Aiiloo ^^3lt,^

NATL INST OF STANDARDS & TECH R.I.C.

A11 1009931 64

TIetsma, G. J/The thermal performance of TA435 .U58 V102;1978 C.1 NBS-PUB-C 1977

A

NBS BUILDING SCIENCE SERIES 102

The Thermal Performance of

:r ^wo-Bedroom Mobile Home

35

0 102

978

2

ARTMENT OF COMMERCE • NATIONAL BUREAU OF STANDARDS
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards^ was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and

technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to pro- mote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology, the Institute for Computer Sciences and Technology, the Office for Information Programs, and the

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THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United States of a complete and consist- ent system of physical measurement; coordinates that system with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry,

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THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measurement, standards, and data on the properties of well-characterized materials needed by industry, commerce, educational insti- tutions, and Government; provides advisory and research services to other Government agencies; and develops, produces, and

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" Located at Boulder, Colorado 80302.
MAR 2 ? 1978 NBS BUILDING SCIENCE SERIES 102

'^"^ The Thermal Performance of

a Two-Bedroom Mobile Home

G. J, Tietsma

Dow Chemical Co.

Midland, Michigan 48640

B. A. Peavy

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

Sponsored by the Federal Energy Adnninistration 1200 Pennsylvania Avenue, N.W.

Washington, D.C. 20461

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued February 1978
Library of Congress Cataloging in Publication Data

Tietsma, G. J. The thermal performance of a two-bedroom mobile home. (NBS building science series ; 102) Bibliography: p. Supt. of Docs, no.: €13.29/2:102

1. Mobile homes—Thermal properties. I. Peavy, Bradley A., joint author. II. United States. Federal Energy Administration. III. Title. IV. Series: United States. National Bureau of Standards. Building

science series ; 102. TA435.U58 no. 102 [TH4819.M6] 690'.021s [629.22'6] 77-608167

National Bureau of Standards Building Science Series 102

Nat. Bur. Stand. (U.S.), BIdg. Sci. Ser. 102, 59 pages (Feb. 1978)

CODEN: BSSNBV

Supersedes NBSIR 76-1182

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1978

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

Price $2.30 (Order by Sd Catalog No. C13.29/2; 102). Stock No. 003-003-01863-5

(Add 25 percent additional for other than U.S. mailing).
CONTENTS

Page CONVERSION FACTORS TO METRIC (SI) UNITS iv ABSTRACT v

1. INTRODUCTION 1

2. DESCRIPTION OF TEST SPECIMEN 3

3. INSTRUMENTATION AND TRANSDUCERS 11

4. PROCEDURE, RESULTS AND DISCUSSION 13

4.1 Thermal Testing Under Steady-State Conditions 13

4.2 Thermal Performance by Thermographic Technique 20

4.3 Air Leakage Analysis 30

4.4 Condensation Potential 37

5. CONCLUSIONS AND RECOMMENDATIONS 45

5.1 Conclusions 45

5.2 Recommendations 46

6. REFERENCES 47

APPENDIX A. Calculations for Heat Transfer 49

APPENDIX B. Rationale for Thermal Resistance of Interior Air Films 53

iii
CONVERSION FACTORS TO METRIC (SI) UNITS

Quantity To convert from To Multiply by

Length inch m (meter) 2.540x10-2

foot m 3.048x10-1

mile m 1.609x103

Area in^ m^ 6.452x10-4

ft2 m2 9.290x10 2 Volume in^ m^ 1.639x10 5

ft3 m^ 2.832x10 2 gallon m^ 3.785x10 3 Temperature °F °C to^ = (toF-32)/1.8 T. difference Atop K ATK = AtoF/1.8

Mass pound kg 4.536x10-1 ounce kg 2.835x10-2

Pressure psi Pa 6.895x103

in H2O Pa 2.488x102

in Hg Pa 3.386x103

mmHg Pa 1.333x102

Energy Btu J 1.055x103

MBtu J 1.055x10"

kWh J 3.600 xlO^

ft-lbf J 1.356x100

kilocalorie J. 4.184x103

Power Btu/h W 2.931x10-1

hp W 7.457x102

Flow gal/min m% 6.309x10-5

ft'^/min m^/s "4. / i7 X iU

Density lb/ft3 kg/m^ 1.602x101

lb/gal kg/m^ 1.198x102

Heat Capacity Btu/(lb • °F) J/(kg-K) 4.187x103

Btu/(ft3-°F) J/(m3-K) 6.707x10^

iv
ABSTRACT

Tests were conducted on a mobile home located in an Environmental Climatic Laboratory for the purpose of evaluating its thermal performance. The heating

demand greatly affected the part-load efficiency of a gas-fired, forced-air, sealed-combustion furnace

system. The practice of installing oversized heating

plants was shown to result in low seasonal operating

efficiencies. Air leakage measurements were

performed using a pressurization technique to quantify the amount of air leakage through the various parts of the mobile home. Separate air infiltration tests using the SF^ tracer-gas technique

showed that somewhat higher air infiltration rates were induced by operation of the mobile home

heating plant. A thermographic survey of interior surfaces showed that the technique used to install the

wall insulation may allow wrinkles formed in the surface of the insulation to form air paths running the height of the wall cavity. Convective air flow through

these paths may create heat leaks on the building

surface which can have an impact on the overall heat-loss rate. Separate tests were also conducted to identify places in the mobile home envelope having

high condensation potential.

Key words: Air infiltration; energy conservation; mobile

home; part-load efficiency; thermography.

1. INTRODUCTION

Rising energy prices and the necessity for conserving our nation's energy resources should provide a significant incentive to increase the effectiveness of energy utilization. For residential and commercial

buildings this can be done by providing more

efficient heating, ventilation, and cooling equipment,

by more efficient operation and sustained

maintenance of buildings and equipment, and by a building envelope with increased resistance to heat

loss or gain. As in other types of buildings, energy

conserving efforts in the mobile home industry and

on existing mobile homes would result in a significant

impact on energy savings. Currently, mobile homes

represent more than 30 percent of new single-family

detached housing, and this percentage is expected to increase over the next few years.

In order to accomplish savings in this significant

segment of building energy consumption, it is necessary to provide the mobile home industry and

present occupants of mobile homes with techniques

and methods for predicting and evaluating energy

utilization. As an effort to upgrade the standards with

regard to energy conservation, the National

Conference of States for Building Codes and

Standards (NCSBCS) asked that the National Fire Protection Association Standard (NFPA) 501B (ANSI

A119.1) Committee develop energy conservation

requirements for mobile homes. NFPA formed an

1
Energy Conservation Task Force to add such

requirements to the mobile home standard and to a proposed new mobile home installation standard. As

part of his initiative in the energy area, the President

has directed the Secretary of the Department of Housing and Urban Development (HUD) to work in conjunction with the Federal Energy Administration

(FEA) and the Department of Commerce (DoC) to ensure that energy conservation standards are included in the mobile home construction and safety standards.

The present report summarizes the findings of an

experimental study conducted at the National Bureau

of Standards (NBS) which was sponsored by FEA. The

objective of this study was to observe measured vs. predicted thermal performance in order to aid in the establishment of energy conservation design

standards with consideration to cost, size, weight,

and performance of mobile homes. The test plan

included the instrumentation and measurement of the thermal performance of a new furnished mobile

home, built in accordance with the Mobile Home

Standard, ANSI A119.1, 1974. The mobile home was

installed in the large NBS Environmental Laboratory,

where it was exposed to a variety of climatic conditions.

Because the final location of a mobile home may not be

known at the time of manufacture and because of a number of variables such as altitude and heating

value of the fuel that may affect the furnace output

capacity, it is a common practice in the mobile home

industry to install oversized heating plants in mobile

homes. In most instances, the heating output capacity

of the heating plant is much larger than the design

heating load for the home in its final location. This

study investigates the effect of heating demand on the

part-load efficiency of a gas-fired forced-air heating

plant.

Heat loss through air infiltration may represent

one-third to one-half the heating load for a mobile

home. Measures taken to reduce air leakage will unquestionably have a significant impact on reducing

heating energy requirements. The points where air leakage occurs may not always be obvious and may

sometimes occur in unsuspected places. A recent air leakage study performed on residential buildings [1]* showed that significant amounts of leakage occurred

through solid waOs and ceilings. These parts were

previously thought to be essentially impervious to air leakage. Similar tests were also conducted on the mobile home of the present study to quantify the amount of air leakage occurring through different parts of the structure. Separate tests were also conducted to determine the effect of the operation of the heating plant on inducing additional air infiltration.

When mobile homes are constructed, the technique

for installation of insulation in the walls may cause the

insulation to wrinkle, creating air paths running the

full height of the wall cavity. In this instance,

significant air flow can be induced due to the thermo-syphon effect which may decrease the

insulation effectiveness. In the present study, heat

leaks due to this phenomenon were investigated.

The condensation of moisture on exposed surfaces and within structural elements may eventually cause

unsightly staining and even wood rotting which may

significantly shorten the life of a mobile home. As part

of the present study, observations were made to identify those parts of the mobile home that have a high condensation potential.

* Figures in brackets indicate the literature references at the end of this paper.

2
2. DESCRIPTION OF TEST

SPECIMEN

The test specimen was a new, factory-produced

two-bedroom mobile home of nominal dimensions of 50 ft by 12 ft with a 7-ft interior height. Nominal

dimensions include a 4-ft hitch, making the actual length 46 ft.

Figures 1 through 12 show construction phases for the mobile home, beginning with the chassis (fig. 1). Floor construction included R-4 insulation over Vi6-in asphalt-impregnated underlayment; 2 x 6-in floor

joists and heating duct running the entire length.

completed with carpet and vinyl floor covering. The

next step was installation of interior partitions and

bathroom and kitchen. Exterior wall sections were

fabricated with R-7 insulation friction-fitted between

2 X 4-in framing, and placed on periphery of floor; the

roof section using R-14 insulation between framing

members and over a 6-mil polyethylene film and a ^A-in fiberboard ceiling material was then lowered

over the walls, and the factory construction was

completed by the installation of corrugated aluminum

siding on exterior walls, and the sheet metal roof.

The mobile home was manufactured in Ephrata,

Pennsylvania, and transported by tractor to NBS (fig. 13). After it was placed in the NBS highbay

3
II
environmental laboratory of approximately 70,000 ft^ in volume, the utilities — electricity, water, natural gas and drainage — were connected. Supports were

placed at various location under the chassis to take weight oft of the tires, for leveling and to reduce

vibration. Also, sheet metal skirting was installed at the outer periphery between the bottom of the mobile

home and the ground, as shown in figure 14. The

home was factory-furnished with refrigerator, stove, two beds, dining-room table and four chairs, couch

and divan, and curtains and draperies at windows.

The warm-air, gas-fired, sealed-combustion furnace was rated at 56,500 Btu/h input and 45,200 Btu/h

output at the bonnet. Air for combustion was taken

from the outdoors at the roof through an outer annulus surrounding the flue pipe and into the combustion chamber. Combustion products were

Figure 3. Installation of 2 x 6 floor joists.

discharged to the outside through the inner annulus

or flue. Air circulation within the mobile home was by

a furnace fan forcing air down through the heat exchanger of the furnace to the heating duct located below the floor. Figure 15 shows air registers in the mobile home plan view. The thermostat was located

in the hallway between the doors to the bedroom and

furnace room. For special test purposes, the furnace

plenum was used for the installation of electric resistance heating elements. The voltage supplied to the elements was controlled by a variable transformer

for varying the electric heat input in tests where

needed.

For expelling exhaust furnace flue gases, a 12-in diameter flexible hose was installed over the flue-gas stack located on the roof and connected to the laboratory outdoor exhaust system. The flexible hose

installation is shown in figure 14.

Figure 4. Longitudinal floor joists with heating ducts

in place.

5
Figure 5. Mobile home floor coverings in place.

Figure 6. Installation of interior partitions.

6
Figure 8. Installed wall sections.

7
Figure 9. Fabrication of roof section.

Figure 10. Roof section being lowered onto walls.

8
Figure 11. Installation of aluminum siding.
Figure 13. Mobile home delivered to NBS.

10
3. INSTRUMENTATION AND

TRANSDUCERS

The electric energy supplied to the mobile home

electrical distribution system was measured using a calibrated 3-wire watt-hour meter equipped with an

impulse generator — a photoelectric device which

generated pulses from revolutions of the disk inside

the meter. The pulses were registered by a counter and printed out at preset time intervals. The same

system was used to measure electric energy input to the electric resistance heating elements installed in the furnace plenum.

The volume of natural gas consumed by the furnace was measured with a domestic gas meter that was

specially modified, so that the passage of 1 ft^ of gas caused an electric pulse to be generated. This was

accomplished by placing an excitation voltage across a micro-switch that rode on the cam shaft of the counter and printed out at preset time intervals.

Ambient air and surface temperatures were measured

with 24-gage copper-constantan thermocouples.

Thermocouple strings, suspended by ^/i6-in diameter

polycarbonate rods, were placed in each room. The

location of these strings is shown in figure 15. Four thermocouples were attached to each rod and located

11
at 3, 30, 60, and 81 inches above the floor. Outside air temperature in the environmental chamber was

measured by eight thermocouples placed one foot from outside surfaces of v^^alls and roof. Two

thermocouples placed one foot below the chassis of

the mobile home measured the air temperature in the space enclosed by the sheet metal skirting. Other

thermocouples measure temperatures at inside

window surfaces, and temperatures of the supply and

return air to the furnace.

©

o

o

aUJ

CO

CLOSET Q

REAR DOOR

© THERMOCOUPLE

Ea AIR REGISTER

SCALE: 1 ft.

0©

BATHROOM

o

©

cn-n REFRIG.

© g

STOVE

© LIVING ROOM

/N„ FRONT DOOR

HOT WATER - STORM DOOR

HEATER

Figure 15. Plan view of 46 ft long by 12 ft wide mobile home showing location of thermocouple strings.

12
4. PROCEDURE, RESULTS

AND DISCUSSION

In the following sections, procedure, results and

discussion are presented for the various aspects of

testing.

4.1 THERMAL TESTING UNDER

STEADY-STATE CONDITIONS

4.1.1 Instrumentation

This series of tests was conducted in an

environmental chamber having the capability of individual controllable temperature and relative

humidity from -50 to 150 °F and from 15 to 85 percent

RH, respectively. The temperature and relative humidity can be held constant or caused to cycle on a predetermined schedule. The air within the chamber

is circulated at a high-change rate (one air change

every two minutes) in order to control the temperature

within the chamber. Control thermocouples are located in the supply and return air.

Forty-eight 24-gauge copper constantan thermocouples were located throughout the interior and exterior of the mobile home so that temperature

measurements could be used for heat flow

calculations. For location of these thermocouples,

refer to section 3 of this report.

13
Although the gas meter provided the gross energy

input to the gas-fired furnace, it was necessary to measure the efficiency of the furnace in order to determine the energy input to the mobile home. The

continuous operating efficiency was determined

using a COj combustion gas analyzer.

The furnace on-and-off cycles were observed using

clocks and strip-chart recorders connected by relays to the furnace. A separate strip-chart recorder was used

to show furnace blower on-time and furnace burner

(or electrical resistance elements) on-time. These

recorders were used to determine the number of cycles per hour. Two electric clocks were also connected to the strip-chart circuitry so that one clock recorded total on-time of the furnace fan during a test and the other clock recorded total burner on-time.

These clocks were used to provide accurate percent

on-time for the fan and the furnace for each test condition.

4.1.2 Test Methods and Conditions

For this series of tests, the mobile home interior temperatures were held constant and the mobile

home energy consumption was recorded. It was

necessary to hold the test chamber at test conditions

for at least 16 hours prior to each test to allow the mobile home to reach equilibrium. A further check on

equilibriurri conditions was made at the end of each

test by comparing energy consumption for each hour.

Any irregularity of energy demand resulted in test invalidation.

The mobile home was tested under several outside air temperature conditions, with both natural gas and

electric energy inputs and with and without storm windows. The various steady-state tests are denoted

by the x's in the following schedule.

Approximate Chamber

Temperature, °F

-15 0 15 30 45

Electric Fumace

With Storm Windows

Without Storm Windows

X X

X

X

X

X

X

Original gas Fumace

With Storm Windows

Without Storm Windows

X XX XX

X

XX

X

XXX

X

Modified gas Fumace

With Storm Windows

Without Storm Windows

X X X X

The average inside air temperature was about 70 °Ffor

all tests. Tests were conducted with the furnace

modified to a lower natural gas input to show the

effect of this variable. Tests with the original gas furnace were repeated to show reproducibility of data. One test was made at -15 °F in order to extend

trends established. The chamber temperatures are described as being approximate since the chamber

controls were set at the value shown but equilibrium was often established with a ±2 °F deviation from the

set point. Actual temperatures were used for all calculations.

After reaching equilibrium, each steady-state test was

continued for a period of seven hours. A complete set of data was recorded every 30 minutes for the duration

of the test.

One series of tests was run with the furnace modified

to a lower input capacity. The original equipment gas furnace was rated at a 56,500 Btu/h input and a bonnet

capacity of 45,200 Btu/h. On special request, the manufacturer provided a replacement burner orifice

to reduce the input capacity to 30,000 Btu/h or a bonnet

capacity of 24,000 Btu/h. In order to keep the furnace

efficiency nearly the same as the original furnace, the manufacturer also provided a small plate which was

used to partially block the burner chamber draft outlet opening. After installing the new burner orifice and

plate, the continuous running efficiency was found to be 75 percent (365 °F and 4.5% CO2) as compared to 72 percent (517 °F and 4.5% CO2) as determined for the

original equipment. Ibr sealed combustion furnaces

the air for combustion is heated by the flue jacket,

whereby the flue jacket losses for the computation of efficiency are ignored. The efficiency is then

computed on the assumption of complete combustion

of the fuel and the temperature and carbon dioxide

content of the combustion products in the flue.

The method used to evaluate the effect of reducing the furnace capacity oversizing in comparison with the heating load was a comparison of energy

consumption with the two different furnace

capacities. The comparative comfort derived from the two furnace sizes was evaluated by determining air temperature gradient from floor to ceiling under the

different test conditions.

4.1.3 Results and Discussion

The actual energy input was observed over 7-hour test periods and averaged to obtain energy input data. The

observed energy readings were adjusted to reflect efficiency losses associated with the combustion

heating systems. The electrical resistance furnace was

14
assumed to operate at 100 percent efficiency and all other electrical inputs were converted directly to heat. The natural gas energy was based on a heat content of 1027 Btu/ft^. This figure was obtained from the local natural gas utility and was reported to be applicable

for the period of time when the tests were run. The

efficiency of the gas furnace was measured using the CO2 gas analyzer previously described. This

efficiency could not be used directly, however,

because of the indeterminate stack losses occurring

during the off-cycle. In order to estimate the efficiency

of the gas furnace under cyclical running conditions,

the energy input to the combustion furnace was

compared to the input to electrical resistance heating

under the same test conditions. The relationship

between the measured energy demands in Btu/h as a function of the inside-to-outside air temperature

difference, AT, for the electric heating tests was found

to be:

for mobile home with storm windows

Qe = 471.3 + 149.2 AT + 1.076 AT^

for mobile home without storm windows

Qe = 120.35 + 177.1 AT + 1.081 AT^

where Qe = total heating load satisfied by

combination of electric resistance heating elements,

lights, and furnace fan.

From tests with the gas-fired furnace, the total rate of gas energy consumption, Qg, rate of electrical energy

consumption from lights and furnace fan, Q„, and the temperature difference as measured, the efficiency, e,

is calculated from the expression:

where Qg is determined from the relationships given

above. These apparent efficiencies as a function of inside-to-outside temperature differences are shown

in figure 16. These efficiencies were used to calculate net energy input to the mobile home for each test condition and compared with the predicted values.

The measured energy input to the mobile home for each of the test conditions is shown in table 1.

For comparison purposes, the predicted heat loss was

calculated for each of the tests conditions. These

calculations as shown in Appendix A were made

using the formula, U factors, and areas shown on table A-1. The construction of the building elements is

15
TABLE 1. MOBILE HOME ENERGY CONSUMPTION.

Total Energy Supplied

Energy Input, Btu/h Temp.

per Unit Envelope Area^ and per unit

°F Furnace Diff. Temp. Difference

Test Conditions °F Input Electric 1 otai F. Btu/h-ft^-F

56,500 Btu/h Input with Storm Windows

-15 25,122.1 1929.6 27,051.7 82.2 0.1718

0 on ceo o 1 Q 1 A n iolU.U ZZ,aDa. J 69.3 .1684

0

1 70 0 on OAS 0 64.0 .1653

15 1 71 A n 17/11/1 n 56.8 .1601 15 14,524.7 1409.2 15,933.9 52.9 .1572 30 11,297.0 1139.6 12,436.6 40.8 .1591 30 11 1 cn 1 1 0 7'^ft 0 IZ, / JO. Z 40.5 .1641 45 Q AQI 7 28.6 .1586 45 7 O/l 0 1 1 "^Q^ 1 Q A'lQ 0 o,4jo.

Z

27.9 .1578

without Storms

15 17,063.6 1275.5 18,339.1 54.7 0.1750

30 12,674.1 1222.5 13,896..6 41.1 .1765 45 8,509.7 1060.6 9,570.3 28.7 .1740

30,000 btu/n Input with Storms

0 19,659.7 1972.9 21,632.6 70.5 0.1601

15 15,062.7 1750.1 16,812.8 55.9 .1570 30 10,783.5 1667.3 12,450.8 44.2 .1470 45 6,333.2 1525.2 7,858.2 28.7 .1429

^Based on a thermal envelope area of 1916 ft^. shown in section 2 of this report. In these calculations the ASHRAE design resistances for interior still-air films were not used in the calculations. This is because of the geometry of the mobile home and its effect on the radiative heat transfer. Appendix B

discusses reasons for the interior air film resistances used.

Actual temperature differences observed during each

test were used in the heat transfer predictions.

Separate temperature differences were used for transmission losses through walls and roof, and floor

areas.

In order to predict heat flow under test conditions, air infiltration rates had to be considered. The air infiltration rates used in the calculations were

determined on the mobile home using sulfur hexafluoride (SF^,) as a tracer gas. Section 4.3 of this report discusses the infiltration rate of this mobile

home as a function of inside-to-outside temperature

differences. Infiltration rates were determined both

with the furnace blower running continuously and

the blower off. In the steady-state heating tests, however, the furnace and blower cycled on and off by

thermostat demand. Therefore, the infiltration rate used in the calculations was estimated to be between

the observed rate with and without fan operation

based on the percent on-time of the blower for each

test condition. The percent furnace on-time for the

tests with the gas-fired furnace operation is shown in figure 17. The estimated infiltration rate as a function

of inside-to-outside temperature with cyclical furnace

operation is shown in figure 18. These infiltration rates were used in the calculations.

Using the method outlined in the calculation form of Appendix A, the predicted overall thermal

transmittance is determined by dividing the heat-loss

rate by the actual temperature difference and the thermal envelope area of 1916 ft^. The measured

heat-loss rate is determined in a like manner, and

table 2 gives the measured and predicted thermal

transmittance values. Generally, at the lower

temperature differences there is good agreement

between the measured and predicted values, but with

increase in temperature difference, the measured

values become greater than the predicted values. This seems to indicate a process taking place that is dependent upon increase in temperature difference,

such as the increased buoyant effect on air. In section

4.2, we note that there is thermosiphoning of air in passageways between the insulation and wall paneling due to loose insulation fits at the top and

bottom of the stud spaces. These effects are difficult to determine from a predictive analysis because they are indeterminate and depend upon the workmanship

and methods used to install the insulation and the magnitude of the inside-to-outside temperature

difference.

16
17
TABLE 2. COMPARISON OF MEASURED AND PREDICTED HEAT LOSS.

Test

* J r

Temp.

Diff. F Efficiency %b

Measured Values

Predicted Overall Thermal Transmittance

Btu/hr Btu/hr ft^ F

Ovprall TTiPrmal Tran Qmi Wtk nr** Rhj/hr ft^ F WE+0 69.5 100.0 IDUJ^. Z 0 121 0,105 WE + 15 56.3 100.0 1 9998 k 1

1

.LVD .104 WE+30 41.5 100.0 OJO/ .

o

108 .102 WE+45 28.7 100.0 ^699 n .101 WG-15 82.2 71.7 .127 .107 WG + 0 69.3 70.8 16549.1 .124 .105 WG + 0 64.0 68.4 118 .104 WG+ 15 56.8 68.2 1 9A99 n 114 .102 WG + 15 52.9 68.8 112 .104 WG+30 40.8 63.9 8^^8 8 107 .104 WG+ 30 40.5 59.9 89AA Q 106 .101 WG+45 28.6 57.9 Rfil 7 109 .104 WG+45 27.9 57.7 U. / . 1lUZ. 09 .104 WG+ 45 27.8 54.0 109 .103 WGM+0 70.5 72.8 121 .105 WGM+ 15 56.1 68.5 12226.6 .114 .102 WGM+30 44.2 68.3 9172.7 .108 .101 WGM+45 28.7 63.7 5640.4 .102 .100 NE + 15 56.1 100.0 13459.0 .125 .111 NE+30 41.4 100.0 9305.7 .117 .110 NE+45 28.3 100.0 5998.3 .111 .110 NG + 15 54.7 68.6 13049.2 .124 .110 NG+30 41.1 63.4 9225.8 .117 .111 NG+45 28.7 59.0 6093.8 .111 .113

•Tor test type W-with storm windows, N-no storm windows, E-electric resistance heating, G gas fired furnace, M-modified gas furnace and remaining numbers denote approximate outdoor temperature.

''The efficiency of 100 percent is assumed for electric resistance heading as delivered to the mobile home.

Gross energy (not adjusted for efficiency) input versus

inside-to-outside temperature difference is shown in figure 19. Throughout the tests, it was observed that the inclusion of storm windows caused an approximate 10-percent reduction in energy

consumption under each test condition. This

reduction was close to the predictions from the heat flow calculations. Previous infiltration studies on this mobile home (refer to sec. 4.3) show that the storm windows had only a small effect on infiltration rates. Therefore, it is assumed that the difference in energy

input with and without storm windows is due to heat

transfer through the windows by conduction and

convection.

Throughout the tests the furnace was observed to be

greatly oversized for the test conditions. The original furnace configuration was rated at 56,500 Btu/h input

and the mobile home was certified to maintain a 70 °F interior temperature with an outside temperature of -50 °F, assuming a maximum 15 mph wind. This

mobile home can be considered to be equipped with a furnace oversized for the New York State marketing

area for which it was produced. The degree of oversizing the original furnace can be seen in figures

17 and 19. When the mobile home was tested in the environmental chamber at -15 °F with the original furnace configuration, the furnace burner was

operating only 43 percent of the time. Energy input to this furnace was 20,5000 Btu/h or 36 percent of furnace

rating when the environmental chamber was

maintained at approximately 0 °F.

In order to evaluate the effect of this oversizing,

measured energy consumption was compared

between tests run with original 56,500 Btu/h input

furnace and the modified 30,000 Btu/h input. The

energy input with the two sized furnaces is shown in table 1. The actual gas and electrical energy inputs (not

adjusted for furnace efficiency) are shown for each of the steady-state tests. It can be seen that the energy

consumption rate was significantly lower for the modified furnace than for the original-equipment

furnace, especially at the lower inside-to-outside

temeperature differences where the effect of oversizing is more pronounced. This is true even though infiltration rates are assumed to be slightly

higher with the smaller furnace because of the increased furnace blower on-time.

18
The reduction in energy consumption can be

attributed to the increased percent on-time shown in figure 17, resulting in decreased time for off-cycle stack losses. The apparent efficiencies shown in figure

16 were determined by comparing the gross gas furnace input to the electrical input under the same

test conditions, assuming the electric furnace to be 100 percent efficient. At a temperature difference of 70 °F, both gas furnaces approach their continuous running

efficiencies. At 30 °F temperature difference, the

difference in efficiency is greater than 10 percent.

These low-load operating conditions are common

during the heating season.

The floor-to-ceiling temperature gradients within the mobile home with each of the furnaces were checked

to determine the effect of furnace sizing on occupant

comfort. Table 3 shows these results. These results indicate that the floor-to-ceiling temperature

differences were not significantly different between

the two sizes of gas furnaces. However, the temperature gradient experienced with the electric furnace was obviously lower. This could be due to longer running time with the electric furnace, but this

is not known because percent on-times were not recorded for the electric furnace tests.

TABLE 3. AVERAGE FLOOR-TO-CEILING AIR TEMPERATURE DIFFERENCES

(F) AS AFFECTED BY FURNACE SIZE.

Approximate

Chamber Temp. F

56,500 Btu/h Furnace, 1st Series

56,500 Btu/h Furnace, 2nd Series 30,000 Btu/h Furnace

Electric Furnace

-15 14.0

0 10.4 13.1 12.9 9.8

15 11.9 11.2 11.3 7.5

30 9.2 10.0 9.7 5.9

45 8.5 9.7 7.8 5.3

19
The lack of significant effect of furnace oversizing on

occupant comfort may be specific to this mobile home

design. Floor-to-ceiling temperature gradients in typical residential construction range from 5 to 20 °F, depending upon the type of air distribution system

used. It is possible that the temperature gradients

would be affected to a greater degree if the ducts and

registers were designed for better air mixing within

the mobile home.

4.2 THERMAL PERFORMANCE BY

THERMOGRAPHIC TECHNIQUE

A method for determining the thermal performance of the components of buildings is use of a thermographic

system which detects infrared radiation emitted from

surfaces. The surface temperature in a relatively narrow temperature range is proportional to the

infrared radiation and relatively warm or cold spots can be quantified and displayed in a scan of surface temperatures if the surface emittances are known.

Such warm and cold spots can help to identify areas of varying thermal transmittance due to thermal bridges

and internal air movement.

The thermographic technique was also successful in identifying locations of air infiltration and air

movement. The relation of warm and cold areas of walls to thermal performance of walls was

investigated.

Refer to figures 20 and 21 and to section 2 of this report

for a description of the wall construction.

4.2.1 Description of Thermographic System

A photograph of the thermographic system used for

this study is shown in figure 22. The equipment from

right to left consists of an infrared camera, black-and-white television monitor, a color television display and a temperature profile adapter. The

long-wave radiation that is emitted and reflected from

a surface is sensed by the infrared camera and

converted into a video signal. The camera consists of an optical scanning system with

liquid-nitrogen-cooled indium-antimonide

photovoltaic detector having a high sensitivity in the

spectral range from 2.0 to 5.6 /x.

The video signal from the camera is processed in the

black and white monitor and converted into a thermal

picture in which the black, gray tones and the white

approximately correspond to surface temperatures

Figure 20. Elevation view of mobile home

wall.

SI

.INSIDE WALL SURFACE - 1/8" PLYWOOD

-CHANNEL FOR PASSAGE OF COLD AIR

ASPHALT/KRAFT PAPER VAPOR BARRIER ON INSIDE SURFACE OF INSULATION

.R-7 GLASS FIBER BATT INSULATION

.2"x4" WOOD STUDS -l6"aC.

OUTSIDE SURFACE: ALUMINUM SHEET

WITH VERTICAL CORRUGATION

PLAN VIEW OF INSULATED MOBILE HOME WALL

Figure 21 . Plan view of insulated mobile home

wall.

20
Figure 22. Thermographic system.

from cool to warm, respectively. The image is displayed on the screen of an oscilloscope for display

and can be photographed with a conventional camera. The video signal is also fed into the color television display where ten individually color-coded isotherms

each represent a step in the temperature gradient. The

temperature profile adapter was not used in this study.

4.2.2 Observations with Thermographic

Equipment

General thermographic observations were made of the inside and outside of the mobile home. During all of the following thermographic observations, the outside temperature was 0 °F and the inside

temperature was 72 °F. On the outside, it was

observed that the skirting around the base was warmer than the wall surface above, (fig. 23). See also figure 24.

Most observations were made of the inside surfaces including several general observations. The location

of wood studs was observable because the inside

surface was slightly colder over the studded areas. Wall surfaces were observed to be colder toward the bottom as compared to the top of the wall (fig. 25). The warm heating ducts created an obvious pattern on the

floor. Figure 26 shows an obviously warm floor

register with a resulting warm wall area caused by the heater air warming the wall surface. The general

thermographic observations also revealed some air infiltration locations. Figure 27 shows a cold (dark) area under a window where outside air is leaking in around the window framing. The wall/floor corner shown in figure 28 is obviously very cold. Closer

observations at this location revealed an air leak at the

wall/floor junction resulting in an interior surface temperature of 29 °F in the corner (inside 72 °F, outside

0 °F).

One thermal characteristic was so dramatic that it created a dominant thermal pattern on the wall. Cold

"streaks" (dark area of fig. 29) occurred along the inside of the wall starting at the base of the wall and

extending up into the upper half. Surface temperature

measurements showed these streaks were about 5.5 to 6 °F colder than the warmer areas. The colder area covered about 25 percent of the surface area between

studs.

From these observations, it was hypothesized that cold air must be in contact with the interior wood

paneling. This early hypothesis was later supported

when it was observed that the cold patterns on the wall correspond directly with locations where the

insulation was not in contact with the wood paneling,

leaving an air channel between the insulation and the paneling.

21
Figure 23. Angular thermograph of exterior sidewall of mobile home. Dark area (colder) is the aluminum wall surface. Lighter area (warmer) below is the skirting at base of mobile

home, and white area in lower left is ground surface. Dark

area found in skirting is an air vent (refer to figure 14).

I I I i I I .2 .4 .6

Figure 24. Warmer horizontal aluminum trim at ivalllroof juncture.

22
Figure 25. Wall along hallzvay looking from kitchen area. Walls are colder at the base of the wall. Cold streaks occur randomly

along wall.

Figure 26. Wall at base of double living room windoiu. Warm air registers at lower left is warming the wall surface under

left window.

23
Figure 27. Windozv with warm curtain (light area) on right side. Dark (cold) area on wall under window is due to perceptible cold air leaking into mobile home.

Figure 28. Living room lualllwalllfloor corner. Perceptible air leak

in comer created very cold spot, out of camera range. Light area in lower left is couch.

24
Figure 29. Wall area to left of living-room side-ivall window.

Picture on wall at upper left, draperies along window on

right side of picture. Dark areas are cold. Note wall studs showing through in upper center. Location as shown on

figure 30.

To test this hypothesis and evaluate various corrective techniques, a series of changes were made in the wall construction and the resulting thermal profiles were

observed. All of these changes were made in one

location as identified in figure 30.

Since gaps were observed at the top and bottom of the

insulation (fig. 31), it was considered possible that cold air could enter the cavity, pass under the

insulation, rise between the insulation and interior paneling and then pass to the outside over the

insulation at the top. The exterior aluminum paneling

allows more or less free passage of outdoor air into and

out of the wall cavity outside of the insulation.

Six-mil polyethylene film''was then attached to the outside surface of the studs at the test location to prevent cold air from penetrating directly into the

insulation cavity (see fig. 32). After the outside surface was sealed, the resulting thermographs (see fig. 33) revealed that the cold pattern still remained with the same configuration as received from the factory. The

surface temperatures were slightly warmer and the

intensity of the cold/warm pattern was not quite as

sharp as shown in figure 29. See table 4 for surface temperatures at locations A and B of figure 39. Location A is a spot where temperatures were lowest,

and location B is the warmest temperature.

After it was determined that sealing the outside

surface of the wall would not prevent the occurrence of cold patterns on the inside surface, it was reasoned

that cold air could be circulating in a closed loop

around the insulation within the stud cavity, without

direct air leakage to the outside, effectively siphoning

heat to the outside. This theory was tested by taping

the inside vapor barrier of the insulation to the adjacent wood studs (see fig. 34). This figure also shows the taped lines outlining areas of the insulation

which were in contact with the back surface of the wood paneling. Those areas in contact had warmer

inside surface temperatures, supporting the theor}' that air movement within the stud space was causing

the cold streaking on the inside wall surface.

After taping the vapor barrier to adjacent studs, more

thermographs were taken. The resulting thermal

profiles showed that the inside sealing process had a

25
REAR DOOR

© THERMOCOUPLE

O AIR REGISTER

SCALE: 1 ft.

© 1QLU

00

CLOSET Q

BATHROOM

o

0

aLU

CO

REFRIG. xxx

© LIVING ROOM

STOVE

yV FRONT DOOR

"^HEATER^" ""^^

Figure 30. Location of thermographic investigation of wall performance denoted by X.
27
TABLE 4. THERMOCOUPLE READINGS ON THE WALL SURFACE AT LOCATIONS A AND B

OF THE WALL TEST ELEMENT.

Condition Location A Location B Observations

1. As received from factory 57.8 63.3 Thermographic observations showed rather sharp contrast between cold and warm areas.

2. After sealing outside surface of stud cavity

59.7 64.0 Intensity of pattern not quite as great Overall temp, of wall slightly warmer. Same pattern still exists.

3. After sealing inside surface of insulation to studs

to prevent thermal si- phoning

61.0 62.2 More significant effect than condition No. 2 Wall warmer

and temp, gradient reduced.

4. Inside surface of insula- tion sealed but outside

stud cavity unsealed

60.0 62.0 Wall slightly colder than condition No. 3 but temp,

gradient still smoother than condition No. 2.

5. Carefully installed full thick insulation

61.8 62.8 Surface temperature very uniform. l>amatic effect.

NOTE: For all temperature recordings outside temperature was 0° F and inside temperature was 72° F.

greater effect on reducing the cold streaks than did

sealing the outside of the stud cavity. Observations were made both with and without sealing the outside

of the stud cavity with polyethylene film. Although

the film did have an observable effect of reducing the cold streaking on the inside surface, sealing the inside

surface to prevent siphoning was obviously more

effective (see table 4).

An alternative way to stop air movement within wall

cavities and subsequent siphoning of heat to the outside would logically be to fill the cavity with

insulation to remove locations where air could move.

This theory was tested by removing the original

insulation (R-7) and replacing with an R-11 full thick

baft. This technique was evaluated with insulation

installed by friction fit (fig. 35) and with the insulation

stapled neatly to the faced of the wood studs (fig. 36).

In both cases, thermographic and thermocouple

observations revealed that the cold streaks were

substantially eliminated with full batts . Without neat, mechanically-attached placement, some streaks did

occur along the studs where the insulation was

wrinkled due to stuffing the stapling flanges of the vapor barrier into the stud space, allowing air passage

(see fig. 37). With neat attachment, cold streaking was

eliminated and surface temperatures were very even,

as shown in figure 38. As changes were made in the insulating system, inside-surface temperatures were

recorded with the thermocouples located at A and B as shown in figure 39. These temperatures are shown in

table 4.

FIGURE 34. Inside of mobile home wall with paneling removed. Insulation vapor barrier has been sealed to stud with duct tape. White

tape outline on surface of insulation outlines area in contact with back of paneling.

28
Figure 35. Inside of mobile home wall with paneling re- moved. Original R-7 batt insulation has been

replaced with R-11 batt. Insulation friction fit.

Figure 36. Same as figure 35 but with R-11 batt stapled

neatly in place.

I I I I .2 .4

Figure 37. Interior thermograph after installing R-11 batt with

friction fit. Note studs and cold streak caused by wrinkle

in friction fit batt in narrow cavity.

29
Figure 38. Interior thermograph with R-11

batts installed as shown in figure

36.

4

Figure 39. Thermocouple locations on inside of wall

paneling. TempeTature readings shown on

table 4.

Some conclusions can be drawn. As currently

installed, a portion of the R-7 insulation in the walls is made ineffective by the passage of cold air through the

insulated cavity, siphoning air around the insulation. Although the surface temperature depression of 5.5 - 6.0° F seems rather small, calculations indicate that

this effectively means a reduction in insulation

effectiveness in the cold spot areas of over 50 percent.

Since cold areas appear to constitute approximately 25 percent of the wall area, movement of cold air within

the wall could result in a 9-percent reduction of overall

wall resistance. Further, since wall losses constitute about 40 percent of the total energy loss of the mobile

home, thermal siphoning could account for 4 percent

of all energy loss at a temperature difference of 70° F. It appears that full, thick, properly installed insulation

substantially reduces or eliminates air movement in insulated cavities. The presence of non-insulating

sheathing such as the polyethylene film did not

effectively stop air movement and thermal siphoning.

Placement of a vapor barrier such as polyethylene film on the outside surface is not a recommended practice,

but was used as an expedient for excluding outside air from entering into the stud walls for the purposes of this test.

4.3 AIR LEAKAGE ANALYSIS

4.3.1 Natural Air Infiltration as a Function of

Stack Effect

A separate NBS study was conducted on the air infiltration characteristics of this mobile home. This

work is described in detail in a report entitled, "Air

Leakage Measurement in a Mobile Home" [2]. For purpose of reference to calculations and observations

in this report, some results from that study are included in this report.

Test method: Infiltration tests were made with sulfur hexafloride (SF^) gas introduced into the furnace fan and distributed throughout the house. Tracer concentrations were measured with a gas chromatograph equipped with an electron capture

detector. Samples were introduced into the chromatograph by means of an automated sampling

system. Replicate samples were collected every 10 minutes from various locations inside the outside of the mobile home. The infiltration rate of the mobile

home was calculated from the decay of tracer gas concentration as a function of time. (See

above-referenced report for further test method

details.)

Observed infiltration rates: The NBS environmental

chamber provided the ideal laboratory for determining infiltration as a function of inside-to-outside temperature difference. It is generally recognized that this stack effect is one of the driving forces. Another driving force is the outside

wind pressure.

The initial series of infiltration tests were run with the furnace blower running continuously. Later tests showed that the furnace blower had a considerable

effect on the infiltration rate. Comparison tests were

30
made between continuous-fan and no-furnace-fan

operation. Without operation of the furnace fan, air infiltration rates were reduced by about 0.2 air changes per hour (fig. 17). In this test, the air within

the mobile home was circulated by means of fans

located within the rooms.

The reason for the large effect of furnace blower

operation on infiltration rate was observed to be an

annular gap around the furnace flue where it penetrated through the ceiling (see fig. 40), allowing

the furnace room to communicate almost

unobstructed with the vented attic space. When the

fan was operated, the furnace room was under a slight

negative pressure which induced outside air from the

attic space.

Figure 40. Ceiling of furnace room showing air gap to attic space at the furnace air intake flue annulus.

little to identify the paths of air infiltration. Some of the infiltration paths were observed with the infrared camera, such as the previously mentioned air leakage

under a window and in the living room corner. This

section discusses another attempt to locate leakage

paths by pressurizing the mobile home interior, and

measuring air flow rates through the mobile home as various suspected leakage paths were blocked and

unblocked.

Test method: Refer to figure 41 for a diagram of the

test equipment. An interior-mounted axial fan (fig. 42) was used to pressurize the mobile home and the

total air flow was measured by means of a pitot

static-air velocity measuring device located upstream

of the fan (fig. 43). The differential pressure was

maintained at a constant 0.20-in water gage by

restricting the size of the open end of the duct

extension as more and more leakage paths were

blocked.

SlK) DUCT

F1GURE41 . Equipment configuration for air leakage tests.

Another suspected source of air leakage was through

the joints in the furnace duct under the floor. This was

checked by drawing air samples out from the crawl

space under the mobile home during infiltration measurements. These air samples were monitored

regularly for SFg traces, but none were found. This

would suggest that the combination of duct and

bottom board provided an effective seal.

Infiltration rate tests were also run with and without

storm windows on the inside. The effect was found to be quite small. This was because window frames are not usually sealed to the interior paneling, and

infiltration can occur through the resulting gaps, and

because the mounting frame for the storm window is not tightly sealed to the inside wall paneling. Air

leaking through or around the primary window is relatively free to pass around the inside mounted

storm sash.

4.3.2 Pressurized Leakage Tests

Although actual infiltration rates were determined

through the SFg tracer-gas technique, this study did

Potential air leaks within the mobile home were

blocked from the inside with the use of duct tape. As

each air leak was blocked the reduction in air flow was

noted. Because the weather outside the environmental test chamber changed rather dramatically during the test, the temperature and

barometric pressure within the test chamber were

recorded at least two times per day and these variables were used in the calculation of air flow. The air density was calculated using the equation:

p = 1.325 —

T

where p = air density in pounds per ft^ Pg = barometric pressure in inches of mercury T = absolute temperature (F + 460)

The air flow rate, Q, in cubic feet per minute,

moving through the duct containing the calibrated

pitot tube was calculated using the equation:

31
Figure 42. Vane axial fan mounted through sealed plywood partition at front entrance door opening.
Q = 1096.7

where A = duct area, .555 ft^ h = pitot tube pressure difference in inches

of water

There was a question whether to use a pressure

or vacuum on the interior in order to create forced

leakage. Previous forced-leakage studies on

conventional homes by G. T. Tamura of the National

Research Council of Canada (NRC) utilized a negative interior pressure and leakage paths which

were blocked from the outside with polyethylene

film and tape. This is a logical way to test conventional homes because it appears that the pressure envelope is on the exterior surface and it is easier to seal areas with the sealing medium

drawn toward the surface rather than pushed away.

Because of the vented exterior wall design used with

this mobile home, the pressure envelope was known

to exist at the inside wall surface. Therefore, the leakage paths were blocked from the inside and

an interior positive pressure was more logical. In addition, equilibrium values were slower in developing and were more erratic when negative

pressure was applied to the mobile home.

Previous studies by NRC were conducted with a constant inside-to-outside pressure difference of 0.300 in water gage while these tests at NBS were

run at 0.200 in water gage. The 0.200-in water gage

pressure was used because it was found that the vane

axial fan was capable of maintaining approximately

0.204 in wg pressure difference without any leakage

paths blocked. With this capacity, it was decided to run all tests at 0.200-in wg pressure.

The decision to test at 0.200-in water gage was

accompanied by a determination of the relationship

between pressure and flow rates. Figure 44 shows

that this relationship is apparently linear down to about 0.02 in wg pressure difference. As the graph

indicated, this relationship was checked with

approximately one half the leakage paths blocked as well as with the mobile home in its original

condition.

The inside-to-outside and pitot-static pressure measurements were made with a micro-manometer

and a standard Hooke gage with micrometer scale.

Results and discussion: The sequence of leak blockage and unblockage as well as their effects are shown in table 5. In general, the consistency of the

33
TABLE 5. MOBILE-HOME FORCED AIR LEAKAGE CHARACTERISTICS, FT^/MIN.a

Sequence

or SealingPotential Leak Location Sealed or Unsealed

Residual

Air Leakage

Reduction

in air Flow Due

To Sealing

of Leak

Sequence

of seal Removal

Increase in

air flow due to removal

of Seal

1 Original condition 1020 13 1039

2 Bathroom ventilation fan 869 151 (14%) 11 156

3 Between ceiling and flue vent 789 80 (8%) 9 88

4 Lower liinge side of rear entrance door 783 6 (<1%) 2 4

5 Two plumbing holes through floor 744 39 (4%) 8 38

6 Duct System (register opening and blower return opening) 744 10 6

7 Wall heater 716 28 (3%) 7 36

8 All exterior outlets and switches 699 17 (<2%) 5 19

9 Windows sealed to inside surface of wall paneling 511 188 (18%) 1 240

10 Front base of furnace 497 14 (1%) 12 -

11 Paneling side joints 420 77 (8%) 4 76

12 Joint between wall and ceiling 282 138 (14%) 3 90

13 Furnace room door 275 7 (<1%) 6

^AU leakage rates were taken with inside-to-outside pressure difference of 0.200 inches water gage.

effect of blocking and unblocking leakage locations was very good, with the exception of two cases; namely, the effect of sealing the wall-ceiling juncture

and the windows. However, if these two leakage

locations are recognized as being parallel leakage

paths through one element (the wall), it is logical that the sequence of blockage of these parallel paths may

have an effect on the reduction or increase of air flow.

It can be seen from the figures below that the total leakage through the wall was virtually the same in both directions.

The sealing process was accomplished with

aluminized duct tape. Figures 45 through 47 show

examples of sealing of suspected air leaks.

After all the obvious and sealable leaks were taped

shut, only approximately 30 percent of the initial leakage remained. At this point, some air was still observed to be leaking at penetrations through the asphalt-impregnated underlayment, through the roof vents and through the exterior-wall-vent openings.

This residual leakage is presumed to be occurring by

such means as:

o permeability through the building materials

o inaccessible holes through the ceiling such as

electrical penetrations within partitions and

under light fixtures, o non-sealable leaks through the floor at the base of the furnace,

o other miscellaneous locations.

Variable

Decrease in

air flow upon sealing

Increase in

air flow upon unblocking

1. Sealing exterior switches and outlets

2. Sealing windows

and rear door

17

188

19

240

3. Sealing plywood

butt joints 77 76

4. Sealing wall/ceil- ing juncture

138 90

Total Effect 420 cfm 425 cfm

34

Figure 47. Plumbing hole through floor under bathtub sealed with duct tape. Access gained through hole cut in end bedroom

wall.

The wall leakage was found to be greater than 40 percent of the total forced leakage. This is a combination of leakage through the window

construction (the largest portion), through the

exterior electrical switches and outlets, through the

wall/ceiling juncture, and through the joints between

plywood panels. Observation of the window

construction leads to the conclusion that much of the

total window loss was caused by sealing the primary

window to the exterior surface, which is not the pressure envelope. It appears that air was leaking

between the inside-mounted storm windows and the

exterior mounted primary window, allowing air flow

into the vented wall cavity. This observation was

supported by the fact that air leakage out of the

exterior wall base and top vents was reduced

substantially when the total window construction was

sealed. Yet, significant leakage through the outside

prime window could not be felt while the window

was unsealed on the inside and the house was under

pressure.

The leakage through wall switches and outlets was

small, considerably less than anticipated. This

tightness is attributed to the closed fit between the

electrical box and the paneling and to the air resistance of the switch or outlet and cover assembly.

The pressurized leakage of the bath exhaust vent may

not be indicative of air leakage through this path

under normal infiltration pressure differences. However, this unit could be designed to provide a more effective seal. As constructed (fig. 48), the

circular duct is dampered with two semi-circular, in-thick sheets of open-cell polyurethane foam glued

to the fan housing at only one point. The fan blows

these flaps up, effectively opening the damper.

Unfortunately, the flaps overlap and do not fit well,

allowing some unrestricted air flow. It was noted that warm air was escaping from the fan vent due to stack

effect only with the interior at 70 Tand the chamber at 0 °F.

Pressurization tests were used to identify air-flow paths as sources for potential air leakage. Results of these tests should not be extrapolated to quantify air infiltration and exfiltration rates. The complex natural convection forces due to wind and temperature

differences are not expected to behave in the same manner as that for the constant internal pressure.

36
Figure 48. Bathroom exhaust fan with foam plastic backfloiu seal- ing flaps.

4.4 CONDENSATION POTENTIAL 4.4.2 Test Methods and Conditions

4.4.1 Instrumentation

Condensation tests on this mobile home were

conducted in the environmental chamber.

Humidification was achieved with a portable power

type wetted element humidifier located in the kitchen

area of the mobile home. The built-in humidistat was

by-passed by setting the humidifier control to continuous operation. The on-off cycling was

controlled using a humidistat with a hair-type

sensing element.

Two full-range humidity transducers were utilized to read the relative humidity in the living space of the mobile home and the attic space. Four narrow-range

(89 to 99%) lithium-chloride moisture sensors were

installed in the insulated wall cavity. Two of the sensors were installed between the interior paneling

and the insulation vapor barrier. The remaining two

were installed between the wall-cavity insulation and

the outside aluminum sheathing.

The condensation-potential tests were conducted

with the environmental chamber maintained at approximately 0 °F and the interior of the mobile

home maintained at 72 °F by means of the mobile-home thermostat-controlled furnace. Mixing

of the humidified air was accomplished with a flexible duct and small fan moving humidified air from the

living room to the rear bedroom and through the mixing action of the furnace blower. The furnace

blower was allowed to cycle on demand.

These tests were conducted with the interior relative humidity maintained at approximately 42 percent RH

(±1%). It is recognized that a 42 percent interior RH is unusual under 0 °F weather conditions, but it is possible with either a portable humidifier (as used in

this test) or a furnace-mounted humidifier. It is also possible to achieve relatively high humidities with

unvented driers, hanging clothes to dry indoors,

showers, cooking, or other high moisture-producing

activities within the mobile home. Therefore, it was

decided to test under conditions severe enough to reveal condensation potential.

37
At the beginning of the test, two full-range humidity

transducers were used to determine consistency of

relative humidity throughout the mobile home. One

of the transducers was located in the center of the

living room area and the other transducer was

installed in various locations such as the rear bedroom, bathroom and hallway. After it was

determined that the interior fan mixing systems

produced even humidities throughout the mobile

home, the second humidity transducer was installed in the attic space near the center of the mobile home.

The transducer was mounted in the free air space of the attic and the lead-in wire was inserted through a ceiling light fixture to the mobile home interior.

The condensation-potential tests were conducted

over a continuous two-week period. During this time,

the full- and narrow-range sensors were read at least four times daily to ensure that the interior relative humidity was being maintained at the set point and to observe whether condensation was occurring in the

attic or wall spaces. The full-range transducers were

also observed closely for variations caused by furnace

on-off cycles by taking readings every minute over two 2-hour periods.

In addition to the instrumentation observations,

visual observations were made throughout the

testing, including checks for surface condensation

and condensation within structural elements. The

visual observation of condensation within the

structural elements was accomplished by opening

sections of the wall and roof at the conclusion of the

test.

4.4.3 Results and Discussion

a. Results obtained through instrumentation: The

narrow-range wall-cavity moisture sensors were used

solely for purposes of determining when relative humidities approached condensation-potential

levels. The sensors located on the inside or the outside

of the insulation showed no reading throughout the

test. This would indicate that relative humidities never exceeded the bottom of the range (89%) for the

sensor. This was also true for the sensor between the paneling and the insulation; however, it was later found that frost had accumulated in the entire area surrounding the sensor between the insulation and

the exterior siding. It is assumed that the narrow-range sensor was not operative at the low

temperature on the outside of the insulation. The

results obtained with the wall-mounted narrow-range

sensors were not considered meaningful and no

conclusions were based on these results.

The full-range RH transducers located in the attic and

living room areas did provide useful information. It was observed that the interior relative humidity was

maintained at an approximately constant level as the humidifier cycled on and off once each 25 minutes.

The attic relative humidity cycled considerably as a

direct function of the furnace-blower operation. It was

concluded that when the furnace blower was on, outside dry air was being drawn through the attic space into the mobile home. This was caused by the annular gap around the chimney as shown in figure

40. This conclusion was later verified by closing the gap with duct tape and virtually eliminating any effect on attic relative humidity by the on-off cycles of the furnace blower. At the location of the RH transducer,

the attic space relative humidities ranged from a low

of 80 to a high of 94 percent over the entire two-week

period. With the gap sealed at the chimney, the attic space RH levels were also within this range but were

held more nearly constant.

b. Results from visual observations: Observations of

interior structural condensation were made by cutting holes through the metal roof to observe attic condensation. Refer to figure 49 for location of roof

cuts. These test cuts were made at the end of the second week of condensation testing while the chamber was stiU at 0 °Fand the interior temperature

and relative humidity were still maintained at 72 °F and 42 percent RH.

Roof test cut No. 1 was made near the furnace flue. Observers walking on the roof in the general area of the test cuts heard cracking noises which were

interpreted as indicating possible frost attachment

between the metal roof membrane and the foam-core

sheathing. Considerable frost was observed on the underside of the sheathing and a single droplet of water was observed on top of the fiberglass roof

insulation.

Roof test cut No. 2 was made far removed from the furnace flue, in an area where the cracking noise

created by roof-top traffic was not predominant. At

this location, very little frost appeared between the metal roof and the sheathing. This frost consisted of fine frost lines on the under surface of the metal with no observable frost on the top side of the sheathing. A

thin film of frost was observed on the underside of the sheathing (more than on the underside of the metal

roofing). The glass-fiber-batt roof insulation

appeared dry at this location.

Roof test cut No. 3 was made near the bathroom

exhaust vent stack. The previously discussed cracking

noises were very predominant in this general area.

38
Frost was observed between the metal and the sheathing (more frost at this location than at either test cut No. 1 or 2). Considerable ice and water were

observed on the underside of the sheathing and water

droplets covered the surface of the batt insulation. See figure 50.

1

(V) i—--! i®T""' i ® V El

1 n®

r

1 1

1

1 1

1 ! ® 1 1

1 Cp) I

C - TEST CUTS

R - RH SENSOR

V - AHIC VENTS

F - FURNACE FLUE

E - BATH ROOM EXHAUST

P - PLUMBING VENTS

Figure 49. Plan view of roof.

Figure 50. Roof test cut numbers showing water and ice on botton surface of sheathing and icatcr droplets on top surface of

roof insulation.

39
These roof test cut observations indicated that attic-space condensation was heavily influenced by

air leakage through the penetrations in the ceiHng of the mobile home rather than general diffusion

through the ceiling construction (which contained a

plastic film vapor barrier). Moisture accumulations were obviously heavier near the penetrations (bath

exhaust and furnace flue) as observed by both the test cuts and by noises created by cracking frost and ice observed while walking over the roof surface.

The bathroom exhaust vent constituted a penetration

into the attic space because the two sections of the exhaust stack were not properly joined, allowing

interior air to go into the attic space as well as to the outside of the mobile home. It appeared that the stack

effect alone was sufficient to force interior air up

through the foam flap "sealed" vent. This deduction was made because of the attic space moisture found

near the vent and because of the frost accumulation

within the outside bonnet covering the vent. This

moisture was observed even though the exhaust fan was never turned on. It was also observed that moisture was condensing on the inside walls of the vent stack and running back onto the surface of the

ceiling vapor barrier. Figure 51 shows the point where

water was observed on top of the vapor barrier at the base of the exhaust vent.

B.l Exterior wall observations: The walls were

inspected for condensation by peeling back the aluminum siding. Since exterior sheathing was not

installed under the aluminum, this revealed the outside surface of the wall insulation. See figure 52. It was observed that considerable frost had accumulated

between the outside surface of the wall insulation and

the metal siding and on the inside surface of the top

fascia. This frost accumulation was very pronounced

on the upper half of the wall but not on the lower half.

These observations indicated that interior moisture was getting into the wall cavity either through a diffusion process, or through leaks. The location of the frost further supports previous observations

concerning convective air currents within the

insulated walls. It appears that air is moving up

within the cavity carrying moisture to the upper half

of the wall and allowing it to escape under the vented

fascia. On this basis, it appears that the vented design

of the wall is functioning to remove moisture from the

wall cavity.

B.2 Surface condensation: Moisture was also observed to be accumulating on the inside surface of the walls. This condensation occurred only at the base

of the walls both in open areas and within enclosed

cupboard and closet areas. Figure 53 shows a close-up

photograph of this condensation at the base of a wall. Note that the condensation pattern was very distinct. Figure 54 is a picture taken of the hallway looking

from the kitchen area. The surface condensation

locations were outlined with white tape to make them

visible in the photograph. Note that the condensation

did not occur in a uniform manner along the wall but

at specific areas only. By comparing the thermographs

of the same area (fig. 25) it was noted that condensation had occurred at those locations where

pronounced cold streaking had previously been

observed. It was concluded that cold outside air which

was convectively looping within the wall cavity was

lowering the temperature of the wall paneling below

the dew point (48 T) of the inside air (72 °F, 42% RH),

resulting in this surface condensation.

Surface condensation also occurred at locations having obvious thermal shorts. This was true

especially for the aluminum frame around the rear entrance door, which showed heavy condensation

and frost and ice buildup.

B.3. Window condensation: Surface condensation was also observed on the windows. This

condensation appeared to be excessive due to the design of the exterior primary window and interior storm window. Figure 55 shows a typical window

with surface condensation. It is important to note that almost all of this condensation occurred on the inside

surface of the outside window, not on the inside

surface of the storm window. It was concluded that the lack of seal between the storm window and the

interior wall surface allowed moisture to move freely

to the outside, primary window.

It was hypothesized that if the inside window were

well sealed and the outside window sealed to a lesser extent (as in typical residential site-built construction

where the storm window is on the outside), the window condensation would not have been severe. To test this theory, the inside storm window was

sealed to the wall surface using duct tape as shown in figure 56, and the outside window was cracked open

approximately Vie in. Under these conditions,

condensation did not occur on the inside surface of the outside window.

40
1

Figure 51. Location of water found on top of plastic film vapor barrier of bathroom exhaust.

Figure 52. Frost accumulation at top of wall and under fascia.

41
Figure 53. Surface moisture condensation at base of wall.

Figure 54. Pattern of surface condensation at base of wall.

42


5. CONCLUSIONS AND

RECOMMENDATIONS

5.1 CONCLUSIONS

(A) For laboratory tests without wind effects, it was

found that the original-equipment furnace was

considerably oversized. Using this originalequipment furnace, the total energy supplied per

unit temperature difference based on a thermal

envelope area of 1916 ft^ ranged from 0.172 to 0.158

Btu/h ft^ F at inside-to-outside temperature differences of 82.2 and 27.9 °F, respectively. Under these same inside-to-outside temperature differences, the apparent cyclical efficiency (determined by compari- son with assumed 100-percent-efficient electric resistance furnace) of the original-equipment gas furnace ranged from 71.7 to 57.7 percent, respectively. After modifying the furnace to reduce its input

capacity by 47 percent, the values ranged from 0.160

to 0.143 Btu/h ft^ F at inside-to-outside temperature

differences of 70.5 and 28.7 °F, respectively.

Under these same inside-to-outside temperature

differences, the apparent cyclical efficiency of the smaller capacity furnace ranged from 72.8 to 63.7 percent, respectively. It is concluded that a greatly

45
oversized furnace will cause a significant increase in seasonal energy consumption. From the calculation procedure in Appendix A, the modified gas furnace

configuration is estimated to meet the heating load at -22 °F with a 15-mph wind; and so is somewhat

oversized for most areas of New York State. Generally,

a certain amount of oversizing is required for recovery due to nighttime setback of the thermostat, reduction

of furnace capacity for mobile homes located in high

altitude areas, and a possible reduction with time of the thermal insulating effect of the mobile home.

(B) The comparison between measured and predicted

heat loss was very close, expecially under conditions

where the outside temperature was relatively high (45 and 30 °F) . It should be noted, however, that predicted

heat-flow calculations included allowances for effect of cupboard and closet areas, furnishings, draperies,

and modified surface air films (refer to Appendices A

and B). If the standard ASHRAE calculations were made ignoring these aspects, the heat-flow

predictions would have been higher than measured.

Thus, standard heat flow calculations would normally

result in some degree of furnace oversizing.

(C) Although the comparison between predicted and

measured heat loss using the calculation procedure

referred to in item B. was close where the approximate

outdoor temperatures were 30 and 45 °F, for lower

outdoor temperatures the measured values were from

5 to 17 percent greater than the predicted values. It should be noted that the predicted heat losses included increased infiltration rates caused by stack

effect and longer furnace percent on-time. It is concluded that this apparent loss of insulating

efficiency at lower temperatures (higher

inside-to-outside temperature differences) is due at least in part to thermosyphoning of air in and around

the insulation within wall stud spaces.

(D) A thermographic survey using an infrared

television camera was useful in identifying locations

of air leakage, thermal bridges and temperature

differences on wall surfaces. Through this analysis, it was determined that, under low outside temperature

conditions, convective air currents were found to exist within the insulated walls, reducing their efficiency.

The thermographic analysis indicated that this thermosiphoning was caused in part by the vented

wall construction which was used, and by the method

of installing the insulation. To reduce this thermosiphoning action, it is recommended that

carefully fitted, full-thickness insulation be used, or some other mechanism be employed to reduce the driving force, or to eliminate locations where

convective air currents can occur through gaps around

the edges of the insulation.

(E) Pressurization tests were used to identify air-flow paths as sources for potential air leakage. These tests showed that 40 percent of the total pressurized

leakage occurred through the walls. The largest

portion of the wall loss was due to the specific

wall-window construction used on this mobile home.

Air leakage at this location was caused by sealing the primary window to the vented outside siding. The

interior storm windows are attached to the inside

paneling (the pressure envelope) but are not sealed in any fashion. Other envelope penetrations such as furnace flue and bathroom exhaust vents were found

to be poorly sealed, resulting in excess leakage.

(F) In this study, it appeared that the combination of a vapor barrier and the attic space vents would prevent

excessive moisture build-up or condensation in the

attic space. However, unsealed penetrations through

the ceiling of the mobile home will allow

moisture-laden interior air to rise into the attic space,

resulting in excessive condensation under high

interior-moisture conditions.

(G) Convective looping of cold air within the wall

cavities will cause interior surface wall condensation

under high interior-moisture conditions.

(H) Excessive surface window condensation was

observed and was found to be the result of putting a non-sealed storm window on the inside surface with a well-sealed window on the exterior.

5,2 RECOMMENDATIONS

(A) It is recommended that this study be continued to provide further information in some areas which have

been investigated and to allow work in some new

energy-related areas. The continued work should

involve:

(1) An analysis of energy consumption

characteristics of this mobile home under

heat-gain (cooling) conditions;

(2) A heat-loss analysis of a similar mobile home

constructed to eliminate thermosiphoning

within walls — to assess the magnitude of this heat loss;

(3) An analysis of alternative wall constructions

such as full-thickness insulation, insulating

sheathing, and others, to clarify their heat

transfer and condensation characteristics;

(4) An analysis of the relationship between

pressurized leakage and natural air infiltration. A standardized pressurized leakage test should

be developed and a maximum pressurized

leakage should be recommended for future construction standards.

46
(B) It is recommended that the following findings

from this study be utilized in future modifications of construction and safety standards for mobile homes;

specifically, these standards should be modified in the following areas:

(1) Construction standards should limit furnace

oversizing in order to increase seasonal

efficiency. This could be done by providing

furnaces adequate for the coldest areas within

the applicable market, along with a dealer-installed kit designed to reduce the excessive capacity of the furnace in warmer

climates;

(2) Also, in order to reduce oversizing of furnaces,

calculations to predict heat loss or gain under

design conditions should include the insulating

effect of draperies, cupboards, closets, and

similar items which add to the thermal

resistance of the exterior envelope of the mobile

home. The resistance of the interior air films used in the calculations should recognize that each exterior wall of a mobile home typically "sees" more cold wall surfaces than for conventional residences, thus reducing the

radiation component of the film heat transfer

coefficient. It is recommended that these factors be included in standard calculation procedures.

(3) Installation methods for insulation should be

specified to reduce potential for thermosiphoning by convective air currents. This should include full-thickness insulation in

walls, smooth installation of mechanically

attached wall insulation, and careful installation

of insulation at all locations to eliminate

locations for convective air movement.

(4) Mobile home construction standards should

include methods to reduce air leakage paths and

to reduce the potential for condensation within

attic and other structural cavities. First of all, the primary window should be sealed continuously

to the pressure envelope of the mobile home.

Secondly, penetrations through the pressure envelope should be made airtight with respect

to the envelope, especially those penetrations

through the ceiling, such as plumbing, vent

stacks, ceiling exhaust vents, and furnace

stacks. Thirdly, connections between wall and floor and between floor and ceiling should be

specifically designed to eliminate air leakage at these locations.

It is recommended that future construction

standards include these methods and enforce them through pressure-leakage tests of completed mobile homes.

6. REFERENCES

[1] Tamura, G. T., "Measurement of Air Leakage

Characteristics of House Enclosures," ASHRAE

Transactions, Vol. 82, Part II, 1975.

[2] Hunt, C. M., Treado, S. J., and Peavy, B A., Air Leakage Measurements in a Mobile Home, NBSIR 76-1063 (July 1976).

47
i

I

I

i

I
APPENDIX A

CALCULATIONS FOR THERMAL TRANSMITTANCE

The following calculations for thermal trans- mittance, U in Btu/h ft^ F, are made for the calculation form (table A-1) from the numbered items using the

resistances in h ft^/Btu of individual components

and weighted area, A in ft^.

l.a) Unobstructed roof area:

Weighted UA =

467.3

9.89

77.10

6.29 = 59.51

L7 =

59.51 544.37 = .1093

2.b) Wall area covered by draperies and valance —

the heavy drapery material reduces the radiant

and convective components of the inside air film resistance in addition to heat transfer through the drapery material. R = 0.99 + 9.15 =

10.14 or U = .099.

At Insulation At Framing

Inside Air Film 0.99 0.99 % in Fiberboard 1.89 1.89

2 in X 2 in Wood Truss 1.88 R-14 Insulation (thickness constructed near edges) 14.00

Partial Air Space .89 2 in X 2 in Wood Truss 1.88 Foam Sheathing, V4 in 1.04 Metal Roofing 0.00 0.00 Outside Air Film .61 .61

18.43 9.18

446.39 48.7 Weighted UA = —— + —- =29.52

^ 18.43 9.18

U = 29.52/495.09 = .0596

2.c) Partition contact area — Refer discussion

of l.b).

Resistance of unobstructed wall area = 9.15 Wood partition resistance = 0.94 Air film obstruction = .50 10.59

U = 0.094

2.d) Window area —Considerations for the window

include exposed glass, drapery, and opaque

and sheer curtains calculated on a weighted

area average basis. The thermal transmittance

of the exposed glass area is based on inside

and outside air film resistances of 1.46.

l.b) Partition contact area — Additional thermal

resistance occurs at partition contact areas:

Resistance of unobstructed roof area = 16.41 Wood partition resistance = 0.94 Air film obstruction = .50 17.85

U = .056

l.c) Bathroom vent — The bathroom vent pipe

forms a thermal bridge from the inside to the outside through the metal of the duct pipe.

From considerations of air films, the thermal

transmittance is estimated to be Li = 0.5.

2. a) Unobstructed wall area:

At Insulation At Framing

Inside Air Film 1.07 1.07 Wood Paneling, Vs in 0.16 0.16 Wood Framing, SVi in 4.38

Insulation, R-7 7.00 Air Space, in 1.01 Siding, Aluminum 0.00 0.00 Outside Air Film .68 .68 9.89 6.29

Area Storm Windows

No Storm Windows

Exposed glass 5.22 0.44 0.73 Sheer curtains 24.26 0.40 0.64 Opaque curtains 4.69 0.37 0.56 Draperies plus

curtains 39.34 0.31 0.44 73.51

Totals UA = 25.93 39.27

Weighted U = .353 .534

The sheer curtains have patterned holes and

add a thermal resistance to the window area by

a decrease in the radiative transfer and a de- crease in the convective air flow. Assuming

that the opaque area of the sheer curtain reduces the radiative transfer by 1/3 and the

curtain reduces convective transfer by 10 percent, the inside surface resistance is then 0.86

or an incremented resistance of 0.18. This gives

U = 0.4.

The opaque curtains are assumed to reduce

radiative transfer by 50 percent and the convective transfer by 25 percent, giving an inside

surface resistance of 1.09 or an increase of 0.41.

49
This gives U = 0.37. The combination of draperies and curtains are assumed to give an incremented resistance of 0.481, due to the thermal resistance of the drapery material

(k = 0.26, Vs).

This gives U = 0.31.

3. b) Floor area obstructed by bed — The area under

beds is considered to have increased thermal

resistance because of the air space trapped by

hanging bedspreads, and the resistance of the

mattress. For the series and parallel heat- flow paths, analysis shows that the equivalent

U - value is

The above calculations are for storm windows.

A similar procedure is followed for windows

without storm windows assuming L7 = 0.73

for exposed glass.

2.e) Front door with storm door;

Inside surface 1.07 Door with storm door 2.18 Outside surface 0.68 T93

U + Umam + Ui,a\,

Uffi = U - value for mattress = 0.05

Ujy = U - value for bedspread = 0.685

U =U -value for unobstructed floor = 0.0797

a = ratio of area of component to area of

floor under beds

or L7 = 0.25

2.f) Rear door:

Inside surface Door

Outside surface

1.07

1.19 0.68 2.94

or L7 .34

3. a) Unobstructed floor area:

At Insulation At Framing

inside air film 3.83 3.83

carpet/vinyl flooring 1.20 1.20 (see note) subfloor, % in 0.98 0.98

joist, 5V2 in 6.88

air space, 4 in 1.23 insulation R-4 4.00 2.00 simplex Board 0.10 0.10 outside air film .92 .92 Totals 12.26 15.91

Weighted UA =

307.76 33.57 = 27.21

12.26 15.91

U = 27.21/341.34 = .0797

Note: Averaging individual resistance for car- peting (192.88 ft^ and vinyl flooring

(148.46 ft2) is

2.08 X 192.

J

.05 X 148.46 341.34

1.20

W

(0.05 X 1.0 + 0.685 X 0.43) 0.0797

0.0797 + 0.05 + 0.685 x 0.43 0.0647

3. c) Floor area under stove — The stove area is considered to have increased thermal resistance,

using the rationale of 3.b, L7 = 0.072

3. d) Floor area under couch and stuffed chairs —

using the rationale of 3.b, U = 0.068.

3. e) Partition contact areas

of l.b, U = 0.071.

using the rationale

Heating ductwork in floor — Heat loss from

duct work occurs through insulation (R-4) and

bottom board downward into the crawl space.

Insulation R-4 = 4.00

Simplex board = 0.10

or !J = 0.20

Closets and cupboards — Heat flow through

closets and cupboards eventually flows through

the walls, ceiling, and/or floor. These areas are not equal to areas of closets and cupboards ex- posed to interior environment of the mobile

home. In order to calculate the heat flow through

these areas, it was necessary to develop an equation based on a heat balance into and out of the

closet and cupboard areas such that

Q

{t, ~ to) + C (to - tc) D

where

50
D =

A,U, + A2U2 + + A^U^ + + A^Uf^ + AyUy + /4„L/s (A, Li. + A,U, + A,U,) (A^U, + A,Us + A,U, + A,U, + A,U,)

and

^3

Subscripts 1, 2, and 3 refer to properties of the

walls, ceiling and floor, respectively. Subscripts

4, 5, 6, 7, and 8 refer to the five possible sides of the enclosure (closet or cupboard) exposed to the

interior temperature, and

f;' = interior temperature

tg = outside temperature

fc = crawl space temperature

For the 16 enclosures in the mobile home, values

of D and C were calculated. The sums are

51
Table A-1. CALCULATION FORM FOR AREA THERMAL TRANSMIT!ANCE

U Area UA

Btu/h ft^ F Ft^ Btu/h F

1. Roof Areas (552 ft^)

a. Net roof area

b. Partition contact area

c. Bathroom vent

Total

2. Wall Areas (812 ft^)

a. Net wall area

b. Wall with drape and valance

c. Partition contact area

d. Window area (storm windows)

e. Front door with storm door

f. Rear door - no storm door

Total

3. Hoor Areas (522 ft^)

a. Net floor area

b. Under bath tub

c. Under beds

d. Under stove

e. Under couch & stuffed chairs

f. Partition contact area

Total

4. Heating Duct Work In Floor

5. Closets & Cupboards

(ti - to)

(to - tc)

Conduction Heat Loss — Q(-= Sum of lines 1-6.

1. 31.97 (tic - to)

2. 96.96 (tj-^v - tg) 110.36 (tj^y — tg) (no storm windows)

3. 33.79 (tif - tc)

4. 11.0 (td - tc)

5. 20.2 (t,w - to)

6. 7.09 (to - tc)

where tjc = interior air temp near ceiling

tjyv = interior air temp near mid-height

tjf = interior air temp near floor

td ~ average heating duct air temp

tc = air temperature in crawl space

to = outside air temp

Infiltration Heat Loss — See section 4.1.

Q, = 55.8 I (ti - to)

I = infiltration rate, air changes per hour

0.060 495.09 29.70

0.056 13.72 0.77

0.5 3.00 1.50 31.97

0.109 544.37 59.88

0.099 20.17 2.02 0.094 9.33 0.89 0.353 73.51 25.95

0.25 16.55 3.97

0.34 13.71 4.32 96.96

0.079 341.34 25.60

0.070 10.22 0.72 0.065 50.25 3.12 0.072 5.00 0.35 0.068 22.20 1.46 0.071 13.72 0.93 33.79

0.20 55.00 11.00

20.20

7.09

52
APPENDIX B

RATIONALE FOR DETERMINING THERMAL

RESISTANCE OF INTERIOR SURFACES

Initial thermal analyses of the mobile home indicated

that the resistance of the interior air film was greater

than design values published in the ASHRAE Handbook of Fundamentals. This is logical because large

portions of the interior wall, ceiling, and floor surfaces face other surfaces that are nearly identical to the temperature of the surface in question. Since

conductance across a surface air film is composed of both convective and radiative heat transfer, and since

radiative transfer does not occur between surfaces of equal temperature and emittance, the interior surface conductance is decreased under these conditions.

The ASHRAE design surface conductance value for

vertical surfaces* is lested as 1.46 Btu/h ft^ F, and consists of 0.54 transferred by convection and conduction, and the remainder, 0.92, transferred by

radiation. Obviously, this is a conservative value for mobile homes where surfaces of exterior walls, floor and ceiling face many similar surfaces of nearly equal

temperature. For example, the living room and

kitchen and the rear bedroom have five sides with

surfaces of exterior walls, floor and ceiling. Each of these five surfaces faces four surfaces with nearly the same temperature, with only one warmer surface, an

interior partition wall. The radiative transfer component of the film conductance is small under

these conditions.

A paper by F. W. Hutchinson, "A Rational Reevaluation of Surface Conductances for Still Air," (ASHRAE

Transactions, Vol. 70, 1964, p. 105) deals with this subject. In this reference, the following surface con- ductances are proposed based on the number of room

surfaces where outside surfaces are exposed to outdoor temperatures.

Position of Surface

Direction of Heat flow Num

1

jer of

2 room

3

surfa 4 cesex

5 posed6

Horizontal

Vertical Horizontal

Upward

Horizontal Downward

1.43

1.21 0,57

1.17 0.96 0.371.01 0.80

0.28

0.88 0.68

0.22

0.78

0.60

0.19

0.71 0.59 0.16

These values were used as a guide in estimating the

surface conductance for different areas of the mobile

home. Obviously, in a furnished mobile home, some

adjustments must be made for furnishings which are considered to be at or near room temperature, and for the heating effect on floor surfaces of the heating

duct. The following surface conductances were used

*ASHRAE Handbook of Fundamentals, p. 357 (1972).

to calculate predicted heat losses for the mobile

home:

A. Walls (vertical surfaces, heat flow horizontal)

1. Living Room and Kitchen.

Five room surfaces are exposed to the outside. Adjustment was made for cupboards,

furniture, heated floor duct areas. Conductance estimated to be 0.90 Btu/h ft^ F.

2. Hallway.

Three sides exposed, but interior non- exposed wall is large compared to exposed

floor and ceiling areas. Conductance esti- mated to be 0.96.

3. Rear Bedroom.

Five sides exposed. Adjustment was made

for bed, closet and heated floor duct areas. Conductance estimated to be 0.90.

4. Bathroom.

Three sides exposed. Adjustment was made

for tub alcove, lavatory cabinet and furnishings. Conductance estimated to be 1.08.

5. Center Bedroom.

Three sides exposed. Adjustment was made

because of bed area. Conductance estimated

to be 1.08.

In order to calculate total heat loss for the mobile

home, it was decided to average the surface resist- ances based on areas involved so that one interior surface resistance could be used for all walls. This was done as follows:

Location % of Total Wall Area

Surface ConductanceWeighted

Average

1. Living room

and Kitchen

46 0.90 0.41

2. Hallway 19 0.96 0.18

3. Rear Bedroom 24 0.90 0.22

4. Bathroom 4 1.08 0.04

5. Center Bedroom 7 1.08 0.08

.93

R = 1.07

B. Ceilings (horizontal, heat flow up):

1. Living Room and Kitchen.

Five surfaces exposed. Adjustment made for

53
cupboard and furniture area. Conductance

estimated to be 0.95 Btu/h ft^ F.

2. Hallway.

Three surfaces exposed. No adjustments.

Conductance estimated to be 1.01.

3. Rear Bedroom.

Five surfaces exposed. Conductance esti- mated to be .95.

4. Bathroom.

Three surfaces exposed. Adjustments made

for limited exterior wall area and for furnishings. Conductance estimated to be

1.17.

5. Center Bedroom.

Three sides exposed. Adjustment made for bed area. Conductance estimated to be 1.17.

Average resistance of ceiling surface was calculated

as follows:

Location % of Total Surface

Ceiling Area Conductance

Weighted

Average

1. Living room

and kitchen 49 0.95 0.466

2. Hallway 9 1.01 00.091

3. Rear bedroom 17 0.95 0.162

4. Bathroom 12 1.17 0.140

5. Center bedroom 13 1.17 0.152

1.011 R = 0.99

C. Floors (horizontal, heat flow down):

1. Living Room and Kitchen.

Five surfaces exposed. Adjustment made for furnishings covering floor and for cup- board areas. Conductance estimated to be

0.22 Btu/h2 F.

2. Hallway.

Three surfaces exposed. No adjustment

made. Conductance estimated to be 0.28.

3. Rear Bedroom.

Five surfaces exposed. Adjustment made for

closet area on one exterior wall. Conductance estimated to be 0.22.

4. Bathroom.

Three surfaces exposed. Adjustments made

for cupboard area furnishings. Conductance estimated to be 0.37.

5. Center Bedroom.

Three surfaces. Adjustments made for closet area on one exterior wall. Conductance

estimated to be 0.37.

Average resistance of floor surface was calculated as follows:

Location % of Total Floor Area

Surface ConductanceWeighted

Average

1. Living room

and kitchen 53 0.22 0.117

11 0.28 0.031 14 0.22 0.031 14 0.37 0.052

8 0.37 0.030

0.261

R = 3.83

Based on the above calculations, the surface resist- ances used to calculate wall, ceiling and floor 'W

factors were as follows:

1. Wall area R = 1.07 h ft^ F/Btu

2. Ceiling area R = .99

3. Floor area R = 3.83

54

U.S. GOVERNMENT PRINTING OFFICE: 1978 0—248-989
NBS-1 14A (REV. 1 1-77)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBS BSS 102

2. Gov't Accession

No.

3. Recipient's Accession Nu.

TITLE AND SUBTITLE

The Thermal Performance of a Two-Bedroom Mobile Home

5. Publication Uate February 1978

6. Performing Organization Code

7. AUTHOR(S)

G.J. Teitsma and B.A. Peavy

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Proiect/Task/Worl< Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

The Federal Energy Administration

1200 Pennsylvania Avenue, N.W,

Washington, D. C. 20461

13. Type of Report & Period

Covered

Final

14. sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 77-608167

16. ABSTRACT (A 200-word or less factual summary of most si^ifleant information. If document includes a significant

bibliography or literature survey, mention it here.)

Tests were conducted on a mobile home located in an Environmental Climatical Laboratory for the purpose of evaluating its thermal performance. The heating demand

greatly affected the part-load efficiency of a gas-fired, forced-air, sealedcombustion furnace system. The practice of installing oversized heating plants

was shown to result in low seasonal operating efficiencies. Air leakage measurements were performed using a pressurization technique to quantify the amount of

air leakage through the various parts of the mobile home. Separate air infiltration tests using the SF^ tracer-gas technique showed that somewhat higher air

infiltration rates were induced by operation of the mobile home heating plant.

A thermographic survey of interior surfaces showed that the technique used to

install the wall insulation may allow wrinkles formed in the surface of the insulation to form air paths running the height of the wall cavity. Convective air

flow through these paths may create heat leaks on the building surface which can have an impact on the overall heat-loss rate. Separate tests were also conducted

to identify places in the mobile home envelope having high condensation potential.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Air infiltration; energy conservation; mobile home; part-load efficiency;

thermography.

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