to which test method is the most

appropriate for that purpose. As described in 4.4.3, significant

differences in test results were obtained in applying the ASTM C236 and the

AAMA 1503 test method to a limited number of windows. Furthermore, it was

apparant that both the ASTM and AAMA test methods had weaknesses that tend

to reduce confidence in the accuracy and repeatability of these methods when

applied to window systems. In this section a number of potential weaknesses

in the current ASTM and AAMA test methods are outlined. These weaknesses

provide the rationale for changes to be addressed in the new uniform test

standard for window and door systems which is currently under developmnet by

an ASTM standards group. A working draft of the new standard practice is

included in Appendix B.

5 .1 RATIONALE FOR CHANGES TO ASTM TEST METHODS

The ASTM C236 Guarded Hot Box and the ASTM C976 Calibrated Hot Box methods

were not developed specifically for window systems but for more general

building construction assemblies, building panels and other applications of

nonhomogeneous assemblies such as walls, ceilings, and roofs, as well as homogeneous assemblies of materials such as thermal insulation used in

building applications. In general, window systems have intrinsically lower

thermal resistance than the other applications and this has led to certain

difficulties in applying the ASTM test methods to window system.

The thermal performance of window systems is much more sensitive to surface

heat transfer effects and to possible air leakage than other building

components, and the current ASTM test methods do not specifically address

these needs. As discussed in 3. 1.4.3, air motion over the surfaces of

window systems has a considerable affect on thermal performance, however,

characterization of the flow field in the ASTM standards is ambiguous, with

parallel forced convection on the outside of the test specimen being left as an option. It was concluded in section 3. 1.4. 5 that simply directing an

airflow stream in a direction parallel to the exterior surface at a specified average speed was not sufficient to produce a desired average

convective surface heat transfer coefficient, but some additional means of

determining the average surface coefficient is required.

The ASTM test chambers are usually configured with baffles parallel to the

specimen surfaces, to confine the airflow to a uniform channel. However,

with the low velocities usually applied to the interior side of the test

windows , non-uniform surface temperatures often occur in the baffle in the

proximity of a low resistance window or glazing unit due to radiative heat

transfer. Non-uniform surface temperatures appear to be a particular

problem in thermal testing of windows because of the prevalence of these

baffles in ASTM test chambers.

Although calibration of the ASTM test chambers is a significant issue that

is addressed at great length particularly in ASTM Standard C976 , the

52
calibration requirements for testing of window systems is not specifically

addressed. Furthermore, since the vast majority of window systems have not

been tested in ASTM hot boxes, there is a lack of operating experience in

this application and the need for a round-robin to establish repeatability,

5.2 RATIONALE FOR CHANGES TO THE AAMA TEST METHOD

In contrast to the ASTM test methods, AAMA Standard 1503 was specifically

developed for thermal testing of window systems. However, one of the

criticisms directed at AAMA has been due to the lack of publication of

technical papers by AAMA in referred journals and at national conferences

sponsored by the technical societies. It is believed that more active

participation in technical society activities, involving open discussion of

test methods and participation in round robins and inter laboratory

comparisons will lead to increased confidence in the AAMA method. The

primary considerations for possible changes to the AAMA test method are due

to calibration, air leakage and temperature sensor placement.

The method used by AAMA to adjust air speed to achieve the ASHRAE winterdesign surface heat transfer coefficient was described in 3.1.3. The

double-glazed panel used in that procedure, in effect, is the calibration

specimen used to determine the overall heat transfer characteristics of the

facility. It is believed that a better characterized test panel and better

defined calibration procedure would increase the accuracy of testing in AAMA

hot boxes, especially with the newer, higher resistance window systems on

the market.

Air leakage has been thought to be a greater problem in AAMA hot boxes than

in ASTM hot boxes because of the dynamic pressure of perpendicular wind

impacting the exterior surfaces of test windows. Although the AAMA standard

requires static pressurization of the warm side to counteract the dynamic

pressurization of the cold side, quantitative measurements of air leakage in

AAMA hot boxes are not available. It is believed that sealing techniques

are available to reduce window leakage to negligible values, however until

comprehensive air leakage measurements are made, air leakage will remain a potential source of error and uncertainty in the AAMA method.

Although the placement of temperature sensors in hot boxes and on test

specimens is often a matter of operator preference, the AAMA standard

appears to require fewer sensors than the ASTM stndards, especially

regarding air temperature sensors. It would appear that with uniform

minimum standards for placement of sensors, the accuracy and installation

requirements could be developed with little difficulty or controversy.

5.3 NEW ASTM TEST STANDARDS FOR WINDOWS

A joint ASTM task group, comprised of the members of Subcommittees C-16.30

and E6.51 was formed in 1984 to develop a new standard for measuring the

thermal transmittance of window systems, using a hot box method. Since two

hot box test methods for measuring U-value and C-value of building

assemblies; ASTM C236 - Guarded Hot Box, and ASTM C976 - Calibrated Hot Box,

were already in existance, it was evident that these standards would form

53
the basis of a new standard which would address the unique features of

window systems, however, giving due consideration to integration of

significant features of the AAMA Standard.

The efforts of this task group have resulted in three documents:

1) Recommended modifications to ASTM C236-80,

2) Recommended modifications to ASTM C976-82 , and

3) Draft standard practice for determining the thermal performance of

window and door systems.

The intention of the task group is to have a uniform test method, which

permits test facilities having either parallel or a perpendicular wind

simulation, to conform to the requirements of an ASTM standard. It is also

anticipated that the ASTM window test standard will conform to the ISO

standard for windows, when both standards are completed.

The current status of the ASTM standards is that the task group is currently

preparing working draft 4, however, balloting within either of the C16.30 or

E6.51 subcommittees has not yet occurred. In its present state, only minor

revisions to ASTM C236 and ASTM C976 have been recommended. These revision

primarily address sections of the standards that either specifically

prohibit perpendicular wind or fail to mention it, when discussing air flow

direction.

The major accomplishment of the task group is the development of a new

standard practice. Appendix B presents the third working draft of this

document. It should be noted that the draft status of these documents

implies that significant revisions or restructuring is quite possible. The

purpose of presenting this information is to provide some benchmarks in the

current format and content of the draft standards. It is also apparant that

a significant effort remains to develop a technical data base of experience

and test results, before the final standard can be made available for

ballot. The next chapter outlines a research program that is aimed at

development of this data base.

54
6. RESEARCH PROGRAM FOR DEVELOPING WINDOW TEST STANDARDS

The authors have prepared an outline for a two-phase research program to

address the needs of the ASTM standards activities described in Section 5

[Goss and McCabe, 1985]. This program addresses specific measurements

deemed necessary to accelerate the development and acceptance of the ASTM

test standards for window systems.

6.1 PHASE 1 RESEARCH

The first phase of the program is jointly sponsored by NBS and DOE and is

currently underway at the thermal measurements laboratory at the University

of Massachusetts at Amherst using the calibrated hot box (hereafter called

the Research Calibrated Hot Box - (RCHB). The first phase focuses on

measuring the U-values of insulated glass units (IGUs) for a range of

environmental conditions. The following sections present some primary

features of Phase 1 research.

6.1.1 Test Facility Description

The RCHB has been modified so that both parallel and perpendicular wind

directions can be simulated. By using different fans with variable speeds,

wind speeds varying between low velocities typical of natural convection up

to the ASHRAE winter design conditions of 6.7 m/ s (15 mph) will be

simulated. With a single test facility providing both parallel and

perpendicular wind directions, a direct comparison will be made of the

effect of wind direction on the Sealed Glazing Unit U-value

.

The mask wall used for supporting the test window and for separating the hot

and cold chambers is constructed of 152 mm (6 in) extruded polystyrene and

covered with 6 mm (1/4 in) plywood faces. The mask wall has a centered

opening where each Insulating Glass Unit is mounted flush with the

environmental side surface.

6.1.2 Heat Flux Transducer

A 1016 x 1016 mm (40 x 40 in) heat flux transducer (HFT) used for the Phase

I research program is identical in design to the HFT used by NRC/ Canada. It

consists of a 13 mm (1/2 in) layer of expanded polystyrene and two sheets of

glass. Type T thermocouple wire in a thermopile configuration is installed

between the glass sheets and the polystyrene. The thermal conductivity of

the polystyrene is accurately measured using the ASTM C177 Guarded Hot Plate

method. This value, along with the measured temperature difference, is used

to determine the heat flux through the HFT.

6.1.3 Temperature Measurements

Small diameter (30-gauge) calibrated thermcouples are used to measure the

IGU surface temperatures and the air temperature near the IGU surfaces. In

addition, the RCHB baffle wall temperatures are measured so that the IGU

surface temperatures can be determined by calculation. Results of this

research will provide a technical data base for thermal performance

55
standards for windows, including both calculation and measurement procedures

for determination of U-value for a variety of applications. In addition,

the IGU will be well characterized and should be quite valuable in the

subsequent Phase 2 research program.

6 .1 .4 Test Specimens

During the Phase 1 program, testing will be performed on the following 1016

x 1016 mm (40 x 40 in) test specimens:

1. Standard, double glazed unit:

6 mm (1/4 in) glass - 13 mm (1/2 in) airspace - 6 mm (1/4 in)

uncoated glass,

2. Low emittance, double glazed unit:

6 mm (1/4 in) glass - 13 mm (1/2 in) airspace - 6 mm (1/4'in)

glass with low emittance coating on inner surface,

3. Triple-glazed, spectrally selective unit:

6 mm (1/4 in) glass - 13 mm (1/2 in) airspace - spectrally

selective, low emittance plastic film - 13 mm (1/2 in) airspace6 mm (1/4 in) glass.

6.1.5 Test Conditions

The following matrix of test condition was selected to obtain data relating

the sensitivity of IGU test specimens to the environmental conditions:

1. Temperature outs ide/inside C (F)

winter: -8/20 (18/68)

summer: 35/20 (97/75)

fall/spring: 3/20 (38/68)

2. Wind speeds m/s (mph)

free convection 0 (0)

summer design 3.4 (7.5)

winter design 6.8 (15.)

3. Wind direction

parallel

perpendicular

4. Position of outside of test specimen (relative to environment side

of mask wall)

flush

recessed

6 .2 PHASE 2 RESEARCH

Specific details for the Phase 2 research program will obviously depend on

the outcome of the Phase 1 research, currently underway. Results of this

research will be a technical data base for thermal performance standards for

56
windows, including both calculation and measurement procedures for

determination of U-value for a variety of applications. A broad-based

testing and analysis program is envisioned for Phase 2, including

continuation of the Research Calibrated Hot Box (RCHB) testing initiated in

Phase 1 and extension of the testing program to include both commercial hot

box testing in laboratories and field testing in outdoor facilities.

In addition to measurement of window U-value in a RCHB facility, a certain

level of wind tunnel testing appears to be appropriate in order to determine

exterior surface convective heat transfer coefficients. Scale model testing

should be performed in a wind tunnel for several window/building

configurations to determine the distribution of surface heat transfer

coefficients with depth of reveal, wind speed and wind direction. Flow

visualization techniques and methods of measuring convective heat transfer

coefficients are available for small scale wind tunnel experiments; however,

they need to be modified for full scale window geometry and typical wind

speeds used in hot box testing.

6.2.1 Research Laboratory Testing and Analysis

The Insulating Glass Units (IGU) tested in the RCHB facility in Phase 1 will

be further tested in order to enlarge the technical data based from which

the thermal performance standards will be develped. In Phase 2, several of

the standard-size, IGU will be fabricated' into windows by adding sash and

frame members comprised of wood, aluminum and PVC plastic materials. These

windows will be tested in the RCHB according to the draft standard practice

described in Appendix B. Heat transfer models will also be prepared for

each window and computer predictions made of window thermal performance.

The computer model predictions and the test results of each individual IGU,

and IGU sash combination will be compared and empirical frame adjustment

factors established for the different frame materials. This research will

assist in development of standard calculation procedures for estimating

frame and sash adjustment factors from IGU test results.

The effects both of test specimen size and of slope angle (deviation from

vertical) on U-value will also be determined. Several IGU of the same

generic configuration as the standard types tested in Phase 1 , except for

differing size will be tested. The specimens will have nominal dimensions

of 610 by 610 mm (24 x 24 in) and 1219 by 2032 mm (48 by 80 in) , corresponding to a smaller window and a larger patio door, respectively.

This testing will include window specimens, with and without an edge frame,

to establish sizing effects of the individual IGU and the IGU plus frame on

U-value. In addition, several standard IGU from Phase 1 will be tested at

various orientation angles between vertical and horizontal, with heat flow

in both the up and down direction. Determination of size and slope

adjustment factors are essential' to establishing whether testing is required

for each unique window size, and whether non-vertical glazing systems, such

as these used in atria and sunspace applications, require special testing.

6.2.2 Commercial Laboratory Hot Box Testing

A coordinated research effort, aimed at obtaining operating experience for

57
the new window testing standards in commercial laboratories, is desired for

measuring the comparability between differing test facilities. Previous

research [McCabe et al., 1986] indicated that substantial discrepancy in

measured U-values for windows exists between different laboratory test

methods. This discrepancy was attributed to the different methods of

simulating wind and was possibly due to air leakage. To avoid ambiguities

in window testing, steps must be taken by the window testing laboratories to

reduce possible air leakage to an acceptable level and to verify that the

residual levels of leakage are within a tolerable range. In addition, a standard method for calibration of the mask wall and a technique for

measurement of surface heat transfer coefficients must be developed. These

are considered key elements in the draft standard practice for testing

windows and doors. A number of commercial testing laboratories will

participate in a round-robin evaluation of the new testing standard for

windows, using the IGU test specimens from Phase 1. At least two testing

laboratories for each type of hot box testing facilities should participate;

including facilities designed according to the ASTM C234, ASTM C976, and the

AAMA test methods.

6.2.3 Field Testing

Several field testing facilities have been constructed, each having

different capabilities to measure the thermal performance of full-sized

building fenestration systems such as windows and doors under carefully

controlled indoor conditions with prevailing outdoor conditions of air

temperature, wind velocity and solar radiation. This method of testing

produces more realistic performance data than possible using simulated

outdoor environments ’in laboratory facilities. However, outdoor testing

inherently results in limited productivity, since relatively long test

periods are often required for each test specimen to reduce data scatter and

the length of the testing season is limited by local climatic conditions.

Extrapolation of the test results to other times of year, weather patterns

or to other climatic regions is required.

The IGU test specimens from Phase 1 will be installed in the participating

outdoor testing laboratories and tested during both the winter-time and the

summer-time testing seasons. The field test results will be compared with

the laboratory test results and the simulation models. Due to complexities

in characterizing the exterior thermal boundary conditions in field testing,

it is apparent that additional air temperature and flow and radiant heat

flux measurements will be required, which may require new sensors and

measuring techniques.

58
7 .0 CONCLUSIONS AND RECOMMENDATIONS

This report reviewed the current status of thermal performance testing for

window systems in the United States. Since there are a number of

alternative objectives for thermal testing, a variety of testing apparatus,

test specimens and test conditions have evolved. Laboratory testing of

windows, skylights and glass doors has traditionally been performed . using

one of the three hot box methods described in Section 3.1. Section 4

discusses many of the test methods used for movable insulations. Industry

estimates conclude that more than 90% of the testing of windows, skylights

and doors are performed according to AAMA Standard 1503. Although no

estimates are available for the movable insulation test methods, WES 1584,

the standard adopted by the fabrics industry of window treatments, is also

based on the AAMA standard.

The primary concern over the current status of window systems testing based

on the AAMA test method is that the AAMA method has never been given

national recognition by ASTM or ANSI. As discussed in Section 5, current

efforts within ASTM are to modify both ASTM C236 and C976 to include

provisions for measuring window thermal transmittance and also to develop a new standard practice for testing of window systems. Since the development

of any new standard within ASTM requires extensive review by a host of

different groups, and the technical basis for the standard requires

extensive testing by users as described in Section 6, it is apparent that a national standard for window systems thermal performance will require

substantial effort and thus, will not be available for possibly several

years . To assist BPA in administration of energy conservation standards related to

window systems in residential buildings, NBS recommendations for an interim

test method are as follows:

1. Thermal transmittance measurements of window systems should be

performed under simulated winter time conditions using either of the

guarded hot box, calibrated hot box or laboratory room test methods

previously described. Either of ASTM C236 , C976 or AAMA Standard 1503

are acceptable for windows, doors or skylights. WES 1584 is acceptable

for movable insulations. A non-standard laboratory room technique is

acceptable provided the following conditions are followed:

a) Heat flow measurement is performed using an electrical energy

transducer with a certified accuracy of 2% at the anticipated heat

flux levels.

b) The warm room heat balance should be determined by using a calibration panel in place of the test specimen. The calibration

panel should be constructed from a homogeneous material of known

thermal conductivity and the mean temperature difference between

the warm and cold surfaces measured to an accuracy of 0.1 C (0.2

F) , using a multiple-junc t ion thermopile or other type of

differential temperature measuring apparatus of similar accuracy.

The use of commercial heat flux sensors, liquid-in-glass

59
thermometers or other non-electrical means of measuring power

dissipation and test specimen heat flux is not acceptable for

measurement of heat transfer through the test specimen. The use

of heat flux sensors or differential temperature sensors is

permissible in measuring heat loss through other surfaces of the

metered room, provided the aggregate heat loss through these

surfaces does not exceed 20% of the measured heat loss through the

test specimen.

2. Simulated winter conditions should be 20 +_ 1 C (68 +. 2 F) interior, -8

±_ 1 C (18 ±. 2 F) exterior, with either 6.7 m/ s (15 mph) simulated wind

in any direction or at essentially still air conditions. Experimental

U-values (air-to-air thermal transmittance), C-values (surface-tosurface thermal conductance) , and the inside and outside surface heat

transfer coefficient should be provided in window thermal performance

test reports. Measurements with free convection conditions on the

environmental (cold) side shall also provide a corrected U-value that

is calculated by adding the inside surface heat transfer coefficient

and the ASHRAE Fundamentals Handbook 6.7 m/ s (15 mph) outside surface

heat transfer coefficient value of 34.1 W./m2.K (6.0 Btu/h.ft2.f) to

the experimental C-value. (Note: use equation 6 to do this

calculation) .

60
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66
appendix A-l

SEALED GLAZING UNITS: THERMAL INSULATION AND SURFACE TEMPERATURE

NBI-138 , 1982

1. Orientation

The method concerns the measurement of the thermal resistance and surface temperature of sealed glazing units. The

measurement is done in a hot box.

The method is a functional testing of sealed glazing units.

The thermal insulation is measured in the entire clearance of

the pane in a way that takes into account the cold bridges at

the sealing edges. The surface temperature is measured so that

the qualities of the pane with respect to formation of condensation can be specified.

The method fulfills the requirement for measurement of

thermal insulation in the building codes.

2. Area of Application

The method is used for all types of sealed glazing units.

The dimension of the pane should preferably be 1120 mm x 1120 mm.

3. References

Norwegian Standard, NS 3161 - Thermal Insulation. Determination of Thermal Resistance by Means of a Hot Box.

NS 3031 - Calculation of Energy and Power Required to

Heat a Building.

NS 8040 - Thermal Insulation. Determination of Thermal

Resistance or Thermal Conductivity of Material with Heat Flux

Sensors.

Building Codes, with emendations as of 4 November 1980,

chapter 54:4 - thermal insulation and tightness.

Directions to the Building Codes, Chapter 54.

4. Definitions

NS 3161

A-1-1
5. Sampling

The usual glass pane size is 1120 mm x 1120 mm. Two

identical panes are supplied. The NBI chooses one specimen

for the testing.

6. Testing

The thermal insulation is measured in the entire clearance

of the pane, so that cold bridges at the sealing edges are

taken into account. Installation in a frame ensures that the

temperature and the heat transfer conditions along the outer

edges of the pane remain such as in ordinary use.

6.1. Methodology

The measurement is taken as per NS 3161 at stationary

heat flux and temperature conditions. The coefficient of

thermal transmission (k value) applies to the entire clearance

of the glass pane.

The measurement is taken* with the pane installed in a

solid wood frame. The frame has a groove for the glass as per

building part sheet A533. 133/134 , but without the bottom drained

rabbet. (The influence of the drained bottom rabbet is assumed

to be included in the frame/molding product in which the pane

is installed.

)

The frame is calibrated as a part of the boundary zone

(separation wall) between the warm and cold room in the heat

box layout.

6.2. Equipment

NS 3161 describes the heat box.

In the measurement of sealed glazing units, both the measurement chamber and the cold room are provided with radiative

screens (internal partition) . The screen is designed so that

the surface maintains the same temperature as the air.

The heat transfer conditions at the cold side are established by means of fan-circulated air. At the warm side,

natural convection prevails.

Appendix 1 shows the placement of the air temperature

sensors.

A- 1-2
6.3. Preparation of Samples

The pane is cleaned at the outer surface, but is otherwise

measured in the as-delivered state.

Equalization of pressure of the sealed glazing unit is

done only if specified. Pressure equalization is not done for

gas-filled glazing units.

Appendix 1 shows the placement of the surface temperature

sensors at either side of the pane.

6.4. Procedure

NS 3161 shows how the stationary heat flux and temperature

conditions are controlled.

The sealed glazing unit is measured at 30 K temperature

difference between the interior and exterior air temperature.

The mean air temperature at the warm side is 20°C±2°C, that at

the cold side -10®C±2°C.

% The measurement interval is 5 hours with a registration

period of 30 minutes.

6.5. Results - Reporting

The heat balance during the measurement is reported along

with the average and weighted temperatures in the air and at

the surface.

The coefficient of thermal transmission (k values) is

calculated from the measured thermal resistance in the pane

and the standardized heat transfer resistances inside and outside (NS 3031).

The surface temperature along the vertical center line of

the pane is indicated, along with the air temperatures.

The normalized surface temperatures are calculated from

the measured air temperatures and the heat transfer resistances

to reference conditions of 20°C/-10°C air temperature and

0.17 m^K/W (NS 3031) cumulative heat transfer resistance.

6.6. Accuracy

The accuracy of the method depends on the calibration of

the hot box.

A-1-3
The sealed glazing units are measured in a solid frame

which is calibrated as part of the boundary zone by means of

a laminated, glass-expanded polystyrene glass and a heat flux

sensor. This heat flux sensor is calibrated in a sheet apparatus

(NS 8040).

The replicability is regularly checked by measuring solid

reference panes.

6.7. Test Report

NS 3161 indicates the framework for the measurement

reporting. Beyond this, reference is made to the general

guidelines in NBI G01/1981.

7 . Appendix

Appendix 1 shows the arrangement of temperature sensors

on the glass surfaces and in the hot box when measuring sealed

glazing units with dimension of 1120 mm x 1120 mm.

The excerpt of the building code and directions of

4 November 1980, Chapter 54:4, indicates the foundation of

the NBI testing methods:

- Indication of the k values can be done by calculation

as per NS 3031, second edition, or by measurement after a

recognized method. The effects of cold bridges as a result

of breakthrough of the insulation in the individual parts of

the building are to be factored in.

- For other structures, e.g., sealed glazing units with

insulating gas in the middle space or with reflecting coating,

the k values must be specified, or both the night k-values and

the equivalent k-values.

A-l-4
NBI-138/1982

The Hot Box

Placement of temperature sensors at the glass surfaces and

in the measurement chamber and cold room in the hot box. All

temperatures measured along the vertical center line of the

glass pane.

Key: a - cold room; b - warm room; c - measurement

chamber; d - clearance; e - glass pane.

Scale: width 1:5, height 1:10

A- 1-5

APPENDIX A2

Swedish Standard SS 81 81 29

Compiled by the Construction Standardization Administration (BST)

WINDOWS - THERMAL RESISTANCE TEST

1. Orientation

This standard reports on a method of determining the

thermal resistance of windows, taking into account the heat loss

through both the glass and the frame and molding. If the

thermal resistance is known, the coefficient of thermal transmission can be calculated. Ongoing developments within this

field may result in modification of the standard as more experience is acquired.

The standard applies to windows (and glass-paned doors)

,

regardless of material, in the ready-to-use condition. The

window to be tested should be % air-tight, corresponding at least

to the requirement for class B in SIS 81 81 03, Windows, Categorization with respect to function. During the testing, the

manufacturer's recommendations for installation and use should

be taken into account. The test does not apply to the joints

between the frame and the surrounding wall.

The test method can also be used to test windows provided

with Venetian blinds, shades, and the like. In this case, the

test is done with these arrangements in the lowered and shut

position.

The testing is done with an air movement in the outside

chamber, which is required to carry away the heat passing

through the test window. A rapid air motion may result in

worsening of the thermal resistance for certain types of

windows

.

2. Definitions

The thermotechnical terms to be used are explained in

SIS 01 61 50, Magnitudes and units. Heat.

3 . Equipment

Two joined rooms, divided by a thermal insulated wall with

an opening for the test window (cf. Fig. 2). In one room (inside chamber) , the interior climate of a house is to be simulated; in the other (outside chamber) , the climate outdoors.

A-2-1
An arrangement that can maintain temperatures of +20±1°C

in the inside chamber and -10±1°C in the outside chamber.

A measurement box as per Fig. 2 with one side open and the

other five sides made of thermal insulated material. The width

and height of the box should at least correspond to the width

and height of the test window. The depth of the box shall be

0.6 m. In the lower part of the box there will be a heat source,

the power of which can be measured and regulated so that the

heat flux and the temperature difference between the box and the

warm chamber remains zero or close to zero. The box shall contain a screen, preventing radiation from the heat source against

the test window, but promoting convection in the box.

An instrument that can measure the power supplied to the

measurement box.

An instrument that can measure the air temperature in the

measurement box at three points, situated at 100 mm from the

window and between the side walls, with one point at the halfheight of the box and the other two points 200 mm from the bottom and top, respectively. * An instrument that can measure the air temperature in the

cold chamber at three points, situated 100 mm from the window

and opposite the points in the measurement box.

An instrument that can measure the surface temperature of

the window at the outside and inside.

(a)

(b)

(c)

Fig. 1. Measurement points (a

dark circle) on the inside and

outside of the test window.

Key: a - frame? b - molding?

c - glass.

/

• A-2-2
4, Preparations for the Test

The test window is installed in the opening between the

frames with the inside against the inside chamber. The joints

between the wall and the frame are made tight and insulated,

so that leakage of air and heat can be disregarded in the test.

The window is to be clean and dry.

The thickness of glass, the distance from the glass, the

type of glass and the installation will correspond to the recommendations of the manufacturer. In default of such, or if the

window is to be used with different glass, the test should be

done with the minimum allowable thickness of glass in relation

to the glass area.

The sensors of the instrument measuring the surface temperature of the window are mounted on the outside and inside of the

window (cf. Fig. 1). The sensors are mounted on the surface of

the glass, molding and frame. Three are mounted on the glass:

one at the center of the glass and the others at a distance of

1/6 of the height of the glass from the upper and lower pieces,

respectively. One sensor is mounted on the frame and molding

in the middle of the top, bottom and side pieces. If these

points are unsuitable on account of the location of fixtures,

or if it is suspected that they produce measured values which

are not representative, other measurement points in the same

vicinity can be used.

The measurement box is arranged centrally outside the

window in the warm chamber (cf. Fig. 2). The joint between

the measurement box and the wall is sealed, so that possible

leakage during the test can be disregarded.

The measurement box should be checked against a wall with

known thermal resistance.

5 . Procedure

The window is exposed to a difference in air temperature

between the outside and the inside.

The temperature in the inside chamber and in the measurement box shall be 20-25 °C, maintained within ±1®C during the

test. The temperature of the outside chamber shall be 30±1®C

lower than that of the inside chamber.

Measurement of the power supplied to the measurement box

is done when the heat flux between the measurement box and the

inside chamber is constant or nearly constant.

A-2-3 i
Horisontalsektion Vertikalsektion • * matpunkt ( i ) Fig. 2. Measurement box. Key: a - test

window; b - outside chamber; c - measurement

box; d - heat source; e - screen; f - inside

chamber; g

- horizontal section; h - vertical

section; i - measurement point.

6. Result

The thermal resistance of the window is:

0

where:

2 M is the thermal resistance m m °C/W;

A is the total area of the test window (outside dimension

of the frame) in m^;

$> is the total heat flux through the test window in W;

A 9 is the average temperature difference between the outer

surface of the window and the inner surface in °C, where

A- 2-4
where

:

iAu * + *b A$b + A9

Atf

g

) !A (°C>

• 2 A^ and A^ is the area in m of the frame or molding,

respectively, projected onto a plane parallel with the window,

and Ag is the clearance of the molding in m2.

A0^,A0^ and A0^ are the median values of the temperature

differences between the inside and the outside of the frame,

molding, and glass, respectively.

The coefficient of thermal transmission of the window is:

M + mt + mu where

:

i 2 k is the coefficient of thermal transmission in W/m °C;

2 M is the thermal resistance in m °C/W

m^ + m^ are the actual heat transfer resistances as per

the applicable norm.

7 . Reporting

The test report will contain the following minimum

information.

A drawing of the test arrangement

A description (details) of the installation of the window

in the test arrangement.

Brief description of the window, containing at least the

following information:

manufacturer

window type

material and surface treatment

total window area (outside dimension of frame)

A-2-5
areas of frame, molding and glass used to compute

the thermal resistance

weatherstrip (manufacturer , type, material and profile shape)

glass (number of panes, type, thickness, distance

between glass and installation)

types of locks and fixtures

Venetian blinds, shades and so forth (manufacturer,

type and material)

A view of the window, indicating the arrangement of the

fixtures: hinges, slide bars, pivots, locks, etc., as well as

horizontal and vertical sections showing the exact position of

the sealing strips.

Median air temperature values at the warm and cold side.

Median surface temperature values of the glass, frame and

mounting at the warm and cold side.

Total power supplied to the measurement box, as well as

the power passed through the window structure.

Calculated thermal resistance for the window structure,

expressed in m^°C/W, with three decimal places, the last rounded

off to 0 or 5.

Calculated coefficient of thermal transmission (k value),

expressed in W/m2°c with two decimal places, the last rounded

off to 0 or 5.

A- 2-
APPENDIX A3

Determination of the Coefficient k of Windows

New Standardized Method (

J. Uyttenbroeck *)

P. Wouters (*)

Within the framework of the preparation of the NBN-B62-QQ2 standard "calculation of the thermal transmission coefficient of the walls of buildings (Ah a new method was developed to calculate the coefficient k of the windows.

This article gives a review of the most important aspects of the new method.

In order not to make this text too difficult to read we refer the reader

to the text of the standard in a certain number of exceptional cases.

1. Introduction

In the past there was no very definite method of calculation to determine

the coefficient k of windows.

Thus the NBN B62-002 standard draft (2) of December 1983 confined itself

2

to Indicating only 2values (windows with single panes k^ = 6.4 W/m K and windows

2 with double panes k^ * 3.8 W/m K) which, in the absence of measured values, were

certainly too pessimistic.

This article presents and explains the new method which will be standardized

through the new standard NBN B62-002 (2) without discussing for all that the complete text retained in the standard in all its details.

2. Preliminary Research

2.1 Series of Tests: Coefficient k of the Windows (***)

On the request of the programming services of scientific policy and with the

additional help of the technical union of the metal joiners (UTMM) and other industrial sectors, the C.S.T.C. carried out extensive series of measurements and

(***)The measurements, calculations and analysis of the results were carried out by

D. L'Heureux, chief technician, P. Yoordecker, technician and P. Wouters, head of

the section.

(*) Civil Engineer-Director of Research V

(**) Civil Engineer-Head of the Hygrotherrry Department
calculations on the thermal transmission coefficient of windows.

The main purpose of this investigation was not to gather a large number of

k values of the windows, but was to arrive at the detailed analysis of the possibilities of achieving a standardized method to determine the k coefficient.

Right from the start we established the following goals:

-the determination of the value k of the window by calculation should be done with

a better position than that of the current method;

-a clear and simple enough method should be achieved;

-an attempt must be made to achieve an acceptable compromise between the desire

for precision and the necessary effort to obtain this precision (cost-measurements-cal cul ations-time )

Altogether 69 tests were conducted according to the standard NBN B62-204 (3)

but also according to the standard DIN 52619 (4).

The parameters studied were:

-the type of framework: the 9 types of frameworks studied were:

4 types in aluminum with thermal cutoff

1 type in aluminum with localized thermal protection

1 type in PVC with 3 air chambers without mental reinforcement

1 type in polyurethane with metal reinforcement

1 type in wood.

-the type of panes: the objects of the tests were of three types:

- ordinary double panes 04/ 12/4)

- improved double panes 04/12/4)

- a panel in synthetic material including 2 air layers and without

peripheral element.

A- 3-2
-the dimensions of the panes or the filling for frameworks with a single opening

where:

-1.2 x 0.6 m^

-1.2 x 1.2 m

2

-1.7 x 1.7 m

2

. For the frameworks with double openings:

-2 x (1.2 x 0.6) m^

-2 x (1.7 x 0.8) m^.

Besides these tests, a large number of bidimensional thermal calculations

were carried out by means of the KOBRU program (5); the purpose was either to

examine the possibilities of this method of calculation or to study the effects

of certain parameters.

The results of these measurements and calculations made it possible for us

to evaluate the relative effect of the characteristics of the windows on the k

coefficient of the latter.

2.2 Draft of ISO Method to Calculate the k Coefficient of Panes (Central Region)

The ISO (International Standardization Organization) established a draft

standard making it possible to calculate the thermal transmission coefficient of

panes in the central region. This method is based on the knowledge of the composition of each gas layer (contained between the panes) as well as the factors of

emissivity in the infra-red region e of the superficial layers of the glass

(coatings).

This ISO draft will be the object of the publication of a new Belgian standard.

r A- 3-3
3. The standardized Method of Calculating the Coefficient k of Windows

3.1 The Coefficient k for Windows

Essentially base of the results of the activity mentioned in paragraph 2,

the following formula was retained to calculate k:

(W/m2

k „ K)

k vc A

vc + k

pc

A

pc

+ k

ch

A

ch + k

L

L

p

A

vc + A

pc

+ A

ch

In this expression:

2

k^ = transmission coefficients of the window (W/m K)

k * coefficient of transmission of the opaque panels possibly present

pc

in a framework (W/m K)

k

yc

= coefficient of transmission of the central region of the panes

(W/m2

K)

2

kc h

= equi'vilent transmission coefficient of the framework (W/m K)

k^ = linear transmission coefficient taking into consideration the

effect of the insert in the panes (W/m K)

2

A * area of the visible portions of the pane Cm ) f V

2

Ap

C= area of the visible portions of the possible opaque panels (m ) A

c^= area of the projection of all the portions of the framework on

a plane parallel to the pane (m )

Lp = length of the perimeter of the glazed and opaque portions.

3.2 Determination of k (Central Portion of the Pane)

V w 3.2.1 General Case

For all the panes which do not allow the passage of infra-red rays, the

coefficient k

yc

may be calculated by means of a method of calculation described

in the standard NBN B62-004.

A- 3-4
The application of this method implies the knowledge of the composition of

each gas layer and the factors of emissivity of the surfaces of the possibly

processed glasses.

These characteristics may be measured with high precision.

3.2.2 Normal Panes with Dry Air Layers and without Processing of the Surfaces

For panes including sheets of clear or colored glass in the mass and one or

two layers of air the application of the method of calculation described in

paragraph 3.2.1 leads to the values of k indicated in table 1.

Y. Ce

TABLE 1

VALUES OF k FOR PANES INCLUDING SHEETS OF CLEAR OR COLORED GLASS IN THE MASS

V • c • AND WITHOUT SURFACE PROCESSING TO DECREASE EMISSIVITY

type of pane

single pane : thickness 5 mm

double pane

thickness of the two sheets of glass:

k

yc

(W/m2

K)

5.76

5 mm Cl)

air layer (mm): 4

6

8

9

10

12

15

3.59

3.28

3.09

3.02

2.96

2.86

2.76

A- 3-5

t
Table 1 continued

type of pane k

yc

(W/m

2

K)

triple panes

thickness of the three sheets of glass:

5 mm (1)

thickness of the 2 identical

air layers (mm):

4

6

8

9

10

12

2.61

2.29

2.11

2.04

1.99

1.90

(1) the same values k may be used for glass sheets whose thickness is between

V • c • 4 and 6 mm.

3.2.3. For special panes, which cannot be calculated in accordance with NBN B62-0Q4,

the measurement of k _ according to NBN B62-201 or NBN B62-203 remains indisv. c. pensible.

In some exceptional cases, the method according to NBN B62-204 may remain the

only applicable one. We refer to the text of the standard (.NBN B-62-002) for

further information about the special cases.

3.3 Determination of k (Opaque Panels)

P • w • The determination of k is possible with the methods described in the stanp • C • dard NBN B62-002 (by calculation or by measurement).

3.4 Determination of k . (of the framework)

W • cl • 3.4.1. All-inclusive Values

For current profiles of the framework (*) the all-inclusive values of kc h r

A- 3-
of table 2 may be used.

TABLE 2

ALL-INCLUSIVE VALUES OF k , c.n.

2

Material of the framework k . W/m K

v • n • Wood: 1.8

Polyurethane: 2.9

PVC:

-with several compartments without reinforcement 1.5

-with several compartments with reinforcement 1.7

-with single chamber without reinforcement 2.8

-with single chamber with reinforcement 3,0

Aluminum or steel

-without thermal cut off 6,0

-with metal connections in points 4.8

-with localized thermal protection including

R£ 0. 14 m2 K/W 4.2

R < 0.14 m2 K/W 4.8

-with continual thermal cut off including

0.035 m

2 K/W 3.5

R < 0.035 m2 K/W 4.0

3.4.2 Determination of kc h by measurement

In some cases the value of k . must be noted, in particular for strucq • n • v tural sections which cannot be taken into consideration according to NBN B62-002

as ordinary structural sections.

A measurement of k . may be recommended also when there are implications

w • n • that a given sectional section should be more insulating than the all-inclusive

values of table 2.

The NBN B62-002 standard recommends in these cases the measurement according

to NBN B62-204 where the element of the tested includes at least 33% of the elements of the framework (measuring the surfaces of the projection of the frame-

*For exact definition see NBN B62-002

A-3-7
work elements on a plane parallel to the measurement rack of the box case) and

where the filling panels are characterized by a coefficient k known precisely

and less than or equal to the coefficient k of the plane which the frame should

recei ve.

An example of the arrangement of the element to be tested is given in fig, 1,

3.4.3 Determination of kc ^ by Combination of Measurement and Calculation

In these exceptional cases where a very high precision is required the

standard provides for a method which combines a measurement on a structural

section with calculations on other structural sections whose k . have to be

c. n. determined.

Seeing that this combined method will be used only rarely, we refer to the

text of the standard for more information.

3.5. Determination of k^ - Effect of the Insert

3.5.1. All-inclusive Values

For ordinary panes the values of table 3 of k^ may be used.

TABLE 3

VALUES OF k

L

(W/mK) IN ORDINARY CASES

Type of pane or filling Type of framework k, CW/mK)

panel

single pane or panel

without edges or insert

All types of panes or

filling panel

Panes with metal insert or

panel with edges or metal

insert k or k > 2 W/m2 K vc pc « idem but k < 2 W/nrK

vc

all types

metal framework without thermal

cut off

All types of framework except

metal frameworks without thermal

cut offs

idem

0

0

0,05

0,07

A-3-8
8

cm

3.5.2 Calculation of k^

For all cases not provided for in table 3 or 4 panes or panels for which the

coefficient k^ is assumed to be better than the values of table 3, it is possible to determine k^ by calculation. In this calculation it is assumed that

the pane or panel is placed in a standardized structural section (fig. 2) and

the heat losses through this element are calculated with and without insert.

The difference between the two results of calculation makes it possible to

determine k^.

4. Average Value of the Transmission Coefficient for all the Windows of a Given

Building

4.1. Simplifications Foreseen

4.1.1. It is permissible to use for certain calculations Glosses, energy require-

*Fig. 1 does not appear in the text

A-3-9
merits) the average value of the thermal transmission coefficient of all the windows of a building instead of the individual values for each window.

This average value is given by:

Y k.^, i * A^,i

kf

, T = (W/m2

K)

1 A

f

,i

In this expression A

fi

is the area of the rack (seen from the outside )of the

window i. See fig. 3.

4.1.2. It is also permissible for small size projects (*) to use a simplified

formula given in paragraph 4.2, unless the author of the project does not allow

this.

This simplified formula may also be used for other projects if the author

of the project mentions this specifically.

4.2. Simplified Formula to Calculate k^, ^

In the cases mentioned ! in paragraph 4.1.2, the average thermal transmission

coefficient for all windows of a building may be determined by the following

method:

-for windows with metal framework:

k

f T

= 0.75 k

yc + 0.25 k

ch + 3 k

L

(W/m

2

K)

-for windows with other frameworks:

k. T = 0.7 k + 0.3 L + 3 k. (W/m2K)

Moreover the panes are of ordinary type (normal panes with dry air layers

and without surface processing, as specified in table 1), and if the frameworks

are of standard type for which the all-inclusive values of table 2 may used, the

*The small size projects on individual houses and apartment buildings with at

least 5 apartments.
previous formula makes it possible to calculate directly the value of k^. The

values thus obtained are given in table 4.

TABLE 4

VALUES OF kn

FOR SMALL PROJECTS (HOUSES + BUILDINGS WITH LESS THAN 5 APARTMENTS)

Material + type of framework

PVC aluminum or metal

wood

pane

T

Y

P

E

thickness

of the

air layer

mm

PVC several with- local- localized thermal

chambers 1 chamber out 1zed protection cut off

cut con- with with with with n?

<r“

out out tlon

0.14 0.14 0.035 0.035

rein- rein- rein- reinforce force force forde

ment ment ment ment

coefficient k

ch (W/m2

K)

1.8 2.9 1.5 1.7 2.8 3.0 6.0 4.8 4.2 4.8 3.5 4.0

S G

I L

-

5,76 4,57 4,90 4,48 4,5*1 4,87 4,93 5,82 5,52 5,37 5,52 5,20 5.32 1

1 N E' (*) 4 3.59 3,20 3,53 3.11 3,17 3.50 3.56 4,19 4,04 3.89 4,04 3,72 3,84

D

0

6 3.28 2,99 3,32 2,90 2,96 3.29 3,35 3.96 3.81 3,66 3.81 3.49 3.61

U

B

8 3.09 2.85 3.18 2.76 2,82 3.15 3,21 3.82 3.67 3,52 3,67 3,35 3,47

i L

E

10 2.96 2,80 3,13 2,71 2.77 3,10 3,16 3,72 3.62 3,47 3,62 3,30 3,42

(*) 12 2.86 2,69 3,02 2,60 2,66 2.99 3.05 3.65 3.49 3.34 3,49 3,17 3,29

15 2.76 2,62 2,95 2,53. 2.59 2,92 2.98 3,57 3.42 *3,27 3.42 3,10 3,22

T

[

; 4 2.61 2,52 2,85 2 , 43 2,49 2,82 2,88 3.46 3.31 3.16 3,31 2,99 3,11

R

I

i 6 2.29 2,29 2,62 2,20 2,26 2,59 2,65 3,22 3,07 2,92 3,07 2,75 2,87

P

L ; 8 2.11 2,17 2,50 2,08 2,14 2,47 2.53 3,08 2,93 2,78 2,93 2,61 2,73

E

*) 9 2.011 2,12 2.45 2,03 2,09 2,42 2,48 3.03 2,88 2.73 2,88 2,56 2,68

10 1.99 2,14 2, *17 2.05 2,11 2,42 2,50 2,99 2,90 2.75 2,90 2,58 2,70

* 12 1 .90 2,08 2,41 1,99 2.05 2.38 2,44 2,93 2,84 2,69 2,84 2,52 2,64

*The thickness of the pane varied between 4 and 6 inn. $

A-3-11
Also for the same small project and assuming that we are dealing with standard

framework whose all-inclusive values (table 2} may be used, table 5 gives the

average va]ues of k^-j. according to the type of framework and as a function of the

coefficient k . vc

TABLE 5

AVERAGE VALUES OF FOR SMALL PROJECTS (W/m2

K)

A FUNCTION OF k

y(;

. PVC

FOR STANDARD FRAMEWORKS AND AS

Aluminum or metal

wood PVC several 1 compart- with local- localized thermal

compartments ment out ized protection cut off

with with with with. ^ > r < r?

out out

01 f t10ns R i R< R? R<

rein- rein- rein- reinforce- force- force- force0.14 0.14 0.035 0.035

k . ment ment ment ment

cn

. 1.8 .2.9 . 1.5. 1.7 . 2.8 3.0 ,.6.0 .4.8 4.2 4.8 3.5 4.0

3 .6 3,21 3,54 3,12 3,18 3,51 3,57 4,20 1,05 3,90 1,05 3.72 3,85

3 .5 3,14 3,17 3,05 3,11 3,44 3,50 4,12 3,97 3,82 3.97 3,65 3,77

3 .4 3,07 3,10 2,98 3,04 3,37 3,13 1,05 3,90 3,75 3,90 3,57 3,70

3 .3 3.00 3,33 2,91 2.97 3.30 3,36 3,97 3.82 3.67 3,82 3.50 3,62

3 .2 2,93 3,26 2,81 2,90 3,23 3,29 3,90 3,75 3,60 3,75 3.12 3,55

3 .1 2,86 3,19 2,77 2,83 3.16 3,22 3.82 3.67 3,52 3,67 3,35 3,17

3 .0 2,79 3.12 2,70 2,76 3.09 3,15 3,75 3,60 3,45 3,60 3,27 3,40

2 .9 2,72 3,05 2,63 2,69 3,02 3,08 3,67 3,52 3,37 3,52 3.20 3,32

2 .8 2,65 2,98 2,56 2,62 2,95 3,01 3.60 3,45 3.30 3,45 3,12 3,25

2 .7 2,58 2,91 2,49 2,55 2,88 2,94 3,52 3.37 3,22 3.37 3,05 3,17

j 2 .6 2,51 2,84 2,42 2,48 2,81 2,87 3,45 3,30 3,15 3.30 2.97 3,10

2 .5 2,44 2,77 2,35 2,41 2,74 2,80 3.37 3.22 3,07 3,22 2,90 3,02

2 .4 2,37 2,70 2,28 2,34 2.67 2.73 3.30 3,15 3,00 3.15 2,82 2,95

2 .3 2,30 2,63 2,21 2,27 2,60 2,66 3,22 3,07 2,92 3,07 2,75 2.87

2 .2 2,23 2,56 2,14 2,20 2,53 2.59 3,15 3,00 2,85 3,00 2,67 2,80

2 .1 2,16 2,49 2,07 2.13 2,46 2,52 3,07 2,92 2,77 2,92 2,60 2,72

2 .0 2,09 2,42 2,00 2,06 2,39 2 , H5 3,00 2,85 2,70 2,85 2,52 2.65

1 .9 2,08 2,41 1,99 2,05 2,38 2,44 2,92 2,83 2,68 2,83 2,51 2.63

1 .8 2,01 2,31 1,92 1,98 2,31 2,37 2,85 2,76 2,61 2,76 2,«3 2.56

1 .7 1 ,91 2,27 1,85 1,91 2,24 2,30 2,77 2,68 2,53 2,68 2.36 2,48

1 .6 1 ,87 2,20 1,78 1 ,84 2,17 2,23 2,70 2,61 2,46 2,61 2,28 2,41

1 .5 1 ,80 2.13 1.71 1,77 ^10 2,16 2,62 2,53 2,38 2,53 2,21 2,33

1 .4 1,73 2,06 1 ,64 1,70 2,03 2,09 2,55 2,46 2.31 2,46 2,13 2,26

1 .3 1,66 1,99 1.57 1.63 1.96 2,02 2,17 2.38 2.23 2,38 2,06 2,18

1 .2 1.59 1,92 1 ,50 1 ,56 1 ,89 1,95 2,40 2,31 2,16 2,31 1 .98 2,11

1 .1 1.52 1.85 1,13 1,19 1,82 1,88 2,32 2,23 2,08 2,23 1,91 2,03

1 .0 1 .‘15 1 ,78 1.36 1 ,42 1.75 1 ,81 2,25 2,16 2,01 2,16 1 ,83 1 ,96

0 .9 1.38 1.71 1,29 1,35 1,68 1,71 2,17 2,08 1.93 2,08 1,76 1,80

0 .8 1.31 1 ,64 1 ,22 1 ,28 1 ,61 1 ,67 2,10 2,01 1 ,86 2,01 1 ,68 1 ,81

0 .7 1,24 1,57 1.15 1,21 1,54 1 ,60 2,02 1.93 1,78 1.93 1 ,61 1,73

0 .6 1.17 1 ,50 1 ,08 1.14 1 ,17 1,53 1.95 1,86 1.71 1 .86 1,53 1,66

0 .5 1,10 1,43 1 ,01 1.07 1,40 1,46 1,87 V8 1,63 1.78 1,46 1,58
4.3. Discussion on the Values k^y for Small Projects

The examination of table 4 and 5 make it possible to draw some conclusions.

1. For windows with wooden, PVC or polyurethane framework, the effect of the

framework on the coefficient k of the window is constant: actually we find:

k (PUR) = k (wood) + 0.33 W/m2

K

k (PVC, several compartments, no reinforcement} = k (wood) - 0.09 W/m K

2

k (PVC, several compartments, with reinfo rcement)=k (wood) - 0.03 W/m K

2

k (PVC, 1 compartment, no reinforcement) = k (wood) +0,30 W/m K

2

k (PVC, 1 compartment, with reinforcement) = k (wood) + 0.36 W/m K

The effect of a metal reinforcement on the coefficient k^ of a window with PVC

2

framework is of the order 0.06 W/m K.

2. Seeing that the simplified formula takes into account a lower percentage

of for metal frameworks (0.25 instead of 0.30), it is not possible to compare

systematically the values of k^ for the frameworks in aluminum with those obtained

for wood.

Nevertheless we find the following orders of magnitude:

2

k (aluminum, without thermal cutoff) = k (wood) + 0.9 to 1 W/m K

2

k (aluminum, with point junctions) s k (wood) + 0.75 to 0.85 W/m K

2

k (aluminum, local protection, R> 0.14) = k (wood) + 0.60 to 0.70 W/m K

2

K (aluminum, local protection, R < 0.14) = k (wood) + 0.75 to 0,85 W/m K

2

k (aluminum, thermal cut off, R > 0.035) « k (wood) + 0.45 to 0.55 W/m K

2

k (aluminum, thermal cut off, R < 0.035) - k (wood) + 0.55 to 0.65 W/m K

3.

The coefficients k^, rounded off to one decimal, for ordinary single or double

glazing (12 mm air layer) are summarized in table 6.

A-3-13
TABLE 6

VALUES OF k

f

OF WINDOWS FOR SMALL PROJECTS, ROUNDED OFF TO ONE DECIMAL (W/m

2

K)

Framework simple double glazing

glazing air layer (12 mm)

wood 4.6 2.7

Polyurethane 4.9 3.0

PVC, several compartments 4.5 2.6 - 2,7

single compartment 4.9 3.0

Aluminum, without thermal cut off 5.8 3.7

with localized joints 5.5 3.5

with localized protection 5.4 - 5.5 3.3 - 3.5

with thermal cut off 5.2 - 5.3 3.2 - 3.3

5. Summary

The new project of chapter 6 of the NBN B62-Q02 standard concerning the calculation of the coefficient k of the windows characterized by the following points:

-an acceptable precision combined with a sufficient simplicity of application;

-the possibility of determining by calculation the coefficient k at the center of

the pane (k vc

);

-the fact that the effect of insertion of the pane and the proposal for a series

of all-inclusive values of k^ is taken into consideration;

-the proposal for a series of all-inclusive values of the coefficient k

c ^ of

the framework;

-for small projects with a presumably constant relationship between the area of

the framework and the panes, in the indication of the practical values of k^;

-the possibility of implementing in special situations by dimensional calculations of heat exchange.

A- 3-14
6. Acknoledgements

The writing of this article and the text of chapter 6 of the standard, as

well as the extensive series of tests and calculations on which these results

are based were made possible by the collaboration and support:

-of the programming services of scientific policy

-the Technical Committee of External Techniques of Hygrothermy, glazing and

Carpentry of the CSTC;

-the Technical Union of Metal Joineries

-the working group of the Thermal Insulation Commission of the IBN, working

group presided over by Mr, H. Remade

-the representatives of the glass industry and the chemical industry.

7. Bibliographical References

A- 3-15
.
appendix b

WORKING DRAFT 3 i i

i APRIL, 1986

ASTM C XYZ/E XYZ

STANDARD PRACTICE FOR DETERMINING

THE THERMAL PERFORMANCE OF WINDOW

AND DOOR SYSTEMS

Please send all comments on this

working draft by July 1, 1986 to:

William P. Goss

Mechanical Engineering Department

University of Massachusetts

Amherst, Massachusetts 01003

(413) 545-2037

ASTM Cl 6: 30 - THERMAL MEASUREMENTS

This document is in the process of development and is for ASTM

Committee use only. It is not to be distributed or quoted without the prior

approval of the ASTM Cl 6.30 or E6.51 Chairperson.

B-l
2

STANDARD PRACTICE FOR DETERMINING THE

THERMAL PERFORMANCE OF WINDOW AND DOOR SYSTEMS

CONTENTS

Page

1 . SCOPE 3

2. APPLICABLE DOCUMENTS 4 2.1 ASTM Documents 4

3. SIGNIFICANCE AND USE 6

4. TERMINOLOGY 7

4.1 Definitions 7

4.2 Descriptions 9

4.3 Symbols 10

5. CALIBRATION 12

5.1 General 12

5.2. Instrumentation 12

5.3 Calibration Tests 13

5.4 Calibration Data Analysis 13

6. EXPERIMENTAL PROCEDURE 19

6.1 Temperature Measurements 19

6.2 Environmental Conditions 20

7. CALCULATION PROCEDURE 22

7.1 General Calculations 22

7.2 Calculations for PROCEDURE A 22

7.3 Calculations for PROCEDURE B 22

8. REPORT 24

9. REFERENCES 25

10. ANNEX 27

10.1 Heat Transducer Calibration Specimen Design. ...... .27

10.2 Radiation Heat Transfer Calculation Procedure 27

B-2
3

1 . SCOPE

1.1 This standard practice provides requirements and guidelines for the

determination of the thermal performance of window and door systems.

The practice specifies the thermal measurements to be made using either

ASTM C 236 or ASTM C 976 and the procedure for calculating the window

or door system thermal performance.

1.2 Thermal performance, as used in this practice, refers to the thermal

transmittance, U, and thermal conductance, C, of a window or door system in the absence of solar and air leakage effects.

1.3 A discussion of the definitions and underlying assumptions for calculating the thermal transmittance and thermal conductance is included.

B-3
2. APPLICABLE DOCUMENTS -

2.1 ASTM Standards

C 236 Steady-state thermal performance of building assemblies by means of a guarded hot box.

C 976 Thermal performance of building assemblies by means of a calibrated

hot box.

C 168 Definitions of terms relating to thermal insulating materials.

C 177 Test method for steady-state thermal transmission properties by means

of the guarded hot plate.

E XXX Test method for rate of air leakage through exterior windows and

doors under specified temperature differences across the specimen (in

draft)

.

C 518 Test method for steady-state thermal transmission properties by means

of the heat flow meter.

C XXX Standard practice for derived thermal transmission properties calculated from steady-state heat flux measurements (in draft).

B-4
E 230 Standard temperature^electromotive force (EMF) tables for

thermocouples.

B-5
6

3. SIGNIFICANCE AND USE

3.1 This practice details the test conditions, procedures, and additional

temperature instrumentation necessary to apply ASTM C 236 or C 976 to

the measurement of thermal performance of window and door systems. It

also provides the calculation procedure to reduce the basic data ob~

tained from guarded and calibrated hot box tests to arrive at the

window or door specimen thermal transmittance, U^, and thermal conduce

tance, C . s

3.2 Standardized test conditions are provided for determining the thermal

transmittance and thermal conductance of the test specimen. Since tem~

perature and surface air film conditions will affect the results, use

of these standard sets of conditions will reduce confusion caused by

comparison of test results under dissimilar conditions. This procedure

can however, be used with other conditions for research or product

development.

3*3 This practice does not include procedures to determine the heat flow

due to air leakage through the specimen under the test conditions.

3.^ Since the size and shape of a specimen will affect the specimen heat

transfer, care must be exercised when extrapolating to product sizes

smaller or larger than the test specimen. Once experience is gained

with this procedure, limitations on the extrapolation of results will

be provided in future editions of this standard.

B-6
7

4‘. -TERMINOLOGY

4.1 Definitions » Definition and terras are in accordance with definitions

Cl 68, from which the following have been selected and modified to apply

to window and door systems:

4.1.3 Specimen thermal conductance, Othe time rate of heat flow through a

unit area of a specimen (window or door), induced by a unit tempera^

ture difference between the specimen surfaces. It is calculated as

follows:

C

s Q /A (t,- t 2 ) s s

1 (1 )

4.1.4 Thermal transmittance, U (sometimes called overall coefficient of

s heat transfer)-the heat transmission in unit time through unit area

of a specimen and its boundary air films, induced by unit temperature

difference between the environments on each side. It is calculated

as follows:

U

s Q /As s V ( 2 )

The transmittance can also be calculated from the thermal conductance

and the surface conductances as follows:

1/U = (1/h ) + (1/C ) + (i/h ) s h s c (3)

B-7
8

4.1.5 Surface conductance, h (often called surface or film coefficient)-the

time rate of heat flow from a unit area of a surface to its- surroundings, induced by a unit temperature difference between the surface

and the environment. Subscripts h and c are used to differentiate

between room side and weather side surface conductances,

respectively. These conductances are calculated as follows:

when t.'h t bl

*

(4a)

when

(4b)

when t t b2

’ c

(5a)

when

(5b)

Specimen thermal resistance R -the mean temperature difference, at

c

B-8

4.1 .6

m
equilibrium between two defined surfaces of a material, or construction that induces a unit heat flow rate through unit area. It is

calculated as follows

( 6 )

4.2 Descriptions of Terras Specific to this Standard:

4.2.1 Surface resistance, r-the temperature difference between an isothermal surface and its surroundings when a unit heat flow per unit area

is established between the surface and the surroundings under steadystate conditions by the combined effects of conduction, convection,

and radiation. Subscripts h and c are used to differentiate between

hot side and cold side surface resistances, respectively. Surface

resistances are calculated as follows:

rh 1/h ( 7 ) n

r 1/h ( 8 ) c c

4.2.2 Overall thermal resistance, R^,-the temperature difference between

the environments on the two sides of a body or assembly when a unit

heat flow per unit area is established through the body or assembly

under steady-state conditions. It is the sum of the resistances of

the body or assembly and of the two surface films and may be calculated as follows:
( 9 ) -R = (t - t ) A/Q = r + R + R =1/U

u h c c c - h s

4.3 Symbols^tjie symbols, terms, and units used in this method are the

following:

bl

bl

area of room side baffle, m

2 = area of weather side baffle, m

2 A

s = rough opening area of specimen, m

= thermal conductance of specimen (surface to surface)

^ W/(wt 2 «K)

2 h = surface conductance, cold side, W/(m .K) c

2 h, = surface conductance, hot side, W/(m *K) h

L = length of path (thickness of specimen),

m

Q = time rate of heat flow, total power through the mask wall-window ordoor system, W.

Q = time rate of heat flow through the mass wall, W m

= time rate of heat flow, total power through the specimen, W

2 q

= heat flux (time rate of heat flow through unit area ), W/m

2 q

s

= time rate of heat flow per unit area through the window system, W/m

q

= time rate of net radiative heat flow per unit area from window sur~

r

2 face, W/m

q = time rate of convective heat flow per unit area from window surface,

c

2 W/m

2 R =* surface to surface thermal resistance of specimen, K«m /W

c

2 r = surface resistance, cold side, K»m /W

c

2 r = surface resistance, hot side, K-m /W

B-10
R = overall thermal resistance of specimen (air to air under test

2 *- conditions), K-m /W

ti = temperature of ambient air, K or °C a

t = baffle surface temperature, room side, K or °G

t; - temperature of weather side air, K or °C

t, _ = baffle surface temperature, weather side; K or °C

bZ

t = temperature of room side air, K or 6 C

t l = temperature of specimen room surface, K or °C

t 2 = temperature of specimen weather surface, K or °C

= thermal transmittance of specimen (air to air under test conditions),

2 W/(m °K)

B-ll
5. CALIBRATION -

5.1 General

5-1.1 The test facility shall be calibrated using a heat flux

transducer calibration specimen constructed as described in

Annex 10.1, containing a core material of known characteristics

traceable to primary standards such as the NBS and NRCC Guarded

Hot Plates. The area of the specimen(s) shall be the same as

•

the specimen sizes to be tested.

5.1.2 Two calibration procedures (calculations) are provided. procedure A is used when test Upvalues are to be determined and

Procedure B when test C-Values are to be determined. In general

procedure A is used when good control on the room side and

weather-side air films can be provided with the facility and

when the test specimen is not likely to have through air leakage

which will cause an erroneous heat transfer measurement.

Procedure B is used when air film are not provided within the

tolerances specified or when through specimen air leakage must

be reduced by lower pressure differences across the specimen.

5.1.3 A mask wall shall be proceeded for mounting the test specimens,

constructed in a layered fashion with homogeneous material

without thermal bridges.

5.2 Instrumentation. In addition to the air and surface area weighted temperature measurements specified in ASTM C236 and ASTM C976, additional

instrumentation is required as follows:
13

5.2.1 Radiative temperatures - the temperature of all surfaces exchanging radiation heat transfer with the window system shall be

measured.

5.2.2 Air Temperature - Temperature measurements should be made in the

room side and weather side air streams in the same locations as

the surface temperature sensors.

Question - should the air temperature be determined in a line down center of

window as in NRC approach?

5.2.3 Chamber Pressures - Two pressure taps are to be provided to

measure the pressure difference across the specimen. They shall

be located at mid height of the specimen at a potential leakage

site and be shielded from direct air impingement. (See Figure

XX).

5-3 Calibration Tests

5.3.1 Install calibration specimen in the center of the mask wall approximately half way between the room side and weather side of

the mask. Seal the perimeter of the specimen to the mask to

prevent air leakage by the specimen/

5.3.2 Balance the pressure between the room side and weather side

chambers and monitor AP. The AP shall be 0±10 Pa (0±0.04) in

H 2 0) for steady state conditions.

5.3.3 Establish steady state temperature conditions for which the

facility is to be calibrated and record measurements of power,

temperature and pressure.

5.4 Calibration Data Analysis - In addition to the requirements of ASTM

C236 and ASTM C976 the following shall be determined:

B-13
5.4.1 PROCEDURE- A * Upvalue Calibration

5. *4.1.1 Total Heat Flow » The time rate of heat flow through the

test assembly (mask wall and calibration specimen), Q, is

determined by the procedures specified in ASTM C236 or

ASTM C976.

5. 4. 1.2 Specimen heat flow, Q

s> is calculated from

Q

s

= C A (TJ - TJ) (10)

C o

2 where: C

c

= conductance of calibration specimen core, W/(m -K)

2 A

s

f ^ i

- Area of calibration specimen, m Average temperature of room side glass/core interface of

calibration specimen, °C

t t 2 = Average temperature of weather side glass/core interface

of calibration specimen, °C

5.4.1 .3 Mask Heat Flow, Q , and conductance C is then

m m Q_m

= Q - Q (11)

s

Cm

Qm (12)

A x(t" . - f ,) m ml Bm2

where A

ra = mask area, m

2

t

ml = weighted room side mask surface temperature, °C

t - Area weighted weather side mask surface temperature, °C

m2

Note: If a mean temperature correction for the mask is required, conduct

calibration tests at at least three sets of mean temperature conditions.

5.^.1 .4 Surface Conductance, h. and h ,

’ h c

hhQ

S a ( V t.)

(13)

where: t = Average room side air temperature, °C
tj = Weighted average room side specimen surface temperature.

where

where

°C, which is calculated from the following:

C

1 1 t (

t

x - t 2 ) (14)

g

C * Conductance of glass facing on calibration specimen

h ~

7=r c A (T T

o

) (15)

t = average weather side air temperature, °C

t 2 = weighted average weather side specimen surface tempera^

ture, °C, which is calculated from

C t " G » » t 2 = t 2 * "

(^i ” t 2 ) g

( 16 )

The surface conductances h^ and h , shall be 8.3 ± 1 and 34 ± 3

W/(m2 »K) (1.5 and 6.0 BTU/h-ft 2 -F) when calculated as above.

Question - should U-value determined using procedure A be corrected back to

standard film conditions if we do not have 8.3 and 34 W/(m2 »K)?

5.4.2 PROCEDURE B - C Value Calibration

5.4.2. 1 Total Heat Flow - The time rate of heat flow through the

test assembly (mask wall and calibration specimen), Q, is

determined by the procedures specified in ASTM C236 or ASTM C976.

5. 4. 2. 2 Specimen Heat flow, Q

s

, is calculated from

Q » C A (t* - t 2 ) (17)

s c s

1 where C = conductance of calibration specimen core,

c

2 W/(m -K)

A^ = Area of calibration specimen, m

B-15
16

tj = Average temperature of room-side glass/core interface of

calibration specimen, °C

* . t 2 = Average temperature of weather-side glass/core interface

of calibration specimen, °C

5. 4.2. 3 Mask Heat Flow, Q , and Conductance C m m Q - Q - Q m s

C = ^

m A x (t - t ) m ml m2

5. 4.2.

4

Calculate the weighted average specimen surface tempera’

( 18 )

(19)

tures t x and t 2

from

t, = t,

t 2 1 2

(t, - t 2 )

(t, t 2 )

( 20 )

(21 ) g

5.

4.2.5

Radiative Heat Transfer, Q

Note : In procedure B the heat transfer to the specimen from the room-side

enclosure is broken down into a radiative, Q , and a convective, Q

c

» component which facilitates the determination of the room side surface

conductance. When a window specimen is being evaluated, the relationship

for Q and Q is used to calculate the equivalent surface temperature of the

r c specimen.

If a baffle or box wall is close to the specimen and parallel

plate heat transfer can be assumed.

= qM

= F r 1b

a (t - t ) b

(21 )

B-16
17

where F

1b

1

i A

1 + — (_L

e

1 A

b

eb

(assuming a view factor of 1.0)

1 )

e

1 = emittance of glass :

e^ = emittance of the baffle or box wall

2 A, = area of baffle, m

b

» area weighted baffle temperature, K

a a = Stefan^Boltzman constant 5.6703x10 W/(m2 -K4

)

NOTE: If the view factor between the window surface and the

baffle surface is not close to 1.0 or if the baffle is not

isothermal, then the radiation heat transfer calculation procedure

in Appendix 10.2 is recommended.

5- 4. 2. 6 Convective heat transfer, Q

Q = Q - Q

c s r

( 22 ) having calculated Q

c

, the constant in the following equation

for the convective heat transfer to the specimen can be

determined.

Q

%

K * ( V V

1.25

(23)

NOTE: The convective heat transfer calculation assumes "near”

natural convection on the room side of the specimen. To ensure

that a single covection, K, can be used for window tests, the

range of heat flows tobe encountered should be tested using the

calibration specimen while the convection constant behavior^/is^^K^

determined.

5.4 .2.7 Surface conductances h^ and h

(

h

h ( t. ~ t,

)

(th~ V

(24)

B-17
The weather side surface conductance can be calculated from the

following simplified formula if the surfaces exchanging radiation

with the specimen are at the same temperature as ± 0.5°C:

h

c A ( t_ - t ) (25) s 2 c If they are not at the same temperature, the radiative and convex

tive components of the heat transfer must be established as done

on the room side.

B-18
19

6. EXPERIMENTAL -PROCEDURE .

6.1 Temperature Measurements - In addition to the air and surface area

weighted temperature measurements specified in C 236 or C 976, ad~

ditional temperature measurements may be made on the mask wall,

the window or door frame, the; window or door glazing and on any

other portions (sills, muntins, etc.) of the window or door system

so as to allow an area weighted determination of the surface to

surface temperature difference of the window or door system. It

should be recognized that there is such a wide range of window and

door designs that it is not possible to specify where to locate

the temperature sensors so as to arrive at a correct area weighted

determination of the window surface temperatures. The experience

of the hot box operator will have to be drawn upon here. Also,

any area weighted window surface temperature determined in this

manner should be compared with the calculated equivalent window

surface temperatures specified in Section 5. Calibration. If

these two values differ by more than 5%, then additional tempera^

ture sensors should be installed on the window surfaces to arrive

at more accurate area weighted window surface temperatures. In

addition, temperature measurements should be made in the cold and

hot air streams in the same locations as the window surface tern’'

perature sensors. This will allow for calculation of the cold and

hot side surface conductances. This technique of area weighting

may be applicable when the frame and glazing conductances are

similar. If they are not, excessive use of temperature sensors

may cause h to differ from calibration tests introducing further

r _

B-19
20

uncertainty in the results. The temperature sensors used may be

special limit thermocouples (24 gage may be used, 30 gage or . smaller are recommended for the window surface temperatures)

,

thermistors or RTD's (resistance temperature detectors).

6.1.1 Radiation effects: * to minimize the effect of radiation induced

error on the temperature sensors, the temperatures of all of the

surfaces exchanging radiation heat transfer with the window or door system must be measured. This includes cold and hot side

baffles facing the mask wall/window system. Any heating and

cooling devices must be shielded from the mask wall/window sys~

tem and the surface temperature of the shield should be

measured. The temperature sensors should be applied to these

surfaces with tape or adhesive which has an emissivity similar

to that of the surface. The air temperature sensors should be

shielded so that they are not significantly effected by surfaces

that they are exchanging radiation with.

6.2 Environmental Conditions » it is recognized that a single environ^

mental condition does not adequately define the thermal

performance of a window or door system. However, to allow for

comparisons of Upvalues of different window and door products a single environmental condition is specified. For windows and

doors where the seasonal thermal performance is desired, addi^

tional environmental conditions are specified.

6.2.1 Environmental condition for Upvalue testing » the test specimen

shall be tested using the following conditions.

B-2.0
21

PROCEDURE A:

t = 21 °C (70°F)

h

t = -18°C (0°F

)

c h = 8o3 W/(m2 -K) h

h = 34 W/(m2 -K) c PROCEDURE B:

t = 21 °C (70°F

)

h

t = - 1 8°C (0°F)

c

h, } as determined in calibration

h

h } as determined in calibration

c NOTE? These conditions represent ASHRAE winter design conditions.

The room side surface conductance h, represents natural

h

convection and the weatherside surface conductance h rep~

c resents a 15 mph wind. If the Upvalue procedure is used,

the test facility will be calibrated to provide these sur~

face conductances. If the C^Value procedure is used, the

effect of these surface conductances will be added to the

OValue to arrive at a Design Upvalue.

2.2 Environmental conditions for seasonal thermal performance

studies’the test specimen shall be tested over a range of condi~

tions which simulate winter, summer and spring conditions with

weather side surface conductances ranging from natural convec~

tion to ASHRAE summer (7.5 mph) and winter (15 mph) design

conditions.

B-21
7.7.1 CALCULATION PROCEDURE

General Calculations

7.1.1 The following shall be calculated for each test:

The time rate of heat flow through the test assembly, Q, is

determined using procedures outlined in ASTM C236 and ASTM C976

7.1.2

Mask heat flow, Q , m

Q = C

ra m m

(t

ml

t

m2 ) ( 26 ) where C is determine from calibration tests

m

7.1.3

Specimen heat flow, Q , 3

Q = Q

s m

(27)

7.2 Calculations for PROCEDURE A

7.2.1 Specimen thermal transmittance, U

U

s A (t - t ) s h c 7.2.2 Specimen Thermal Conductance, C

1 C =

s 1

. h

h c

( 28 )

(29)

Question - should actual values for h. and h as determined in the calibra^

h c

2 tion test be used or use values of 8.3 and 3^ W/(m *K)?

7.3

Calculations for PROCEDURE B

7.3.1 Equivalent room side surface temperature of specimen, t x , is

calculated by solving the following 3 equations for Q , q^ and

1

•

Q

= Q

+ Q

r c Q

-

Q

=

A *F -o- ( t - t ) sib b 1 A -K-(t - t ) s h 1 1.25

(30)

(31 )

(32) .

B-22
23

where K is determined from the calibration tests discussed in section

5.

Note: One way to solve these equations is by iteration.. Assume a value for

t in equation (31), calculate Q , determine Q from equation (30),

1 r c then calculate a new t

1 from equation (32 )«, If this new value is

different than the first assumption, use the average of the two t^

values in equation (31 ) and repeat the calculation until the t^

values agree to within 0.1 °C.

7.3.2 Equivalent weather side surface temperature, t^.

h

where h is determined from the calibration tests discussed in

section 5. 7.3-3 Specimen thermal conductance, C , 3

c - s A (t

s 1 V

7.3.^ Specimen thermal transmittance, U

s

,

(3*0

U = s — + C

s

1

’ + 1, ST3 3

1!

(35)

B-23
24

8 . REPORT

8.1 The report shall include all of the information specified in C976

Section 11 or C236 Section 10.

8.2 In addition, the thermal conductance, C , and thermal transmittance,

2

U^, and the hot and cold side surface conductances, h^ and h

c

, of the

window or door system shall be reported and their estimated uncertainty

specified.

B-24
27

10. ANNEX

The following annexes will be added to Draft 4:

10.1 Heat flux transducer calibration

flux transducer is used in the calibration of the surface conductances

and for checking the mask wall conductance.

10.2 Radiation heat transfer calculation procedure - this calculation procedure is to be used when the assumption that the window and baffle

surfaces are parallel surfaces and the wi?low only exchanges radiation

heat transfer with the isothermal baffled. However, in many situations, the window also exchanges radiation heat transfer with the mask

wall opening surfaces and with nonisothermal baffle surfaces. In those

situations, the radiation calculation procedure described in this annex

is recommended.

pecimen design - this large heat

B-25
26

Wilkes, G.B., and C.M.F. -Peterson, "Radiation and Convection From

Surfaces in Various Positions", ASHVE Transactions, Vol. 44, 1938, pp.

513-522.

as, W. , M. McCabe, R. Cholvibul, and P. Wormser, "U-Value : Measurements for Windows and Movable Insulations from Hot Box tests in

Two commercial Laboratories", Submitted to ASHRAE for Publication,

1985.

Bowen, R.P. and K.R. Solvason, "A Calorimeter for Determining Heat

Transmission Characteristics of Windows", Submitted to ASTM for publication, 1985.

Raber, B.F. and F.W. Hutchinson, "Radiation Corrections for Basic

Constants Used in the Design of All Types of Heating Systems", ASHVE

Transactions, Vol 51, 1945, pp. 213-226.

Rennekmp. S.J., "U-Value Testing of Windows Using a Modified Guarded

Hot Box Technique", ASHRAE Transactions, Vol 85, Pt. 1, 1979.

Low inski, J.F., "Thermal Performance of Wood Windows and Doors", ASHRAE

Transaction, Vol 85, pt. 1, 1979.

McClure, Merle, "Guarded Hot Box Test of Single-and Double-Glazed

Windows", HPAC-ASHVE Journal Section, May, 1942, pp. 313-316.

ASHRAE Handbook, 1985 Fundamentals Volume, American Society of Heating,

Refr igerating, and Air Conditioning Engineers, Inc.

B-26
REFERENCES

BS$74: 1973 » Methods for Determining Thermal Insulation Properties

witjh Definitions of Thermal Insulating Terms, British Standards

Institution, British Standards House, 2 Park Street, London W1A 2BS,

England.

ISO/TC 1 63/SCI /WG1, Thermal Insulation Determination of Thermal

Transmission Properties: Calibrated and Guarded Hot Box, International

Standard Organization.

Rowley, F.B., and A.B. Algren, "Heat Transmission Through Building

Materials", Bulletin of the University of Minnesota, No. 8, Aug., 1932.

Rowley, F.B., and W.A. Eckley, "Surface Coefficients as Affected by

Direction of Wind", ASHVAE Transactions, Vol 38, 1932, pp. 33-^6 (also

HPAC Journal Section, Vol 3 No. 10, Oct., 1931, pp. 870-874.

Rowley, F.B.,A.B. Algren, and J.L. Blackshaw, "Effects of Air

Velocities on Surface Coefficients", ASHVAE Transactions, Vol. 36 , 1930, pp. 123-136.

Rowley, F.B., A.B. Algren, J.L. Blackshaw, "Surface Conductances as Affected by Air Velocity, Temperature and Character of Surface", ASHVE

Transactions, Vol. 36 , 1930, pp. 429-446 (also HPAC Journal Section,

Vol. 2, No. 6, June, 1930, pp. 501-508.

Parmelee, G^V., and R.G. Huebsciner, "Forced Convection Heat Transfer

from Flat Surfaces: Part I - Smooth Surfaces", ASHVE Transactions,

Vol. 53, 1947, pp. 245-284.

Parmelee, G.V. and W.W. Aubele, "Overall Coefficients for Flat Glass

Determihed Under Natural Weather Conditions", ASHVE Transactions, Vol.

55, 1949, pp. 39-60.

Parmelee, G.V. and W.W. Aubele, "Heat Flow Through Unshaded Glass:

Design Data for Use in Load Calculations", ASHVE Transactions, Vol 56,

1950, pp. 371-398.

Parmelee, G.V., "Heat Transmission Through Glass: Part I-Overall

Coefficients of Heat Transmission of Windows and Glass Block Panels"

ASHVE Research Bulletin, Vol. 53, No. 1, July, 1947, pp. 5-88.

Parmelee, G~V., "Heat Transmission Through Glass: Part II - Solar Heat

Transmission by Windows and Glass Panels", ASHVE Research Bulletin, Vol

53, No. 1, July, 1947, pp. 89-158.

Houghton, F.C. and P. McDermott, "Wind Velocity Radients Near a- Surface

and Their Effect on Film Conductance", ASVHE Transactions, vol. 37,

1931 , pp. 201-322.

:

B-27
N BS«1 14A (REV. 2«8C) U.s.S DEPT. OF COMM.

BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date

SHEET (See instructions)

NBSIR 87-3569 MAY 1987

4. TITLE AND SUBTITLE

Interim Procedure to Measure the Thermal Performance

of Window Systems

5. AUTHOR(S)

M. E. McCabe and W. A. Goss

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/ Grant No. DE-A179-83BP13452

S. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

U.S. Department of Energy

Bonneville Power Administration

P.O. Box 3621 - EMRI

Portland, OR 97208

10. SUPPLEMENTARY NOTES

| j Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey , mention it here)

The purpose of this report is to review current and historical sources of information

on U-values for windows and window systems and to describe the current state of thermal

test methods used for windows in order to provide the Bonneville Power Administration

with some general guidelines in applying thermal test data in Model Conservation

Standards

A review of heat transfer theory and sources of U-value data for windows are presented

along with descriptions of laboratory and field test methods from the technical

literature. Draft test standards for window systems currently being developed by ASTM

are presented, and current research and planned future research efforts to provide the

technical information deemed necessary for approval of the standards are described.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) calibrated hot box; fenestration; guarded hot box, heat transfer; movable

insulation; U-value; window system

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

| X 1 Unlimited

j 1 For Official Distribution. Do Not Release to NTIS

H Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

128

IS. Price

;JC| Order From National Technical Information Service (NTIS), Springfield, VA. 22161 $18.95

U S COMM'OC 8O43-P80