USING HIGH-RESOLUTION HAND-HELD RADIOMETERS

TO MEASURE IN-SITU THERMAL RESISTANCE

DOUGLAS M. BURCH AND DONALD F. KRINTZ

BUILDING PHYSICS DIVISION , CENTER FOR

BUILDING TECHNOLOGY, NBS

NATIONAL BUREAU OF STANDARDS INTERAGENCY

REPORT 83-2764

THIS WAS THE ONLY COPY AVAILABLE AT TIME OF

BINDING

24

Using high-resolution hand-held radiometers to measure in-situ thermal resistance

Douglas M. Burch and Donald F. Krintz

Building Physics Division, Center for Building Technology,

National Bureau of Standards, Washington, D.C. 20234

Abstract

A field study was carried out to investigate the accuracy of using high-resolution

radiometers to determine the in situ thermal resistance of building components having conventional residential construction. Two different types of radiometers were used to determine the thermal resistances of the walls of six test buildings located at the National

Bureau of Standards. These radiometer thermal resistance measurements were compared to

reference thermal resistance values determined from steady-state series resistance predictions, time-averaged heat -f low-sensor measurements, and guarded-hot-box measurements.

When measurements were carried out 5 hours after sunset when the outdoor temperature

was relatively steady and the heating plant was operated in a typical cyclic fashion, the

following results were obtained: for lightweight wood-frame cavity walls, the radiometer

procedures were found to distinguish wall thermal resistance 4.4 h*ft^*°F/Btu (0.77 m^K/W)

systematically higher than corresponding reference values. Such a discrimination will permit insulated and uninsulated walls to be distinguished. However, in the case of walls

having large heat capacity (e.g., masonry and log), thermal storage effects produced large

time lags between the outdoor diurnal temperature variation and the heat-flow response at

the inside surface. This phenomenon caused radiometer thermal resistances to deviate

substantially from corresponding reference values.

This study recommends that the ANSI/ASHRAE Standard 101-1981 be modified requiring the

heating plant to be operated in a typical cyclic fashion instead of being turned off prior

to and during radiometer measurements.

Introduction

Radiometers are currently used by building energy auditors to assess the thermal

resistance of building components. When used for such a purpose, the heat-flow rate at the

inside surface of a building component is measured radiometrically , and the in situ thermal

resistance is determined by dividing the temperature difference across the component by the

measured in situ heat-flow rate. Such a measurement procedure is attractive because of its

simplicity and the speed at which the measurement can be carried out. Furthermore, the

method is non-destructive and does not require sensors to be mounted onto the subject

surface. However, the procedure has been criticized as being inaccurate.

The National Bureau of Standards previously carried out a laboratory and field study [1]

to assess the accuracy of such a procedure using low-resolution (i.e., ± 0.5°F or ± 0.3°C)

radiation thermometers . That study found the procedure be inaccurate because the radiation

thermometers did not possess sufficient accuracy to resolve adequately the temperature

difference between a subject surface and the ambient air in contact with it and because

transient heat flows due to diurnal temperature variations and heating plant operation

interfered with the measurements. The surface-to-air temperature difference is usually

quite small, particularly for insulated building components. The inability to resolve

this temperature difference can largely be overcome by using a high-resolution (± 0.1°F

or ± 0.05°C) radiation thermometer.

This report presents the results of a field study to investigate the accuracy of highresolution radiation thermometers and radiant flux radiometers to determine the in situ

thermal resistance of a building component. Using two commercially-avai lable high-resolution

radiometers, in situ thermal resistance measurements were carried out on the walls of six

test buildings located at the National Bureau of Standards in Gaithersburg, Maryland. These

measured in situ thermal resistance values were compared to corresponding reference thermal

resistance values determined by steady-state thermal resistance predictions, time-averaged

heat-flow-sensor measurements, and guarded-hot-box measurements. This study was sponsored

by the Navy Civil Engineering Laboratory located at Port Hueneme, California.

Radiation thermometers are referred to as spot radiometers in the building industry.

24

De scription of test buildings

The six test buildings were 20 ft (6.1 m) wide and 20 ft (b.l m) long one-room buildings

with a 7 . 5 ft (2.3 m) high ceiling. A photograph of one of the test buildings is given in

Figure 1. These buildings had the same floor plan and orientation. They were identical,

Figure 1. Photograph of one of the test buildings

except for the wall construction, which was as follows: No. 1. Insulated lightweight wood

: ame; No. 2. Uninsulated lightweight wood frame; No. 3. Insulated masonry (outside mass);

No. 4. Uninsulated masonry; No. 5. Log; and No. 6. Insulated masonry (inside mass). The

interior surfaces of the test buildings were painted with an off-white latex paint. Two

double-hung, insulating glass (double pane) windows fitted with storm windows were located

on the south-facing walls and two on the north-facing walls. Each test building had a 19.5 ft 2 (1.8 m

2

) hollow metal door insulated with perlite on the east wall. Two inch

(5.1 cm) thick polystyrene insulation was installed over the s lab-on -grade floors. Each

test building was equipped with a centrally located 4.1 kW electric forced air heating

plant equipped with a 13,000 Btu/h (3,800 W) split vapor-compression conventional

residential air conditioning system.

A detailed description of the walls of the test buildings is given in Table 1. The

reference thermal resistance values for the walls of the test buildings, determined by

three independent methods, are summarized in Table 2. The steady-state thermal resistance values given in Table 2 were calculated using the

series-resistance method as outlined in reference [2]. These predicted steady-state values

apply to the portion of the wall between structural members, since this is the region where

radiometer thermal resistance measurements were carried out.

The heat-flow-sensor thermal resistance values given in Table 2 were determined as follows: a heat-flow sensor, consisting of 2x2 in (5*5 cm) wide and 1/8 in (0.32 cm) thick

wafer containing an emDedded thermopile, was spot glued at the center inside wall surface

(between structural members). Tne heat-flow sensor produced a D.C. signal proportional to

the instantaneous heat-flow rate passing through the device. Tne heat-flow sensors were calibrated oy exposing them to a known uniform heat flux in a thermal conductivity measuring apparatus. Inside and outside surface thermocouples were installed in the immediate

vicinity of the neat-flow sensor. The heat-flow rate (q), inside surface temperature (T 1S ), and outside surface temperature (T os ) were recorded at hourly intervals over a 5-day period

using a data acquisition system. The overall thermal resistance (R^) was determined from

the following running average relation:

N

l ( T j

_T os )

j

R , II 4- 3=1 4- R

N ( 1 ) i N o

I q-j

1 = 1 where is a i r film thermal resistance at inside surface, 0.68 h*ft 2 *°F/Btu (0.12 m

2

• K/W

and

Ro

15 air film thermal resistance at outside surface, 0.4 h*ft^*°F/Btu (0.07 m

^

• K/W

25

Table 1

.

Construction Deta il s of Wall

s

—r~fn~s~uTaTe d~ Lightweight Wood Frame

‘ ~ oT^HT. gypsum board

0 . 002 -in. polyethylene film

2x4 studs placed 16 in. o.c. with R-ll glass-fiber insulation installed between the studs

b/O-in. exterior plywood

Nn. 2 Uninsulate d Lightweight Wood Frame

TsanTe as No. 1, except no insulation)

No. 3 Insulated Masonry (Outside Mass)

oTT-ffT. gypsum board

0 . 002 -in. polyethylene film

2

-

in. -thick extruded polystyrene insulation placed between 1-1/2-in. -wide wood furring

strips placed 24 in. o.c.

1/4-in. air space

4-in. * 2-core, hollow concrete block (105 lb/ft 3

) 4-in. face brick

No . 4 Uninsulated Masonry

0 . 5- i n . gypsum board

0.002-in. polyethylene film

3/4-in. air space created by 2 x 3/4 in. furring strips placed 16 in. o.c.

8-in., 2-core, hollow concrete block (105 lb/ft 3

) No. 5 Log

7-

in. square lodge-pole-pine logs (butt jointed and caulked at the front and rear joints)

No. 6 Insulated Masonry (Inside Hass ) 0.5-in. plaster

8-

in., 2-core, hollow concrete block (105 lb/ft 3 )

3-

1/2-in. perlite insulation in the space between block and face brick

4-

in. face brick

Table 2. Thermal Resistances for Walls of

Thermal Resistance*, h*ft

Test Buildings

2 *°F/Btu

Building

* bteady-State

Predictions

Heat -Flow- Sensor

Measurements

Guarded -Hot -Box

Measurements

1 13.2 12.3 12.2

2 3.2 3.3 3.6

i 14 .

1

14.8 13.7

4 3.6 4 . 1 4.6

5 9 .

8

9.8 10.3

6 13.3 12.1 12.4

* These thermal resistance values include air films with thermal

resistances of 0.68 h*ft^*°F/Btu (0.12 m^-K/W) at inside surface and

0.4 h*ft2.°F/Btu (0.070 m^-K/W) at outside surface.

** Values apply to the center portion of the wall between structural

members

.

The index j is the hourly time index. Thermal resistances determined using this procedure

for the north wall of building no. 5 are plotted in Figure 2.

26

oI

M£

ulo

M

Figure 2. Thermal resistances obtained from heat -f low-sensor measurements for no. 5

The yuarded-hot-box thermal resistance values given in Table 2 were determined as follows

Six 6x6 ft (1.8x1. 8 m) wall specimens having the same construction as the walls of the test

buildings were sent to an independent testing laboratory where they were sandwiched between

a hot box maintained at approximately 70°F (21°C) and a cold box maintained at approximately

35°F (1.7°C) and permitted to reach thermal eguilibrium. The resulting heat-transfer rate

(q) through a 48*32 in (1.2xU.81 m) metering section was measured. The overall steady-state

thermal resistance (R) of each test wall was determined from the relation:

R T - T

R + -is as. + R

1 q °

(2)

Here T 1S and Tos are the inside and outside wall surface temperatures, respectively.

Theory and equipment

The apparent thermal resistance (R a ) of an exterior wall is given by the relation:

(3)

where T 1 is instantaneous indoor air temperature, °F (°C) T0

is instantaneous outdoor air temperature, °F (°C)

q is instantaneous heat flow-rate at the inside surface, Btu/h*ft 2 (W/m 2 ). Under steady-state conditions, the apparent thermal resistance is equal to the steadystate thermal resistance (R). Two different radiometer procedures for determining the in

situ thermal resistance are described below. Both procedures determine the heat-flow

rate (q) at the inside wall surface, and compute thermal resistance (R) from Equation 3. Procedure using a radiation thermomet er

In this procedure, the heat-flow rate (q) is determined from the relation:

q

= h •(T i -r is ) ( 4 ) where h is surface heat-transfer coefficient at the inside wall, Btu/h*ft 2 *°F (W/m 2 *K);

and

T^ s is inside wall surface temperature, °F (°C).

The surface heat-transfer coefficient (h) is estimated from the relation:

h = 4 »E • o *T 3 + 0.19. (T

i -T is ) 0 * 33

(5)

27

where o is Stef an-Boltzmann constant, 0.1714*10* 8 Btu/h*ft 2 *R 4 (5.67*1U -8 W/m 2 *K 4 );

T s is average temperature of surrounding surfaces, R(K); and

E is emissivity factor (see Equation 6).

The first and second terms are the radiative and convective components of the surface

heat-transfer coefficient, respectively. For the present report, the average temperature

of surrounding surfaces (T s ) was taken to be equal to the inside wall surface temperature

(Tis ) •

The emissivity factor (E) is estimated from the relation:

1/es * ^/ e w

~ 1

( 6 )

Here e s and e w are the surface emissivities for the subject and other room enclosure

surfaces, respectively. Taking the emissivities e s and e w to be 0.9, the emissivity

factor (E) is 0.82.

The inside wall surface temperature (T^g) was measured by pointing the radiation

thermometer shown in Figure 3A at the wall surface and depressing the on-off trigger. The

device senses the radiance exiting the surface- and is calibrated to read out the apparent

radiance temperature on a digital scale to within a resolution of ± 0.1°F (± 0.05°C). The

indoor air temperature was estimated by using the radiation thermometer to determine the

radiance temperature of a polystyrene board painted with opt ical ly-black paint (

s > 0.98)

placed in front of the subject surface. flanuf acturers generally recommend using an object

having low heat capacity and high surface emissivity placed in front of the subject surface

to determine the inside air temperature.

Procedure using a radiant-flux radiometer

In this procedure, the heat-flow rate (q) is estimated from the manufacturer's equation:

q = ( 1 + f c

’

I • ( J i J w where f c is 0.29, f J

i is incomi

J wis outward

The incoming radi

at a po]v c tyrene boa

face . . . e polysty

incoming radiant flu

at the wall surface,

is computed internal

»actor for the effect of

g radiant flux, Btu/h*f

radiant flux, Btu/h‘ft

ant flux (J^) is sensed

rd placed in the center

ene board is painted wi

x ( J i ) is stored in the

and the outward radian

ly utilizing a computer

(7)

convection (developed by manufacturer);

t

2

( W/m 2

) ; and

2

( W/m 2

)

by pointing the radiometer shown in Figure 3B

of the room and facing the subject wall surth optically-black paint (

g

> 0.98). The

device. The radiometer is subsequently pointed

t flux (

w ) is sensed. The heat-flow rate (q)

within the device.

A. Radiation thermometer

B. Radiant-flux radiometer

Figure 3. Photograph of radiometers

Here it is assumed that the thermal radiation exiting the polystyrene board will

essentially equal to the incoming radiation to the subject surface from surrounding

surfaces, since T s

= T^ = T^oard’

28
* P fl Br !
Me asurement s with heating plan t turned oft

The ANSI/ASHRAE Standard 1U1-1981 entitled "Application of Infrared Sensing Devices to

Assessment of Building Heat Loss Characteristics" [J| sets forth recommended procedures

£ e ca rrying out radiometer thermal resistance measurements. This standard recommends that

!he heatiny plant be turned off at least 1 hour prior to carrying out radiometer measuret nts • For the measurements reported in this section, the heatiny plants of the test

buildinys were turned off 30 to 60 minutes prior to the radiometer measurements.

E xperimental Procedure

Within the six test buildings, radiometer thermal resistance measurements were performed

at the center of the north and south walls (between structural members) using the two radiometer procedures previously described. For all radiometer measurements, three instrument

readings were taken, and the average of the three readinys was reported. The measurements

were commenced at about 11:30 p.m. (about 5 hours after sunset) on March 22, 1982. During

the measurements, the outdoor temperature varied from 39 (3.9) to 29°F (-1.7°C), and the

outdoor wind speed was less than 1 mph (1.6 m/s). Prior to turning off the heatiny plants,

the indoor temperatures of the test buildings were maintained at approximately 68°F (20°C).

Results

Radiometer thermal resistance values are compared with corresponding reference values

obtained from time-averayed heat-flow-sensor measurements in Table 3. The heat-flow-sensor

values were used as reference values because they were measured in situ at approximately

the same locations where the radiometer measurements were carried out. In addition, they

agreed with the corresponding values predicted using the steady-state theory and measured

using the yuarded-hot-box apparatus (see Table 2).

Table 3. Comparison of In Situ Thermal Resistances (Heating Plant Turned Off)

Thermal Resistance, h«ft^«°F/Btu

Building

Heat -Flow

Meter

Rad iat ion

North

Wall

Thermometer

South

Wall

Radiant-Flux

North

Wall

Radiometer

South

Wall

1 12.3 21.9 13 .

10.7 11.4

2 3.3 4.8 4.5 3.3 3.1

3 14 .

61.8 26.1 34.6 14 . 3 4 4 .

9.1 7.7 6.8 7.6

5 9.8 87.5 22.3 23.5 26 .

6 12.1 95. 3 195. 86. 5 -38. 8

The radiometer thermal resistance values are generally higher than the known values.

This trend is more apparent in the heavyweight walls (i.e., nos. 3-6). It is interesting

to note that one of the values for building no. 6 is negative.

It was hypothesized that the procedure of turning off the heating plant produced

transient heat flows in the walls, thereby causing high apparent thermal resistance measurements. To investigate this possibility, inside wall surface and indoor air temperature

thermocouple readinys were analyzed. It was found that after the heating plant was turned

off, the indoor air temperature of the buildings decayed more rapidly than the inside wall

surface temperature due to infiltration and window heat loss. This effect is illustrated

for building no. 6 in Figure 4A. During the period from 4:00 to b:00 a.m., when the

heating plant is operating in a typical cyclic fashion, the inside surface and indoor air

temperature remained approximately steady, and more important the difference between tnese

temperatures remained approximate ly constant. After the heating plant was turned off, both

the inside surface and indoor air temperatures decreased. However, the indoor air temperature decreased more rapidly than the inside wall surface temperature. The difference

between these temperatures approached zero and subsequently became negative. Since the

heat-loss rate at the inside wall surface is proportional to this temperature difference,

these results indicate that after the heatiny plant is turned off, the heat-loss rate at

the inside wall surface approaches zero and subsequently becomes negative.

Substituting the measured thermocouple measurements given in Figure 4A in Equations 3,

4, and 5, apparent thermal resistance values were computed as a function of time for building no. 6 (see Figure 4B). From 4:00 to 5:00 a.m., the apparent thermal resistance is seen

to remain approximately steady. After the heating plant is turned off, the apparent thermal

resistance increases rapidly, passes through a discontinuity, and subsequently becomes

negative. Similar results were obtained for the other test buildings.

From these measurements, it was concluded that the heating plants of buildings should

not be turned off prior to carrying out radiometer thermal resistance measurements. This

finding is inconsistent with the ANSI/ASHRAE Standard 101-1981 which recommends that heating

29

F igure 4

B. Variation in apparent thermal

resistance

Illustration of the effect of turning off heating plant on apparent

thermal resistance

plants be turned off prior to radiometer measurements. It is recommended that the ANSI,/

ASriRAH Standard 101-1981 be modified to reflect a change in this measurement procedure.

In situ heat-flow rates (q) determined with both types of radiometers were generally

found to agree with corresponding coincident values measured with heat-flow sensors. Measurements w ith heating pl ant o p era ted in typi ca l cy clic fashion

For the measurements reported in this section, the heating plants of the test buildings

were not turned oft prior to carrying out radiometer measurements. The heating plants

instead were controlled by their thermostats and operated in a typical cyclic fashion.

E xperimental procedure

Within the six test buildings, radiometer thermal resistance measurements were performed

at the center (between structural members) of the north wall using only the radiation

thermometer. Three separate radiometer readings were taken and averaged for each of the

individual measurements reported. The measurements were commenced at about 11:30 p.m.

(about 5 hours after sunset) on November 10, 1982. During the measurements, the outdoor

temperature varied from 45 (7.2) to 42°F (5.6°C), and the outdoor wind speed was less than

2 mph (3.2 m/s). Outdoor temperatures which occurred prior to radiometer measurements are plotted in Figure 5. Approximately 1 week prior to the measurements, the thermostats of

the test buildings were adjusted to maintain an indoor temperature of about 82°F (28°C),

in order to provide a large inside-to-outside temperature difference.

Results

Radiometer thermal resistance values are compared with corresponding reference values

obtained from time-averaged heat-flow-sensor measurements in Table 4. Five individual measurements were carried out at randomly different times with respect

to the cyclic operation of the heating plant. The mean values given in Table 4 are the

arithmetic averages for the five individual measurements. Tne means for the sets or five

radiometer measurements are consistently higher than corresponding reference values. The

systematic errors, the difference between the means and reference values, are less than

4.4 n*ft 2 *F/Btu (0.77 m^K/W) for the lightweight wood-frame walls (nos. 1 and 2). Such a discrimination will permit the presence or absence of cavity insulation (typically R-ll)

to be distinguished. However, the systematic errors are considerably larger for the walls

having large heat capacity (nos. 3-6).

30

TIME OF DAY, HOURS

Figure 5. Outdoor temperature variation prior to radiometer measurements

Table 4. Comparison of In Situ Thermal Resistances (Heating Plant Operated in

Typical Cyclic Fashion)

Thermal Resistance, h-ft^« °F/Btu

Building

Radiometer Measurements

Reference*

Value

Systemat ic

Individual Measurements Mean Error

Sample Estimate

of Standard

Deviation

1 15 . 2 , 18.0, 13.2, 18.7, 18.3 16.7 2.4 12.3 4 .

2 4.4, 4.7, 4.9, 4.6, 4.6 4.6 0.18 3.3 1.3

3 21.8, 23.7, 22.4, 25.9, 26.3 24.0 2.0 14 .

9.2

4 6.5, 7.5, 8.1, 7.8, 8.2 7.6 0.68 4.1 3.5

5 22.2, 23.9, 23.7, 22.2, 21.3 22.7 1.1 9.8 12.9

6 43.6, 30.6, 24.9, 23.1, 32.5 30.9 8.1 12.1 18.8

* Obtained from time-averaged heat flow-sensor measurements (see equation 1) ** Systematic error = mean - reference value

In analyzing the systematic errors, it was found that they appeared to depend upon the

"lag times" for the walls. Here "lag time" refers to the elapsed time between the minimum

outdoor temperature and the maximum wall heat-loss rate. Lag times for the walls of the

six test buildings were determined by analyzing hourly heat-flow-sensor and outdoor temperature measurements over a diurnal period. The systematic errors given in Table 4 are plotted as a function of wall lag times in Figure 6. This plot indicates that a relationship exists between the systematic error and the lag time. The systematic error increases

as the lag time increases. As the lag time approaches zero, the systematic error approaches zero.

Figure 6. Systematic errors plotted as a function of lag

31

The diurnal variation in apparent thermal resistance is of considerable interest

because such information would provide insight on the effect of thermal storage characteristics on the apparent thermal resistance. Hourly radiometer measurements over a 24-hour

period were not available. However, hourly thermocouple temperatures for the inside wall

surface, indoor air, and outdoor air were recorded for similar test days. Substituting a diurnal set of hourly thermocouple readings into equations 3, 4, and 5, apparent thermal

resistance values were derived for the north wall of building nos. 1 and 5 (see Figure 7).

From Figure 7, it is seen that the apparent thermal resistance for the lightweight woodframe wall (no. 1) approaches tne reference steady-state thermal resistance after 11:00 p.m.

However, the apparent thermal resistance for the log wall (no. 5) is considerably greater

than steady-state thermal resistance during the night period. This is because the night

heat-flow rate for the log wall depends to a greater extent upon previously warm day outdoor temperatures. The log wall never reaches a quasi-steady condition because of its

large heat capacity.

A. Outdoor temperature.

BUILDING 1

TIME OF DAY. HOURS

B. Apparent thermal resistance for

building no . 1

BUILDING 5

TNE OF DAY. HOURS

C. Apparent thermal resistance for

building no. 5. Figure 7. Illustration of the variation in apparent thermal resistance during a diurnal period

In Table 4, the sample estimate of standard deviation ( s

) * provides an estimate for the variability of the individual measurements comprising each data set. Ninety-five percent

of the individual measurements are estimated to lie within the range (mean ± 2s). A major

where y mean value; y^ individual measurement; and N = number in set.

32