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NATIONAL SUtEAU OF STANDARDS REPORT

WiS PROJECT

42103-40-4215101 May 11, 1966

NBS REPORT

9309

THERMAL INSULATION IN BUILDINGS

Frank J. Powell

Environmental Engineering Section

Building Research Division

Institute of Applied Technology

NOT FOR PUBLICATION

Preprint for the May 23, 1966

meeting of

The Society of American Military Engineers

at the National Bureau of Standards

Gaithersburg, Maryland

NATIONAL BUREAU OF ST

for use within the Government, and review. For this reason, thi whole or in part, is not authoi Bureau of Standards, Washingti

the Report has been specifically

IMPORTANT NOTICE

Approved for public release by the

Director of the National Institute of

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on October 9, 2015.

ss accounting documents intended

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the Government agency for which

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U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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Thermal Insulation in Buildings

by

Frank J. Powell

1 . INTRODUCTION

The usefulness of thermal insulating materials for making buildings

more comfortable has been recognized since the time of primitive man.

Typical examples of applications range from the insulating effect of the

snow of an Eskimo's igloo to the shade-cooling relief offered by a

thatched roof in the tropics.

The purpose of this paper is to review and discuss from the

viewpoint of the engineer the presently used thermal design procedures

including practical limitations, the environmental and applications

factors that influence the performance of a thermal design and some

current research problems that involve insulated constructions. Also,

the benefits of insulated constructions and their importance to a nation

will be mentioned.

2 . DESIGN PROCEDURES

The engineer and architect must produce a thermal design for a

building shell that is compatible with the structural, architectural,

mechanical and electrical designs of the building. Sometimes the requirement for compatibility restricts the designer in regard to dimensions and

space available for insulation and must be recognized from the start.

Also, a predetermined cost limitation may become a primary consideration

in the production of a thermal design.

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The usual thermal design approach is to view the building shell as

an insulating envelope whose walls, roof, and floor retard the flow of

heat into or out of the building and provide a protective barrier from

the elements. The thermal resistance of each building component or

element to the flow of heat is determined using published or measured

coefficients of thermal conductivity and conductance for candidate building

materials and insulations, and selected dimensions of the components.

Several alternative designs are usually possible and heating and cooling

loads for the building should be calculated for each design. Loads can

then be compared and used as the basis for selecting the best design for

the building, rather than using a comparison of individual coefficients

of heat transmission, or U-values, because the total load includes

infiltration and ventilation losses which can vary with the individual

thermal design. The calculation procedures for a single design are given

in detail in reference [l] and are well known to engineers responsible

for the design or sizing of mechanical equipment for heating or cooling.

The currently used design procedure is largely based on a steadystate heat transfer concept. The thermal coefficients for insulation and

building materials as used by the designer are given in materials

specifications that are based on steady-state test methods. The procedure

thus presumes static or steady thermal conditions when in fact the thermal

behavior of a building in use is quite dynamic. Outdoor temperatures

rise and fall each day and the response of the building, depending upon

its ability to store heat, lags the weather so that a truly steady-state

heat flow situation rarely exists in a building. A suitable, simple

design procedure that more closely matches the dynamic character of

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buildings would allow the engineer to more closely predict the thermal

performance and loads of a building.

The design procedure sometimes demands as much art as science

because considerable latitude for judgement and experience is evident in

the procedure. Two general design solutions for providing a desirable

indoor environment can be used. The more usual practice is to provide

temperature and humidity control by using heating, ventilating and airconditioning equipment in whatever quantity is necessary, sometimes at

large operating costs. Alternatively one can design the building shell

to greatly retard the flow of heat. The latter solution is preferred

not only because the lowered costs for heating and air-conditioning more

than offset the additional cost of insulation but because a well-insulated

building provides a more comfortable and more easily controlled indoor

environment. A typical example of the utilization of well-insulated

building shells is in the application of electric heating and cooling.

However, the principles of good thermal design are applicable regardless

of the source of energy for heating and cooling be it electric, gas, oil,

coal, or atomic energy.

3. ENVIRONMENTAL FACTORS

The engineer must carefully assess both the indoor and outdoor

environments for their bearing upon his thermal design.

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Indoor temperature and relative humidity are most important factors.

For example , too little insulating effect in a construction in winter

could produce indoor surface temperatures that are below the dewpoint of

the indoor air resulting in the formation of surface condensation. A

typical case is condensation on single glass windows in cold climates.

The typical solutions for this case are to lower the indoor dewpoint, or

to increase the interior glass surface temperature by raising the indoor

temperature or by using double or triple glass windows.

The question of how much insulation should be used faces the

engineer for each design. Economic and dimensional considerations aside,

the bare minimum should always be sufficient to prevent surface condensation.

However, use of minimal amounts of insulation sometimes produces uncomfortable rooms because cold walls, ceilings, or floors promote cold

drafts and increase the amount of heat transferred by thermal radiation

from occupants to the cold building surfaces even though the air temperature

at or near the thermostat is maintained at the design value. The maximum

amount of insulation that can be used is usually controlled by the

practical limitations of the dimensions and thicknesses of the construction.

A guide for design with a view toward comfort of the occupant is that t’..

amount of insulation used be sufficient to restrict the average difference

in temperature between the indoor air and the surface of exposed walls,

ceiling, and floor areas, figured at the selected indoor and outdoor design

temperature conditions. The suggested average difference in temperature

should not exceed from 5 to 10 degrees F, the lower the better. This

average value usually cannot be achieved for windows in cold climates,

so that the percentage of glass area must be controlled or draperies or -4-
window coverings must b© used to maintain comfort if large areas of glass

ars involved. The suggested design guide will usually prove to be

economically justifiable,

A water vapor pressure difference usually exists between the

interior and exterior of a building. Ibis pressure difference causes

wafer vapor to migrate foilwing a path of least resistance from the

highest to the lowest pressure. Since most building materials are

permeable to water vapor one means by which vapor may enter the construction

is by the process of diffusion of vapor through the building materials,

in winter, if water vapor permeates the construction, condensation and

accumulation of water or frost on cold areas can occur. However, widespread use of vapor barriers such as paint films or membranes which often

are an integral part of the insulation, has largely controlled the

accumulation of concealed condensation via this process. A second, and

much faster, means for water vapor to enter a construction is by its

transport in air flows into the construction via avenues opened by cracks

under baseboards, passages in walls between a crawl space and an attic

space, openings around pipes, ducts, electrical fixtures, trap doors, etc.

Natural convection forces arising from indoor-outdoor temperature differences,

or wind forces, promote large flows of air if even small passages exist.

Large amounts of moisture can be transported to cold regions by this

means, often leading to excessive condensation and accumulation of moisture.

For example, 100 cubic feet of air at 70® F and 35 percent relative humidity

will release about 150 grains of moisture if its temperature is lowered to

20^ F. A one-square inch passage through a ceiling at a light fixture can allow, into an attic overhead, an air flow of more than one cubic foot

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per minute, which in one month can deliver to the attic as much as 9

pounds (over one gallon) of water condensable at 20® F. Much can be done

by the designer and applicator to alleviate this source of damage by

requiring simple stuffing and taping of holes to block convection air

paths

»

The susceptibility of the design of the insulated construction to

water ingress is very important . Water staining of painted surfaces,

dripping from ceilings, and similar damage poses the problem as to whether

the source of the moisture is from an accumulation of condensate originating from indoors or from an exterior roof or wall rain leak, or water

from leaking pipes. Outdoor conditions of wind-driven rain, solar-heating

and night-time cooling, freezing and thawing, etc., are forces that help

deteriorate outdoor surfaces allowing ingress of rain or melt water that

wets insulation and damages other building materials. Ideally, in winter,

water vapor that does enter the building shell from indoors should be free

to pass outdoors without condensing and at the same time the outdoor

surface should prevent rain or blowing snow from entering the construction.

The effects of possible improper application of insulation and

building materials on the performance of the thermal design should be

evaluated. For example, storage of insulation in the open where it can

get wet and subsequent use of the wet insulation between a vapor barrier

material and built-up roofing greatly reduces insulating value and ultimately

deteriorates the roofing. Installation of insulating materials with the

vapor barrier on the cold side instead of the winter-warm side of heated

buildings invites accumulations of moisture. Gaps, tears, fishmouth

shaped apertures at the edges of insulation that allow convection of air

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within the construction should be assessed. An inspection should be made

at the site during construction to assure that insulation and building

materials are installed according to specification or manufacturers

recommendations

.

4. RESEARCH PROBLEMS INVOLVING THERMAL INSULATION

All insulations will produce the insulating value expected of them

if they are properly installed and maintained in a dry condition.

There are available to the designer materials specifications, methods

of test, and products of good quality so that the basic technology of

proper definition of insulations and of means to assure quality materials

has been developed. Practically all problems involving insulation can

be classified as problems that deal with components of construction and

buildings where several building materials are combined and used

simultaneously rather than problems of individual materials. For

example, design procedures are needed that simultaneously account for

the transient nature of heat transfer as well as air and moisture

transfer, and allow a designer to be able to predict the performance

of his design for a variety of exposure conditions. A standard

method of measurement that simultaneously includes determination of

heat flow, moisture flow and accumulation, air flow, wetting from

rain, and expansion and contraction has not been developed. Some

efforts along these lines are currently being pursued at the

National Bureau of Standards. Performance specifications that are

based on the transient nature or periodic behavior of a building or

construction rather than steady-state tests on individual materials

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need to be developed. For example, recently a laboratory method of

measurement for insulated flat roofs containing moisture was developed

at the NBS f2], and 73 designs were evaluated. From this work, a

proposed test specification and criteria for roofs that should dry

out in place and recover insulating value, if wetted, was developed.

The self-drying principle appears practicable and may save the high

cost involved in complete replacement of wet insulation and roofing.

The performances of designs produced using such criteria need to be

checked in the field on full-size roofs to confirm the principle and

allow a performance specification to be written that is based on both

laboratory data and full-scale field experience.

Improved in-situ methods to measure the performance of a building

and its elements in respect to heat flow, moisture content, expansions

and contractions, deteriorations, cracking, corrosion, mechanical

systems performance, and acoustical behavior, need to be developed and

used to provide a basis for characterization and definition of a

realistic performance standard and for evaluating the engineering

behavior of a building. The Building Research Division is presently

in the planning stages for one such endeavor.

Other technical problems and their solutions useful to the designer

have been completed or are being considered. For example, the problem

of outdoor columns on high rise buildings expanding when heated by

the sun and contracting when cooled which produces differential movements in respect to the constant temperature inside columns. An

analytical solution, programable in a computer, was prepared at NBS [3],

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that provides, for the first time, a means to predict temperatures

within the outdoor column which must be known quantitatively before

an evaluation of potential movement can be made. Also, a study is

under way to help make the design decision as to just where in the

thickness of an exposed building wall the insulation can best be

located. It may be advantageous in terms of peak air-conditioning

cooling loads to place insulation outboard of a masonry wall rather

than at the usual inboard location. The large heat capacity of the

masonry wall should stabilize the indoor air temperature and the

masonry would not be subjected to daily and seasonal temperature

extremes and to weather and water, all of which contribute to deteriora

tion

.

A need exists to critically evaluate the total indoor environment.

Factors other than temperature and relative humidity affect indoor

environment. Generally, criteria for indoor environmental conditions

are needed and stem from human needs for well-being and comfort, but

necessarily involve auxiliary criteria for operational equipment,

systems, and functions of the building. Optimal human comfort and

performance depends upon an integrated system that provides temperature

and humidity conditions, light, air, desirable acoustics, communication

and transportation, for which performance standards are needed. Thermal

insulation will probably play an important role in the development

of such standards.

9
5. BENEFITS OF THERMAL INSULATION

Much has been written about savings in fuel or energy for heating

or cooling for a fully insulated house as compared with a similar but

uninsulated house. It is not difficult to show a reduction in heat

transfer of over 50 percent. Thus, though the initial cost of insulation

requires an outlay of money the investment rapidly pays for itself in

fuel or energy savings and becomes very worth-while for the consumer.

An interesting example of the impact of the economics of this is

illustrated by what was done in 1957 in Great Britain [4], Because

Britain must import much fuel, energy costs are high. Huge national

savings were estimated to be forthcoming by restricting the loss of

heat and thus a law was passed making it mandatory to provide insulation in the roofs of all new industrial construction. The importance

of this concept was rather forcibly brought home during this past

unusually severe winter in England during which the electrical capacity

of the country was overburdened through the use of supplementary

electrical heaters, etc., to the point where several power plants

had to be temporarily shut down.

Perhaps as important as monetary economy is the improvement of tne

indoor environment and comfort conditions that can be achieved by the

use of insulation. This aspect is difficult to assess in terms of

dollar benefits. First the problem of controlling temperature and

relative humidity becomes simpler because the magnitude and fluctuations

of the loads are reduced. Secondly, the differences in temperature

from floor to ceiling and throughout the room are reduced and uncomfortable

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sensations experienced because of cold walls and drafts can largely

be eliminated.

On a national basis , energy conserved by the use of insulation

will help to conserve our fossil fuel reserves which have been predicted

to have finite life-times, and to restrict entry of their products of

combustion into the country's atmosphere.
6. REFERENCES

[lj Guide and Data Book, Fundamentals and Equipment, 1965-1966,

Chapters 24-27, American Society of Heating, Refrigerating and

Air-Conditioning Engineers, New York, New York.

[2] F. J. Powell and H. E. Robinson, Measurement of Effect of Moisture

on Heat Transfer Through Insulated Flat-Roof Construction,

Special Technical Publication No. 312, Symposium on Methods of

Testing Building Constructions, American Society for Testing and

Materials, 1961.

[3] B. A. Peavy, Steady-State Heat Conduction in an Exposed Exterior

Column of Rectangular Cross Section, J. Research NBS, C.

Engineering and Instrumentation, Vol. 69C, No. 2, April-June (1965)

[4] Thermal Insulation (Industrial Buildings) Act, 1957; Thermal

Insulation (Industrial Buildings) Act, 1957, Explanatory

Memorandum; Thermal Insulation (Industrial Buildings) Act, 1957,

Thermal Insulation (Industrial Buildings) Regulations, 1958,

S.I. 1958 No 1220; Her Majesty's Stationery Office, London, England

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