ENERGY RESEARCH FOREST PRODUCTS LABORATORY 

PAPER FOREST SERVICE 

EFFlClENCY FPL 317 U.S. DEPARTMENT OF AGRICULTURE 

MADISON, WIS. 

IN LIGHT-FRAME 

WOOD 

CONSTRUCTION 1979 
ABSTRACT 

This report presents information needed 

for design and construction of energyefficient light-frame wood structures. The 

opening section discusses improving the 

thermal performance of a house by careful 

planning and design. A second section of the 

report provides technical information on the 

thermal properties of construction materials, 

and on the basic engineering design principles applicable to light-frame wood structures. Moisture condensation problems are 

discussed in relation to the effects of increased building insulation and more effective air leakage control. 

NOTE 

This paper replaces U.S. Forest Products Laboratory Report 1740, “Thermal insulation made of wood base materials–its application and use in houses” (L. V. Teasdale, 

1949) and U.S. Forest Service Research Paper 

FPL 86, “Thermal insulation from wood for 

buildings: Effects of moisture and its control” (Wayne C. Lewis, 1968). 

ACKNOWLEDGMENT 

The following tables have been adapted 

with permission from the 1977 Fundamentals 

Volume, ASHRAE Handbook & Product Directory: tables 1, 2, 2A, 3, 4, 6, 6A, 11, 12, 13, 

14, 15, 15A. 

Drawings in figure 3 are reprinted with 

permission from Solar Dwelling Design Concepts, prepared by the AIA Research Corporation for the U.S. Department of Housing 

and Urban Development, May 1976 under 

Contract IAA H-5574. 

Acknowledgment is made to the following for additional material adapted in certain 

tables: National Research Council of Canada; the 1977 Minimum Property Standards 

of the U.S. Department of Housing and Urban 

Development. 

i 
CONTENTS 

Page 

Objectives and Scope 1 

Architectural Design for Energy Efficiency 2 

Building code considerations 2 

Integrated systems design 2 

Energy balance considerations 3 

Heat losses and gains 3 

Zoning for energy balance 3 

Utilizing solar gain 6 

Window management 6 

Windbreaks and air leakage control 7 

Human comfort considerations 7 

Further design perspectives 10 

Engineering Design and Construction 11 

Modes of heat transfer 13 

Thermal performance of material 15 

Thermal resistance and capacitance 15 

Building insulation 15 

Environmental conditions 28 

Reflectance and surface effects 35 

Heating and cooling load computations 35 

Design temperature 35 

Heat loss computations 44 

Heat gain computations 46 

Thermal performance of assemblies 48 

Enhancing thermal performance 48 

Code requirements 54 

Summary 55 

Literature Cited 57 

i i 
ENERGY EFFICIENCY IN LIGHT-FRAME 

WOOD CONSTRUCTION 

By

GERALD E. SHERWOOD, Engineer 

and 

GUNARD E. HANS, Architect 

Forest Products Laboratory, Forest Service 

U.S. Department of Agriculture 

Recent changes in building code requirements and construction standards have 

made the poorly insulated house obsolete. 

Compared to past building practices, the 

modern house is more compactly planned, 

better insulated, and weatherstripped to 

minimize heat losses associated with air 

leakage. These steps toward energy conservation are relatively easy to accomplish in 

new construction, although there are practical limits to energy savings by insulation 

alone (10).2 The interactions between 

climatic and occupancy factors must also be 

considered, as well as relationships between 

landscape design and construction. 

OBJECTIVES AND SCOPE 

This report provides information helpful 

for the design and construction of energyefficient light-frame wood structures that are 

scaled to human needs. 

The opening section of this report 

stresses the role of housing design and landscape planning in building energy efficient 

homes. Topics discussed include orientation 

of the home, placement and design of windows, general configuration of the house for 

1 Maintained in Madison, Wis., in cooperation with the 

University of Wisconsin. 2 Numbers in parentheses refer to literature cited at end 

of report. 

minimal heat loss in winter, and architectural 

strategies for comfort through natural use of 

sun and shade. The role of landscaping is 

also discussed as a mode of natural energy 

conservation. Strategic placement of shading vegetation, and the use of plants or 

fencing to alter wind-flow patterns, can 

reduce heat loss in winter, or minimize need 

for air conditioning in the summer. In addition, human comfort is discussed as an important criterion in home construction. 

The second major part of this report 

deals with engineering for energy efficiency. 

The section begins with a discussion of 

basic principles of heat transfer. Also 

discussed are performance characteristics 

of insulating materials, effect of alternative 

wall constructions on energy efficiency, 

calculations to determine the heat transmission of structures, and a discussion relating 

infiltration control and insulation to moisture 

condensation in the structure. Energy-efficient design is also related to current standards and building codes. 

Undoubtedly, most houses will continue to 

be designed on the basis of code requirements or rules-of-thumb rather than a detailed engineering analysis. Nevertheless, 

discussion of the more detailed analytical 

design procedures underlying present code 

requirements can be useful. Such a discussion may assist the builder and designer in 
providing energy- and cost-efficient construction above the level of minimum code 

requirements. 

One should realize that the overall 

energy efficiency of a house is also greatly 

influenced by the efficiency of its heating 

system and controls. Proper sizing of furnace 

or boiler, correct adjustments for the heat 

distribution system, and good burner maintenance are important. They can increase 

fuel conversion efficiency well above that encountered in most houses which have not 

received the needed attention in design or 

upkeep. Discussion of heating systems,. 

however, falls beyond the scope of this 

paper. 

ARCHITECTURAL DESIGN FOR 

ENERGY EFFICIENCY 

“Architectural design” is not confined 

to appearance factors but is used here in its 

wider sense, as applicable to all design considerations. The term covers not only the arrangement of building masses, but also the 

relationships of a structure to its surroundings, the climate of the locale, and the sun. 

Human comfort requirements and occupancy patterns also influence design. These 

factors all affect the actual energy consumption in a home under occupied conditions. 

Building Code Considerations 

Minimum performance requirements for 

houses are defined by building codes. However, building codes can only partially influence energy usage in houses, because 

code requirements apply primarily only to 

construction specifications, and thus affect 

houses only under essentially unoccupied 

conditions. Occupant responsibilities such 

as ventilation and maintenance of heating 

equipment, which in well-insulated homes 

emerge as key considerations, have remained outside building code jurisdiction. 

Thermal performance codes serve to reduce energy usage without sacrificing living 

standards. In some instances, code requirements compromise between performance desired and performance attainable 

using common and cost-effective construction methods. Thus the knowledgeable 

designer should use code requirements only 

as rules-of-thumb, recognizing that meeting 

code requirements is only the first step in 

energy efficient designs. 

Many energy-saving improvements may 

be made in a home beyond meeting the minimum code requirements, and often at 

modest expense. Fuel costs are likely to continue rising more rapidly than the general inflation rate, making any investment in energy 

efficiency features attractive. However, 

some options may yield a higher return than 

others. When designers or builders seek new 

ways for increased energy efficiency, their 

decisions must be based on both construction costs and expected operating costs. 

Energy efficient construction may appear costly, but seeming difficulties of 

amortizing it over a relatively short time 

period do not necessarily prove it to be a bad 

investment–such comparisons show only 

that our priorities in apportioning construction costs lie elsewhere. The amortization of 

a house is related to its mortgage life, and 

any reasonable energy efficiency measure 

should start paying handsome dividends well 

before the end of that period. 

Integrated Systems Design 

Current house design practices still 

reflect indifference to energy waste of past 

years, when low fuel costs permitted compensation for design deficiencies by brute 

force with oversized heating and cooling 

equipment. However, mechanical systems 

are operating at only part load conditions for 

most of the time. These systems must be 

correctly sized and maintained to assure 

their long-range efficiency. 

Residential heating systems convert primary fuels to usable heat with an efficiency 

well below what is theoretically possible. For 

example, improved furnace design may result in far greater efficiencies than would improvements in construction quality. Discussion of mechanical systems falls outside the 

scope of this publication, but the designer 

must consider the relationships between 

heating system and home design. For instance, an outdoor air intake for the heating 

2 
plant that prevents the loss of warm room air 

up the chimney may be more cost-effective 

than many other improvements. 

For increased operating efficiencies, 

the house and its heating plant must be 

designed as an integrated system. The ultimate objective is not to heat the house, but 

to warm its occupants. Moreover, choice of 

heating system involves much more than 

merely the output of the unit. Different types 

of systerns–e.g., hot water, warm air, electric radiant panels–and different control 

systems can lead to different comfort conditions in homes of otherwise similar design, 

and with differing degrees of energy- and 

cost-efficiency. 

Energy Balance Considerations 

Effective use of energy from alternate 

sources–primarily the sun–can significantly reduce fuel usage. Performanceoriented building codes reward design which 

utilizes solar or other nondepletable sources. 

Seasonal solar benefits can be estimated accurately through computerized performance 

simulation. However, solar benefits can also 

be estimated based on simplified design 

methods (1, 13, 16) 

The heat losses of a house are compensated by energy gains from three sources: (a) 

from the heating plant, (b) from the sun 

through windows, and (c) from appliances 

and occupants as waste heat (i.e., “internal 

gain”). (Heat gained from a fireplace or a 

wood-burning stove is included here under 

(c) because it assumes occupied conditions.) 

To reduce fuel consumption, the heating 

system should operate at the lowest possible level consistent with the comfort of the 

occupants. Solar and internal heat gained 

during the day can permit cutbacks in the 

operation of the heating system, provided 

that the house design allows for a reasonable 

temperature distribution throughout occupied areas. South-facing windows can supply significant amounts of heat. 

The energy balance design concept, 

therefore, considers the total magnitude and 

distribution of heat gains and losses. 

3 

Heat Losses and Gains 

An estimated energy balance for a 

24- x 48-foot reference house with southerly 

orientation, under conditions typical for 

Madison, Wis., is given in figure 1. Heat loss 

computations were based on construction 

quality suggested by “The Arkansas Story” 

(11). The window areas, however, have been 

allowed to remain at a 10 percent glass-tofloor area ratio typical of past design practices. Solar gain is estimated using data from 

National Bureau of Standards (NBS) publication BSS 96 (4), and internal gain schedules 

follow projections of NBS BSS 57 (12). 

The 100 percent utilization of solar and 

internal gain assumed in figure 1 is obviously 

unattainable; a 67 percent level could represent a far more reasonable assumption. On 

the other hand, fuel usage is also likely to be 

lower at reduced thermostat settings, which 

are becoming increasingly common, than at 

the code-required 72° F temperature that 

serves as the basis for these calculations. 

Due to the wide range of possible variables in 

occupancy patterns, the estimates of potential solar and internal gain credits have 

also been allowed to stand at the 100 percent 

utiIization level. 

For the energy balance depicted in figure 1, the potential solar and internal gain 

credits can reduce the demand on the heating system to only one-third of the cumulative seasonal heat loss. Undoubtedly, the 

solar gain will vary greatly with different localities, and the internal gain level may be 

considerably reduced by choice of more 

energy-efficient household appliances. 

Nevertheless, these credits are often sufficient to make them important to more 

energy-efficient design. 

Zoning for Energy Balance 

Home planning to follow the daily path 

of the sun represents sound, energy-efficient 

zoning (fig. 2). Solar gain and internal gain 

sources (major household appliances) are 

separated to permit more even temperature 

distribution throughout the house during the 

periods when the heating plant is not operating. Internal gain sources are confined to 
Figure 1 .–Heating season energy balance for a 6 Btu/sq.ft.-degree day house at Madison, Wisconsin conditions. 

(M 146 363) 
Figure 2.–Optimal home orientation for best use of internal and solar gain effects in northern latitudes. 

(M 146 362) 
rooms with northerly exposure for two 

reasons: (a) if south-facing rooms stay 

cooler during the night hours, they can absorb more solar heat the following day, and 

(b) occupants will tolerate more solar gain in 

rooms with no competing internal heat 

sources. 

Bedrooms with an easterly orientation 

not only offer the psychological benefits of 

morning sunshine but are also sheltered 

from the sun during the afternoon hours, and 

therefore remain cooler at bedtime in summer. Living areas with a southwesterly exposure gain solar heat during the winter 

afternoon hours, and are still warm in the 

evening, while the summer gain during the 

same hours can be controlled by a roof overhang of appropriate width. North-facing 

areas of household activity remain at a 

relatively constant temperature during the 

entire day, and are subjected to less overheating during the daytime hours of highest 

activity in both winter and summer. 

Departures from this pattern of orienting 

the living areas to the south are possible with 

sophisticated heating systems and high windows or skylights in north side rooms. Such 

openings must be provided with closure devices to control nighttime heat losses. The 

concept of zoning for energy balance, therefore, is entirely flexible, and is not necessarily limited to the orientation shown in 

figure 2. Nevertheless, the operation of the 

heating system is determined by temperatures in the coldest occupied part of the 

house, and thus a satisfactory temperature 

distribution is essential to energy efficiency. 

Deviations from the general pattern of southerly orientation for living areas require considerably more careful study for satisfactory 

results. 

Utilizing Solar Gain 

Design for direct gain of solar energy 

through windows is the most basic of all 

solar design concepts. A house so designed 

is essentially a lived-in solar collector, requiring adequate thermal mass to stabilize room 

temperatures and to store the excess energy 

received during periods of high solar gain. 

This thermal mass may be in the form of 

masonry interior walls, a concrete floor slab, 

or similar ceiling construction. Designs relying on natural radiation and convection effects for energy storage and distribution 

without mechanical assist are generally 

known as passive solar designs. 

In light-frame wood construction, the 

potential for stabilizing room temperature 

and storing energy in the mass of the building is limited by the relatively light weight 

and the low heat diffusivity of typical interior 

finish materials. Technologies which effectively use solar gain in light-frame structures 

are still only in development. For more detailed information, the designer must consult 

other publications on this subject 

(1,4,8,9,13,14,16). 

Window heat collection and storage, 

with recovery by fan systems, seems compatible with existing light-frame construction 

and heating technology. Pebble beds may 

serve during the day as effective air cooling 

media with subsequent recovery of the 

stored energy through radiation or convection effects. Reliable criteria for passive 

solar design in the light-frame field, however, 

still are not available, and practical design 

still is based largely on experienced but intuitive judgment. 

Window Management 

Occupants of houses affect heat 

transfer through windows by installing and 

adjusting drapes, shades, awnings, or blinds, 

or even by periodically opening and closing 

windows. The occupants’ effect on the heat 

transmission of windows is termed “window 

management.” The effectiveness of passive 

solar designs depends upon intelligent window management. Regardless of the heat 

storage capacity provided in a house, the actual energy balance for glass areas will depend on the ratio of heat received during the 

daytime hours and lost at night. With indiscriminate use of curtains, shades, or venetian blinds for the protection of furnishings, 

solar gain may be reduced to an insignificant 

level. Freely hanging draperies that allow air 

circulation across the face of the glass at 

night are far less effective as insulating 

6 
devices than tightly fitting shades or shutters. 

Venetian blinds prevent the damaging 

effects of ultraviolet radiation on delicate 

fabrics by effectively deflecting sunlight to 

the ceiling. From there the scattered light 

can be absorbed in other surfaces. Where 

protection of furnishings is less important, 

sheer curtains may control glare, scattering 

sunlight for more diffuse illumination and 

more even energy absorption in room surfaces. With sheer curtains, reflection back 

through the glass is reduced. 

For protection against unwanted summer sunlight, exterior shading devices are 

more effective than interior devices. 

However, solar energy is received through 

windows in forms other than direct or beam 

radiation. Depending on sky and ground conditions, substantial quantities of energy may 

also be received as diffuse radiation, scattered by moisture and dust in the air, or as 

reflected radiation received from the ground 

and adjacent houses. 

Roof overhangs are effective only in 

controlling direct radiation through south 

windows. Windows at any other orientation 

require other means of shading. Deciduous 

trees provide shade in the summer with 

minimal loss of the desired winter sunlight. 

Broad-leaved ground cover is more effective 

in scattering ground reflections than a 

smooth lawn. The surrounding vegetation 

also helps to reduce summer air temperatures by evaporative cooling. For sun control, 

landscape design becomes an integral part 

of the overall design task. Advantageous 

positioning of trees can lead to significant 

improvements in the thermal performance of 

a house. 

Windbreaks and Air Leakage Control 

Use of plant material for windbreaks is a 

familiar concept in the prairie States. More 

recent studies have helped to quantify the 

relationships between fuel usage and wind 

protection, demonstrating the importance of 

effective air leakage control (6). 

The positioning of windbreaks, such as 

plantings and fences, is important for 

creating the most desirable wind-flow patterns. Where snowfall is heavy, improperly 

located windbreaks can lead to undesirable 

drift accumulations. A pocket of calm air on 

the leeward side of a windbreak is known as 

a wind shadow. Design for wind shadow effects can lead to more effective use of outdoor living areas in mild but windy climates. 

A continuous enclosure on the downwind 

side, on the other hand, can result in a cold 

air trap (fig. 3). Where site conditions do not 

lend themselves to external wind protection, 

the house design must provide internal protection. For example, air-lock vestibules for 

entrance doors are an essential energyefficiency feature in northern climates. 

Wind control and solar design concepts 

are integrated in the three-bedroom ranch 

house plan of figure 4. This reference house 

design was developed as a demonstration 

project for a new construction system, rather 

than as an example in energy-efficient planning. Nevertheless, all of the previously 

discussed wind control concepts are clearly 

evident. The house is positioned on a gentle 

south-facing slope in rolling Wisconsin farmland, with no external protection to the northwest. The number of north- and west-facing 

windows is minimized. The house benefits 

from maximum exposure to winter sun 

through its 20 degree south-southwest orientation, facing the sun with the longer 

diagonal of the plan rather than with the 

shorter front wall, for added solar gain in the 

bedrooms. The projection of the garage both 

protects living room windows against late 

afternoon sun and casts a wind shadow for 

the outdoor deck. Enclosed spaces at both 

entrances function as air-lock vestibules. 

Human Comfort Considerations 

With effective air leakage control, a 

house may be ventilated only as needed for 

human comfort. The fresh air requirements in 

homes have not been conclusively defined. 

Need for odor control, which varies greatly 

with the living habits of the occupants, appears to be a more significant factor than 

depletion of the oxygen supply or accumulation of carbon dioxide. 

Problems of moisture control are often 

misunderstood. The moisture balance in a 

7 
VEGETATION MAY BE PRESERVED OR PLACED 

IN SUCH A WAY AS TO CHANNEL OR BLOCK 

DAILY OR SEASONAL AIR FLOW PATTERNS 

FENCES, WALLS OR VEGETATION CAN BLOCK 

NATURAL AIR FLOW PATTERNS. CARE MUST BE 

TAKEN DURING SITE DESIGN TO PROVIDE THE 

NECESSARY VISUAL CONTROL WHILE AVOIDING 

ADVERSE CLIMATIC CONDITIONS. AS COOLER 

AIR FLOWS DOWNHILL IN THE EVENING. 

FENCES, WALLS OR PLANTINGS SHOULD NOT 

UNINTENTIONALLY DAM THIS FLOW AND THUS 

CREATE A COLD AIR POCKET WHERE IT IS NOT 

WANTED. 

VEGETATION PROPERLY PLACED CAN DEFLECT 

RATHER THAN DAM COLD AIR FLOW 

DWELLING UNPROTECTED FROM COLD AIR COLD AIR TRAPPED BY FENCE 

FLOW 

FENCE DESIGN CAN PROVIDE FOR COLD AIR 

DRAINAGE 

Figure 3.–Wind flow effects show the contribution of landscaping to home energy efficiency. (By courtesy of AIA Research Corp.) 

(M 146 365) 
FRONT ELEVATION 

FLOOR PLAN 

Figure 4.–Design for one type of energy-efficient, light-frame house. (M 146 366-1) 

9 
house can vary greatly with occupancy factors. A younger and more active family may 

generate more moisture than an elderly couple by cooking, laundry, and other means, but 

a more frequent use of outside doors by the 

active family results in a higher air exchange 

rate and faster dissipation of this moisture. 

Houses with the most serious moisture accumulation problems are often found to be 

occupied by elderly persons. 

The human body is relatively insensitive 

to variations in indoor moisture levels within 

the 30 to 60 percent relative humidity (RH) 

range. Lower levels lead to dryness of skin 

and nasal membranes; higher levels retard 

normal evaporation of body moisture. Within 

the RH range of 30 to 60 percent, the sensation of added warmth associated with the 

higher moisture levels is not significant 

enough to warrant mechanical humidification. Reducing thermostat settings will, of 

itself, raise the RH level. 

At any given moisture content of indoor 

air the RH changes markedly with changes in 

room temperature. A 30 percent RH at 72° F 

translates to 35 percent RH at 65° F. For increased comfort and energy efficiency, reducing the thermostat setting and wearing a 

sweater and a pair of warmer socks are preferable to mechanical humidification. 

Often, houses are just as sensitive to excessive moisture as the human body. Darkening of ceiling surface at nails or walls in cold 

corners is the result of moisture accumulation rather than temperature differentials as 

such. Even a seemingly reasonable 40 to 50 

percent RH level can lead to mildewing in 

poorly ventilated closets and moisture accumulation within the exterior wall construction. The heating season moisture level target, therefore, appears to fall in the 30 to 40 

percent range. 

The body is also relatively insensitive to 

temperature variations in the 68 to 72 degree 

range, particularly when considered in view 

of our ability to make adjustments by minor 

changes in clothing. With typical residential 

heating systems, temperatures near the floor 

are always lower than those at shoulder level, 

but such differences are often inconsequential. Nevertheless, the overall sensation of 

comfort is determined primarily by the skin 

temperature of body extremities. Thus, in a 

space where air temperatures are noticeably 

stratified, the thermostat setting is likely to 

be determined by the temperature near the 

floor required to keep ankles warm. 

Yet heat losses through the ceiling and 

by air infiltration are dependent on air 

temperatures in the upper part of the space. 

Consequently, a heating system should provide uniform temperature distribution–or 

even inverse stratification with highest 

temperatures near the floor–so that the 

average room temperature can remain at the 

lowest possible level consistent with comfort requirements. 

The body loses heat not only by air convection but also by radiation to windows and 

other cold surfaces. Higher air temperatures 

compensate for such radiation losses, but 

only at a corresponding compromise in the 

energy efficiency of the house as a unit. 

Double or triple glazing not only helps to 

reduce heat losses through the glass but 

also leads to higher glass surface temperatures which, in turn, permit lower room air 

temperatures for comparable human comfort. 

Contrary to past practices, in energyefficient design the primary objective is not 

heating the house but keeping its occupants 

comfortable. Even if code requirements are 

oriented toward steady state (unoccupied) 

conditions, design must be based on occupied conditions with their resulting 

dynamic interactions of heat loss and gain. 

Further Design Perspectives 

Until more specific guidelines become 

available, architectural design for energy efficiency may have to remain as intuitive as in 

the past. The art of passive solar design is 

still in developmental stages and many past 

references may prove unreliable. Reasonably 

accurate qualifications of the various heat 

gain and loss factors have become possible 

only with the introduction of computerized 

analytical models. 

The development of solar heating systems–i.e., active solar technology–should 

lead to considerable reduction in domestic 

energy usage in future years. To permit 

future addition of solar equipment, the 

heating systems of all new houses should be 

10 
designed for the air temperatures and 

velocities associated with solar designs. 

Solar energy will always remain a lowintensity source and thus will require effective conservation to become a practical alternative. Therefore, design recommendations 

are based on high levels of insulation and on 

effective use of passive solar benefits as a 

prerequisite for cost-effective active solar installations. 

Added insulation, however, has a limited 

potential for energy conservation. Heat 

losses through the building envelope are proportional to the indoor-outdoor temperature 

differential and are inversely proportional to 

insulating value (thermal resistance). These 

relationships are indicated by the asymptotic 

curve of figure 5. 

Heat loss by conduction through the 

building envelope is a function of the thermal 

transmittance of a construction assembly, U, 

indicated on the vertical axis. The reciprocal 

of U, the resistance R, is shown on the horizontal axis. Within the range of values applicable to light-frame wood construction, 

the curve flattens rapidly in both directions 

from the chosen reference point at a U-value 

of 0.10. 

Along the vertical portion of the curve, 

which is applicable to windows and uninsulated assemblies, adding insulating materials even in minimal thicknesses reduces 

heat losses significantly. Along the horizontal portion, however, the gains in efficiency with added insulation decrease markedly. While design optimization also requires 

consideration of economic factors (the cost 

of fuel and building materials), the physical 

relationships indicated by this curve remain 

unchanged. 

The heat loss distribution pattern shown 

in figure 1 is based on thermal resistance 

values of R-20 for walls and R-40 for the ceiling/roof assembly. Construction using one 6­

inch glass fiber blanket in the walls and two 

such blankets in the ceiling exceeds these 

ratings. Nevertheless, these points are 

already so low on the curve that a small difference in assumed R-values results in only 

minor variations in the computed heat loss. 

Beyond these points, improvement in the 

thermal properties of the assemblies with added insulation alone becomes increasingly 

difficult. 

Introduction of new building materials 

will undoubtedly offer new design possibilities, particularly for collection and storage of 

solar energy gained through windows. New 

developments to control heat loss, on the 

other hand, seem less promising. The potential for significant further improvements 

in the energy efficiency of homes by more insulation alone, therefore, is relatively limited. 

While rising energy costs may make increased quantities of insulation more cost effective, their thermal effectiveness will not grow 

correspondingly. And, more than one-third of 

the total heat losses in a well-insulated 

house may be caused by air leakage effects, 

so that the magnitude of the heat loss component that could be affected by added insulation also keeps getting smaller. 

The houses of the future may be considerably different from those of the past. Attached and clustered housing types may gain 

popularity because of reduced exterior wall 

surface area. Detached homes may make 

greater use of ground effects, not only as 

below-grade floors or earth berms against 

outside walls, but also as roof covering. 

Wood foundations (7) permit such construction with light-frame wood technology. Although relatively little is still known about 

the thermal properties of wood systems in 

ground contact, they do represent a readily 

available design alternative. 

The solution to the challenge of more 

energy-efficient house design obviously lies 

in more imaginative approaches to this complex problem than in the past. More efficient 

design will also require more careful engineering, rather than just selection and specification of materials on the basis of outdated 

rule-of-thumb methods. 

ENGINEERING DESIGN AND 

CONSTRUCTION 

Design of a home, its orientation, and its 

site and landscaping all affect its energy efficiency; however, construction decisions will 

also make a great difference in fuel requirements for heating and cooling. 

11 
Figure 5.–Plot of thermal transmittance (U) against thermal resistance (R); at U values below 0.10, 

increases in efficiency become increasingly expensive. 

(M 146 364) 
To select materials and construction 

electives intelligently, it is advantageous to 

understand the mechanics of heat transfer. 

Without such knowledge, it is impossible to 

determine relative efficiencies of different 

materials and types of construction. This 

section of the report will also discuss effectiveness of various insulation materials, and 

will discuss alternative constructions for low 

heat transfer. 

Modes of Heat Transfer 

Heat seeks to attain a balance with surrounding conditions, just as water will flow 

from a higher to a lower level. When occupied buildings are heated to maintain inside temperature in the comfort range, a difference in temperature exists between inside 

and outside. Heat will therefore be transferred through walls, floors, ceilings, windows, 

and doors at a rate related to the temperature 

difference and to the resistance to heat flow 

of intervening materials. The transfer of heat 

takes place by one or more of three methods-conduction, convection, and radiation. 

Conduction is the mode of heat flow 

through solid materials; for example, the 

conduction of heat along a metal rod when 

one end is heated in a fire. Convection involves transfer of heat by air currents; for example, air moving across a hot radiator carries heat to other parts of the room or space. 

Heat also may be transmitted from a warm 

body to a cold body by radiation. Heat obtained directly from a heat source, such as a 

fire, is radiant heat. 

Heat transfer through a structural assembly composed of a variety of materials 

may be by one or more of the three methods 

described. Consider a frame house with an 

exterior wall composed of gypsum lath and 

plaster, 2- by 4-inch studs, sheathing, sheathing paper, and bevel siding (fig. 6). In such a 

house, heat is transferred from the room atmosphere to the plaster by radiation, conduction, and convection, and through the lath 

and plaster by conduction. Heat transfer 

across the uninsulated stud space is mainly 

by radiation and convection. By radiation, it 

moves from the back of the gypsum lath to 

the colder sheathing; by convection, the air 

warmed by the lath moves upward on the 

warm side of the stud space, and that cooled 

by the sheathing moves downward on the 

cold side. Heat transfer through sheathing, 

sheathing paper, and siding is by conduction. Some small air spaces will be found 

back of the siding, and the heat transfer 

across these spaces is principally by radiation. Through the studs from gypsum lath to 

sheathing, heat is transferred by conduction 

and from the outer surface of the wall to the 

atmosphere, it is transferred mainly by convection and radiation. 

The thermal conductivity of a material is 

inversely proportional to the insulating value 

of that material. Heat conductivity in a 

homogeneous material is customarily measured as the amount of heat (in British thermal 

units) that will flow in 1 hour through a layer 

of the material which is 1 foot square and 1 

inch thick, where the faces of the layer have a 

temperature difference of 1° F. Heat conductivity is usually expressed by the symbol 

k. 

Where a material is not homogeneous in 

structure, such as one containing air spaces 

like hollow tile, the term conductance is used instead of conductivity. The conductance, 

usually designated by the symbol C, is the 

amount of heat (in Btu's) that will flow in 1 

hour through 1 square foot of the material or 

combination of materials per 1° F temperature difference between surfaces of the 

material. (A dead air space with or without a 

reflective surface may also be rated for conductance by the same method.) 

Resistivity and resistance (direct measures of the insulating value) are the reciprocals of transmission (conductivity or conductance) and are represented by the symbol 

R. Resistivity, which is unit resistance, is the 

reciprocal of k and is given the same symbol 

R as resistance in the technical literature 

because resistances are added together to 

calculate the total for any construction. This 

is the same R commonly used to rate commercial insulation. The overall coefficient of 

heat transmission through a wall or similar 

unit air to air, including surface resistances, 

is represented by the symbol U. U defines the 

transmittance in Btu's per hour, per square 

foot, per 1° F temperature differential. Thus, 

the total equivalent resistance of a construc13 
Figure 6.–Modes of heat transfer in light-frame wall. (M 139 201) 

14 
1 tion section is the R = U , where air film 

values are expressed as equivalent R’s. 

Figure 5 illustrates the relationship between R and U. As R is increased, the incremental reduction in rate of heat loss 

becomes progressively less. Common construction components are identified on the 

curve. Note that at R = 10, the slope of the 

curve begins to change rapidly. Beyond 

R = 20, the curve becomes nearly flat. 

Thermal Performance of Material 

The rate of heat transfer through a material depends on the material composition, 

its density, and factors such as the material’s 

mean temperature and moisture content. 

Another important aspect of a material’s 

thermal performance is capacitance, or its 

ability to store heat. This depends largely on 

the mass of the material. The overall rate of 

heat transfer through a combination of materials is further affected by surface conditions such as roughness, color, and reflectivity, as variations in these properties will 

lead to variations in the component of the 

total heat loss by convection, conduction, 

and radiation. 

Thermal Resistance and Capacitance 

Materials used for construction and insulation of buildings can be divided into 

several types depending on characteristics 

which have a major influence on their thermal properties. Fibrous materials are in the 

form of loose fill, blankets, or batts 

specifically for use as thermal insulation. 

Foams are available in rigid sheets, loose 

beads, or for forming in place. Materials of 

construction include wood, metal, glass, and 

various panel products. Masonry materials 

are considered in a separate category because of their weight and consequent special heat capacitance characteristics. Soil is 

not generally considered as a construction 

material, but it does contribute to insulation 

of buildings, especially where basements are 

used. Heat transmission coefficients (3) for 

building and insulating materials are presented in table 1. Transmission coefficients 

for door and glass areas are shown in tables 

2 and 3. 

Building Insulation 

Fibrous Materials 

Fibrous insulating materials are commonly rock wool, glass fiber, or cellulose. 

These materials are often further classified 

in two categories: (1) vegetable fiber, such as 

wood fiber, is made from plants; (2) mineral 

fiber is derived from nonorganic materials, 

and includes rock wool and glass fiber. 

Fibrous materials are available in the 

forms of blanket, batt, and loose fill. Blankets 

and batts are generally in a specific thickness and have a higher resistance to heat 

transfer than the same material as loose fill. 

Batts are semirigid, purchased flat, and may 

be installed in structural spaces with a friction fit which holds them in place. Blankets 

are quite flexible, and are purchased in rolls. 

Loose fill is available in bags for pouring, as 

into an attic by the homeowner, or it may be 

blown into structural cavities by commercial 

crews. The quality of application affects the 

resulting density and thus the thermal resistance of the end product. 

A maximum resistance value for fibrous 

insulations is obtained when the fibers are 

spaced uniformly and are perpendicular to 

the direction of heat flow. The fiber diameter 

also affects the insulation value; smaller 

fiber diameters at the same density have 

higher resistance values. The type and 

amount of binder, which influences the bond 

or contact of the fibers, may also affect the 

thermal conductivity. 

Density affects the thermal resistance 

of all materials, but fibrous insulating materials are especially influenced by density 

because they can usually be compressed or 

expanded at the time of construction. The 

resistance to heat flow depends mainly on air 

trapped in and between the fibers. As the insulation is compressed, more fibrous materials are required to achieve the same 

thickness; the compression creates more air 

spaces and thus more resistance to heat 

flow. However, at some density part of the air 

spaces close and the fibrous material begins 

to lose insulating value. The optimum density varies with the fiber diameter, and in 

place is achieved when the insulation can 

support its own weight without settling. 

15 
Table 1.--Thermal properties of typical building and insulating materials--design values from ASHRAE Handbook, 19771/ 

Resistance (R)2/ for Resistance (R)2/ for 

Conduc- Conduc- customaryunit 

Specific SI unit 

Description Density tivity tance Per inch For thick- heat (m · K) (m2 (k) (C) · K) thickness ness listed W W (1/k) (1/C) 

Lb/ft3 Btu/lb/

°F 

BUILDING BOARD3/ 

Boards, Panels, Subflooring, Sheathing,

Woodboard Panel Products 

Asbestos-cement board 

0.125 in. 

0.25 in. 

Gypsum or plaster board 

0.375 in. 

0.5 in. 

0.625 in. 

Plywood, Douglas Fir 

0.25 in. 

0.375 in. 

0.5 in. 

0.625 in. 

Plywood or wood panels 0.75 in. 

Vegetable Fiber Board 

Sheathing

Regular density

0.5 in. 

0.78125 in. 

Intermediate density, 0.5 in. 

Nail-base, 0.5 in. 

Shingle backer 

0.375 in. 

0.3125 in. 

Sound deadening board, 0.5 in. 

Tile and lay-in panels, plain or acoustic 

0.5 in. 

0.75 in. 

Homogeneous board from repulped paper

Hardboard 

Medium density

High density

Service tempered, service underlay

Standard tempered 
Table 1. --Thermal properties of typical building and insulating materials--design values from ASHRAE Handbook, 19771/--con. 

Resistance (R) 2/ for Resistance (R)

2/ 

for 

Conduc- Conduc- customary unit SI unit 

Description Density tivity tance Per inch For thick- heat

(k) (C) thickness ness listed 

Specific 

(m · K) (m2 · K)

W W (1/k) (1/C) 

Lb/ft3 

Particleboard 

Low density

Medium density

High density

Underlyment, 0.625 in. 

Wood subfloor, 0.75 

Vapor

Permeablefelt 

Seal 

Two layers of mopped 15-lbfelt 

Plastic film 

Carpet and fibrous pad

Carpet and rubber pad

Cork tile, 0.125 in. 

Terrazzo, 1 in. 

Tile 

Asphalt, linoleum, vinyl, rubber 

Vinylasbestos 

Ceramic 

Wood, hardwood finish, 0.75 in. 

Blanket and Batt 

Mineral fiber, fibrous form, processed from 

rock, slag, or glass 

Approximately 4/ 2 to 2.75 in. 

Approximately 4/ 3-3.5 in. 

Approximately 4/3.50-6.5 

Approximately 4/ 6-7 in. 

Approximately 4/ 8.5in. 

Btu/lb/

°F 

BUILDING BOARD3/- - con. 

BUILDING MEMBRANE 

FINISH FLOORING MATERIALS 

INSULATING MATERIALS 
Table 1.--Thermal properties of typical building and insulating materials--designvalues from ASHRAE Handbook, 19711/--con. 

Resistance (R)2/ for Resistance (R)2/ for 

Conduc- Conduc- customaryunit 

Specific SI unit 

Description Density tivity tance 

Per inch For thick- heat (m · K) (m2 (k) (C) · K) thickness ness listed W W (1/C) 

Lb/ft3 Btu/lb/

°F 

Board and slabs 

Cellular glass

Glass fiber, organic bonded 

Expanded rubber, rigid

Expanded polystyrene extruded 

Cut cell surface 

Smooth skin surface 

Smooth skin surface 

Expanded polystyrene, molded beads 

Expanded polyurethane6/ , R-11 exp., thickness 

1 in. or greater

Mineral fiber with resin binder 

Mineral fiberboard,

Wet felted 

Core or roof insulation 

Acoustical tile 

Acoustical tile 

Wet molded, acoustical tile7/ 

Wood or cane fiberboard, acoustical tile7/ 

0.5 in. 

0.75 in. 

Interior finish, plank, tile 

Wood, shredded, cemented in preformed slabs 

Cellulosic insulation, milled paper or 

wood pulp

Sawdust or shavings

Wood fiber, softwoods 

Perlite, expanded

Mineral fiber, rock, slag, or glass

Approximately 4/ 3.75-5 in 

Approximately 4/ 6.5-8.75 in 

Approximately 4/ 7.5-10 in 

Approximately 4/ 10.25-13.75 in 

INSULATING MATERIALS--con. 

LOOSE FILL 
Table 1.--Thermal properties of typical building and insulating materials--design values from ASHRAE Handbook, 19771/--con. 

Resistance (R)2/ for Resistance (R)2/ for 

Conduc- Conduc- customary unit 

Specific SI unit 

Description Density tivity tance 

Per inch For thick- beat (m · 

W K) (m2 (k) · K) (c) thickness ness listed W (1/k) (1/C) 

Lb/ft3 Btu/lb/

°F 

LOOSE FILL--con. 

MASONRY MATERIALS 

Vermiculite, exfoliated 

Vermiculite, exfoliated 

Roof Insulation, preformed, for use 

above-deck8/ 

Cement mortar 

Gypsum-fiber concrete--87.5 pct gypsum,

12.5 pct wood chips

Lightweight aggregates including expanded

shale, clay or slate; expanded slags;

cinders; pumice; vermiculite; also 

cellular concretes 

Perlite, expanded 

Sand and gravel or stone aggregate

Oven dried 

Not dried 

stucco 

MASONRY UNITS 

Brick 

Common9/ 

Face9/ 

Clay tile, hollow 

One cell deep

3 in. 

4 in. 

Two cells deep

6 in. 

8 in. 

10 in. 

Three cells deep, 12 in. 
Table 1.--Thermal properties of typical building and insulating materials--design values from ASHRAE Handbook, 19771/--con. 

Resistance (R)2/ for Resistance (R)2/ for 

Conduc- Conduc- customary unit 

Specific SI unit 

Description Density tivity tance Per inch For thick- heat (m · K) (m2 · K) (k) (C) thickness ness listed W W (1/k) (1/C) 

Lb/ft3 Btu/lb/

°F 

MASONRY UNITS--con. 

Concrete blocks 

Three oval core 

Sand and gravel aggregate

4 in. 

8 in. 

12 in. 

Cinder aggregate

3 in. 

4 in. 

8 in. 

12 in. 

Lightweight aggregate, expanded shale,

clay, slate, or slag; pumice

3 in. 

4 in. 

8 in. 

12 in. 

Concrete block, rectangular core *10/ 

Sand and gravel aggregate, two cores, 

8 in., 36 lb11/* 

Same with filled cores 10/* 

Lightweight aggregate, expanded shale,

clay, slate or slag; pumice 

Three cores, 6 in., 19 lb

11/* 

Same with filled cores 12/* 

Two cores, 8 in., 24 lb11/* 

Same with filled cores 12/* 

Three cores, 12 in., 38 lb11/* 

Same with filled cores 12/* 

Stone, lime or sand 

Gypsum partition tile 

3 x 12 x 30 in. solid 

3 x 12 x 30 in. four cells 

4 x 12 x 30 in. three cells 
Table 1.--Thermal properties of typical building and insulating materials--design values from ASHRAE Handbook, 19771/--con. 

Resistance (R)2/ for Resistance (R)2/ for 

Conduc- Conduc- customary unit 

Specific SI unit 

Description Density tivity tance 

Per inch For thick- heat (m 

W 

· K) (m2 (k) (C) · K) thickness ness listed W (1/k) (1/C) 

Lb/ft3 Btu/lb/

°F 

Cement plaster, sand aggregate

0.375 in. 

0.75 in. 

Gypsum plaster

Lightweight aggregate

0.5 in. 

0.625 in. 

On metal lath, 0.75 in. 

Perlite aggregate

Sand aggregate

0.5 in. 

0.625 in. 

On metal lath. 0.75 in. 

Vermiculite aggregate 

Asbestos-cement shingles

Asphalt roll roofing

Asphalt shingles

Built-up roofing, 0.375 in. 

Slate, 0.5 in. 

Wood shingles, plain and plastic film faced 

Shingles

Asbestos-cement 

Wood 

16 in., 7.5-in. exposure

Double, 16 in., 12-in. exposure

Plus insulating backer board, 0.3125 in. 

Siding

Asbestos-cement, 0.25 in., lapped

Asphalt roll siding

Asphalt insulating siding, 0.5 in. bed. 

PLASTERING MATERIALS 

ROOFING 

SIDING MATERIALS ON FLAT SURFACE 
Table 1..--Thermal properties of typical building and insulating materials--design values from ASHRAE Handbook, 19771/--con. 

Description Density 

Resistance (R)2/ for 

Conduc- Conduc- customary unit 

Specific 

Resistance (R)2/ for 

SI unit 

tivity tance Per inch For thick- heat (m 

W 

· K) (m2 (k) (C) · K) thickness ness listed W (1/k) (1/C) 

Lb/ft3 Btu/lb/

°F 

SIDING MATERIALS ON FLAT SURFACE--con. 

Wood 

Drop, 1 x 8 in. 

Bevel, lapped

0.5 x 8 in. 

0.75 x 10 in. 

Plywood, 0.375 in., lapped

Medium density, 0.4375 in. 

Aluminum or steel13/, over sheathing

Hollow-backed 

Insulating-board backed, nominal 

0.315 in. 

Foil backed 

Architectural glass 

WOODS 

Maple, oak, and similar hardwoods 

Fir, pine, and similar softwoods 

0.75 in. 

1.5 in. 

2.5 in. 

3.5 in. 
1/ Representative values for dry materials were selected by ASHRAE TC4.4, Insulation and Moisture Barriers. They are intended as design

(not specification) values for materials in normal use. For properties of a particular product, use the value supplied by the manufacturer 

or by unbiased tests. These constants are expressed in Btu per hour per square foot per °F temperature difference). Conductivities (k)are 

per inch thickness, and conductances (C) are for thickness or construction stated, not per inch thickness. All values are for a mean temperature of 75° F, except as noted by an asterisk (*) which have been reported at 45° F. 

2/ Resistance values are the reciprocals of C before rounding off C to two decimal places.

3/ Also see Insulating Materials, Board. 

6/ Conductivity varies with fiber diameter. Insulation is produced by different densities; therefore, there is a wide variation in 

thickness for the same R value among manufacturers. No effort should be made to relate any specific R value to any specific thickness. 

Commercial thicknesses generally available range from 2 to 8.5. 

5/ Does not include paper backing and facing, ifany. Where insulation forms a boundary (reflective or otherwise) of an air space, see 

tables 6 and 7 for the insulating value of air space for the appropriate effective emittance and temperature conditions of the space.

6/ Values are for aged board stock. 

7/ Insulating values of acoustical tile vary, depending on density of the board and on type, size, and depth of perforations.

8/ The U.S. Department of Commerce Simplified Practice Recomendation for Thermal Conductance Factors for Preformed Above-Deck Roof 

Insulation, No. R 257-55, recognizes the specification of roof insulation on the basis of the C values shown. Roof insulation is made in 

thicknesses to meet these values. Different roof insulations are available in different thicknesses to provide the design C values listed. 

Consult individual manufacturers for actual thickness of theirmaterial. 

9/ Face brick and common brick do not always have these specific densities. When density is different from that shown, there will be a 

changein thermal conductivity.

10/ Data on rectangular core concrete blocks differ from the above data on oval core blocks, due to core configuration, different mean 

temperatures, and possibly differences in unit weights. Weight data on the oval core blocks tested are not available. 

11/ Weights of units approximately 7.625 in. high and 15.75 in. long. These weights are given as a means of describing the blocks 

tested, but conductance values are all for 1 ft2 of area. 

12/Vermiculite,perlite, ormineral wool insulation. Where insulation is used, vapor barriers or other precautions must be considered 

to keep insulation dry.

13/ Values for metal siding applied over flat surfaces vary widely, depending on amount of ventilation of air space beneath the siding;

whether air space is reflective or nonreflective; and on thickness, type, and application of insulating backing-board used. Values given are 

averages for use as design guides, and were obtained from several guarded hotbox tests (ASTM C236) or calibrated hotbox (BSS 77) on hollowbacked types and types made using backing-boards of wood fiber, foamed plastic, and glass fiber. Departures of ±50 pct or more from the 

values given may occur. 
Table 2.--Coefficients of transmission (U) of windows, skylights, and light

transmitting partitions from ASHRAE Handbook, 19771/ 

Exterior2/

Description 

Winter Summer Interior 

PARTA--VERTICAL PANELS (EXTERIOR WINDOWS, SLIDING PATIODOORS, 

glass 3/ 

AND PARTITIONS)--FLAT GLASS, GLASS BLOCK, AND PLASTIC SHEET 

Flat 

Single

Insulating glass--double 4/ glass 

0.1875-in. air space5/ 

0.25-in. air space5/ 

6/ 0.5-in. air space

emittance coating7/ 

Insulating glass--triple 4/ 

0.5-in. air space, low 

e = 0.20 

e = 0.40 

e = 0.60 

0.25-in. air spaces5/ 

0.5-in. air spaces8/ 

1- to 4-in. air space5/ Storm windows 

Plastic sheet 

Single glazed

0.125 in. thick 

0.25 in. thick 

0.5 in. thick 

Insulating unit--double 4/ 

0.25-in. air space5/ 

0.5-in. air space4/ 

Glass block9/

6 x 6 x 4 in. thick 

8 x 8 x 4 in. thick 

With cavity divider 

12 x 12 x 4 in. thick 

With cavity divider 

12 x 12 x 2 in. thick 

24 
Table 2.--Coefficients of transmission (U) of windows, skylights, and light

transmitting partitions from ASHRAE Handbook, 19771/--con. 

Exterior2/

Description 

Winter Summer Interior 

PART B--HORIZONTAL PANELS (SKYLIGHTS)--FLAT GLASS,

GLASS BLOCK, AND PLASTIC DOMES10/,11/ 

Flat glass6/ 

Single glass

Insulating glass--double4/ 

0.1875-in. air space5/ 

0.25-in. air space5/ 

0.5-in. air space4/ 

0.5-in. air space, low 

emittance coating7/ 

e = 0.20 

e = 0.40 

e = 0.60 

Glass block9/ 

11 x 11 x 3 in. thick with 

cavity divider 

12 x 12 x 4 in. thick with 

cavity divider 

Plastic domes12/

Single-walled 

Double-walled 

1/ These values are for heat transfer from air to air, British thermal 

units per hour per square foot per degree Fahrenheit. 

types. 

2/ See table 2A for adjustment for various window and sliding patio door 

3/ Emittance of uncooled glass surface = 0.84. 

4/ Double and triple refer to the number of lights of glass.

5/ 0.125-in.glass.

6/ 0.25-in. glass.

7/ Coating on either glass surface facing air space; all other glass

surfaces uncoated. 

8/ Window design: 0.25-in. glass-0.125-in. glass-0.25-in. glass.

9/ Dimensions are nominal. 

10/ Winter: For heat flow up.

11/ Summer: For heat flow down. 

12/ Based on area of opening, not total surface area. 

25 
Table 2A.--Adjustment factors for various window and 

sliding patio door types1/,2/ 

Storm 

windows Description Single

glass 

Double 

or triple

glass 

Windows 

All glass3/ 

Wood sash 

80 pct glass 

60 pct glass 

Metal sash 

80 pct glass 

Sliding patio doors 

Wood frame 

Metal frame 

1/ Multiply U values in parts A and B of table 2 by

these factors. 

2/ These values are for heat transfer from air to air,

British thermal units per hour per square foot per degree

Fahrenhit. 

3/ Refers to windows with negligible opaque area. 

4/ Values will be less than these when metal sash and 

frame incorporate thermal breaks. In some thermal break 

designs, U values will be equal to or less than those for 

the glass. Window manufacturers should be consulted for 

specific data. 

Although compressing a fibrous blanket 

or batt may increase the thermal resistance 

per unit thickness of the resulting material, it 

will decrease the overall resistance. For example: A 6-inch blanket compressed into a 

31⁄2-inch wall cavity may have a slightly 

higher resistance than a standard 31⁄2-inch 

blanket; however, its resistance will not be as 

high as it would have been in a 6-inch space. 

Foams 

Commonly used foams in building construction are polystyrene, polyurethane, and 

urea formaldehyde. The insulating value of 

foams is derived from cells filled with air or 

other gases. Heat transfer is mainly by conduction across these cells. Optimum thermal 

resistance is achieved by a specific combination of cell size and density. The thermal 

resistance is also greatly affected by the kind 

of gas in the cells. 

Polystyrene is commonly manufactured 

as rigid sheets and may be extruded or molded. Extruded polystyrene, with closed cells 

which trap gases in the material, has a higher 

resistance than molded polystyrene with 

open cells. Polystyrene beads are also 

26 
Table 3.--Coefficients of transmission (U) for slab doors in British 

thermal units per hour per square foot per degree

Fahrenheit (adapted from ASHRAE Handbook, 1977) 

Winter Summer 

Nominal Solid 

thickness wood, Steel door1/ Storm door2/ No Steel door1/ 

no storm 

storm A B C Wood Metal door 

A B C 

door 

In. 

1/ A = mineral fiber core (2 lb/ft3); B = solid urethane foam core 

with thermal break; C = solid polystyrene core with thermal break. 

2/ Values for wood storm doors are for approximately 50 pct. glass;

for metal storm door values apply for any percent of glass. 

available as a loose-fill insulation which has 

larger cells with lower resistance than the 

rigid forms. 

Polyurethane may be produced in rigid 

sheets, foamed in place into cavities, or applied to surfaces of building components as 

a fast-setting foam. This foam has 90 percent 

or more closed cells, and retains fluorocarbon gases for extended periods of time. 

However, as air permeates into cells and 

dilutes the fluorocarbon gas, the thermal 

resistance slowly decreases. Some factors 

affecting the rate of air permeation are environmental temperature, thickness, and surface protection. Encasing the foam in gasimpermeable membranes greatly reduces 

the rate of air permeation. 

The urea-formaldehyde currently used in 

building construction is a cold-setting, lowdensity, resilient foam. There is no further 

expansion of the material after it leaves the 

applicator gun, so it can be used to fill voids 

without danger of subsequent pressure 

buildup. Rated shrinkage during the dryingout period is about 1.8 to 3 percent. Rapid 

drying may increase the amount of shrinkage, and slow drying will decrease shrinkage. 

Thermal resistance values for this material 

were not available at the time table 1 was 

assembled. Values listed by manufacturers 

for a density of 0.7 pound per cubic foot 

(Ib/ft3

) are: an “R” factor of 5.0 at an average 

temperature of 70° F, and an “R” factor of 5.5 

at an average temperature of 35° F. (The effect of the temperature at which a material is 

tested is discussed in a later section.) Tests 

by the National Bureau of Standards indicate 

that an in-place R of 4.2 at 75° F should be 

used to compensate for shrinkage and variations in density. 

Materials of Construction 

Density of construction materials is the 

major factor influencing their thermal con27 
ductivity. Very dense materials such as steel 

and glass have high conductivities. Wood 

and fiber panel products have lower conductivities because of a porous structure 

that includes voids in and between fibers. In 

most construction systems, a high percentage of insulation is provided by nonstructural 

insulating materials, so conductivity of the 

construction materials is not critical. 

However, the high-conductivity materials 

must be used with some discretion to avoid 

thermal bridges–that is, dense materials extending all the way through the building 

component–between warm and cold faces. 

Masonry 

Most masonry has a relatively high density, and consequently a high thermal conductivity. However, because it is often used 

in relatively large masses, its capacitance, or 

ability to store heat, may offer some advantage for certain climates especially during the air-conditioning season. Where there 

is a large diurnal cycling of temperature, the 

material can slowly collect heat during the 

day and release heat to the inside during the 

night. If the masonry is sufficiently cooled 

during the night cycle, the building interior 

will remain cool during the day while the 

masonry is slowly storing heat that would 

otherwise be transferred to the interior. 

Thermal conductivity of concrete or 

manufactured masonry units, such as concrete block, can be reduced by using a 

lightweight aggregate, but this also reduces 

heat storage capacity. 

SoiI 

The insulating value of soil varies with 

type, density, and moisture content. Of 

these, moisture content is the most critical 

factor. Although the thermal' resistance is 

low, in practice soil is used in large quantities that result in an overall low conductance. 

Basement floors and walls below the 

frostline generally do not require additional 

insulation because of the protection provided by soil. Where a basement is built partially out of the ground with berms added 

against the walls, soil still adds greatly to the 

insulation. With the growing concern for 

energy conservation, underground structures are seriously being considered, so soil 

in combination with below-surface insulation 

will play a greater role as insulation in the 

future. 

Environmental Conditions 

The Effect of Temperature 

Thermal conductivity is affected by the 

mean temperature of an insulating material 

during rating under test conditions. Most of 

the insulating values in table 1 were obtained 

at 75° F; however, product literature often 

lists conductivity values obtained from tests 

conducted at other temperatures, making 

comparison difficult. For a meaningful comparison of thermal efficiency of materials, 

data must be from tests at the temperature 

which the material will achieve in service. 

Each type of insulation varies differently with 

temperature. Some have a decreasing conductivity with decreasing temperature, whiIe 

the reverse effect occurs in others. Figure 7 

shows the relationship between temperature 

and thermal conductivity for some insula 

tions. 

The Effect of Moisture 

Water or ice in insulation impairs or destroys its insulating value, and may cause 

structural damage by decay, corrosion, or the 

expansive action of freezing water. The extent that moisture affects the material 

depends on the physical structure of the 

material. Most insulations are hygroscopic, 

so they gain or lose moisture in proportion to 

the relative humidity of the air in contact with 

the insulation. Closed-cell foams are essentially nonhygroscopic, so their insulating 

value is not appreciably affected by moisture; however, urea-formaldehyde foam has 

open cells and is more absorbent. 

One of the main sources of moisture in 

insulation is from water vapor being driven 

by vapor pressure differences from the warm 

to the cold side of insulation. Air leakage 

through building components also transports moisture. This vapor condenses as free 

water when it reaches its dewpoint tempera28 
Figure 7.–Typical variation of thermal conductivity with mean temperature (ASHRAE Handbook, 

1977). 

ture. If the insulation has a high permeability, 

this moisture moves on through the insulation to the coldest surface. Such condensation occurring inside a structural component, 

such as a wall, is referred to as concealed 

condensation. Where condensation occurs 

on an exposed surface, such as a window or 

uninsulated wall, it is called visible condensat ion. 

Moisture Control 

Moisture control in structural spaces is 

generally accomplished by the use of vapor 

barriers, ventilation, or a combination of 

these. In addition, reduction of moisture in 

living spaces may be required in order to 

reduce the vapor pressure drive through the 

exterior envelope of the building. Air change 

(M 146 477) 

from normal infiltration is usually enough to 

remove the water vapor generated by a family. However, in some cases, bathroom and 

kitchen vents may be necessary. Condensation on windows is one indication that humidity is too high. In extreme cases, such as 

a large family living in a small, tight house, 

mechanical dehumidification may be necessary. The same requirement may exist in 

buildings heated electrically since there is 

no chimney draw to increase air infiltration. 

The usual fuel-burning heating systems require air for combustion. This creates a 

negative pressure in the building, which accelerates air exchange and consequently 

lowers relative humidity in the building. 

Vapor barriers are in the form of structural sheets, membranes, or coatings. The 

structural sheets may be reinforced plastics, 

29 
aluminum, steel, or rigid insulation. Membranes are the most commonly used vapor 

barriers, and include metal foils, laminated 

foil and treated papers, coated felts and 

papers, and plastic films or sheets. Coatings 

may be semifluid, mastic type, or paint type. 

Most vapor barriers do not stop the passage 

of all water vapor, but reduce the rate of 

vapor movement so moisture can escape 

from the structural component as fast as it is 

moving through the barrier. 

The vapor barrier should always be applied as near as possible to the warm face of 

a wall or other building component. An exact 

correlation between climatic conditions and 

vapor barrier requirements is not available; 

how ever, current recommended practice requires a vapor barrier on the inside face 

of walls in all zones shown in figure 8. The 

shaded areas of the map, figure 8, show 

areas where vapor barriers should not be 

used on the inside face of the wall. Because 

of long periods of air-conditioning, vapor barriers may be required on the outside face of 

walls. Local recommended practice should 

be followed. 

The vapor barrier should be continuous 

with all joints lapped. Usually a 2-inch lap 

over a framing member will make a sufficiently tight joint when the interior finish is applied. When the vapor barrier is a backing for 

blanket insulation, flanges should be lapped 

over the framing member rather than being 

stapled to the sides of framing. While stapling the barrier to the sides of the studs may 

be adequate in moderately cold climate 

where the vapor pressure drive is not severe, 

this practice can lead to high levels of condensation in colder climates. It is nearly impossible to attach the vapor barrier flanges 

continuously tight against the stud and prevent the transfer of water vapor at that joint. 

Also, where full-thickness insulation is used, 

attachment to the side of the stud cannot be 

accomplished without compressing the insulation near the stud and thus reducing insulating efficiency. The barrier should be 

carefully cut to size to fit closely around 

Figure 8.–Condensation zones in the United States (from ASHRAE Handbook, 1977). Zones include areas with design temperatures about as follows: Zone 1, – 20° F and lower; zone 2, 0° 

F to – 20° F; and zone 3, above 0° F. (Note that cross hatched areas are outside of zones 1 

and 3.) (M 146 479) 

30 
openings such as electrical outlets and windows. 

Ceiling vapor barriers are required in 

colder climates as indicated in table 4. Ventilation of attic or roof spaces is also 

necessary because most roofing is quite 

resistant to the passage of water vapor. 

Where practical, outlet vents should be 

provided at a higher level than inlet vents, 

and all ventilation should be well distributed 

to move air through all parts of the structural space. Amounts of ventilation required 

for various types of roofs are presented in 

table 4. 

Ventilation is also required in crawl 

spaces. At least four openings, one at each 

corner, as high as possible, should be provided. A minimum value for their total net area 

may be calculated by the formula: 

where 

L is the perimeter of the crawl 

space, linear feet, 

A is the area of the crawl space, 

square feet, and 

a is the total net area of all vents (or 

the gross area if a 4-mesh 

screen is used), square feet. 

Ventilation of the crawl space can be reduced to only 10 percent of that calculated 

by the above equation if a ground cover that 

resists moisture transmission is laid over the 

soil. Materials commonly used for this are 

roll roofing and 4- or 6-mil polyethylene film. 

The ground cover should be laid with all 

edges lapped at least 4 inches. 

Where roof decks also serve as ceiling, 

such as in plank-and-beam construction, 

there is no structural space to ventilate. 

Here, the concept of a limited vapor barrier 

rather than a near absolute barrier may be 

considered. A limited barrier restricts the 

flow of water vapor during periods of condensation to the amount that can be tolerated by 

the construction without deterioration or 

measurable loss in insulating or other efficiency. Yet moisture can move back through a 

limited barrier during the noncondensation 

periods. Criteria do not exist to determine 

the perm rating for this type of application; 

however, construction using barriers with 

permeance values in the range between 0 

and 0.05 perm have shown cumulative buildup of moisture in unventilated decks. This 

suggests vapor barriers with permeance 

values in the range of 0.15 to 0.50 perm may 

be more suitable. 

Adjustment for Moisture in Wood 

Some moisture exists in all insulating 

materials; however, most heat loss calculations are based on materials at zero percent 

moisture content. For accuracy a moisture 

content adjustment should be made. Adjustment factors are not available for most 

materials; however, the chart shown in figure 

9 has been developed for determining thermal conductivity of wood at various moisture 

contents. To use this chart, obtain the average specific gravity from table 5 for the 

species under consideration. Determine or 

assume the moisture content of the wood. 

When the actual moisture content is not 

known, assume it is 15 percent for wood 

siding or sheathing and 7 percent for inside 

woodwork or finish. On the chart, find the 

moisture content of the wood and follow the 

vertical line upward until it intersects the 

sloping line that corresponds to the specific 

gravity of the wood. The reading on the vertical scale at the left of this intersection 

point is the desired thermal conductivity, k, 

for the wood at the assumed moisture content. 

The specific gravity data of table 5 are 

average values for species listed. There are, 

of course, appreciable variations in specific 

gravity between boards and even between 

shipments of the same species. Substances 

such as gums and resins, and such defects 

as checks, knots, and irregular grain, may 

also influence conductivity, but for purpose 

of calculation those factors may be ignored. 

The conductivity value for plywood is 

essentially the same as that for solid wood of 

the same thickness. 

31 
1

Table 4.--Recommended good practice for loft and attic ventilation (adapted from ASHRAE Handbook, 1977) 

Roof type Condensation zone2/ 

and slope I II III 

Flat;

3 in 12 

or less Total net area3/ of ventilation should be 1/300th

distributed uniformly at the eaves plus a vapor

barrier in the top story ceiling. Freecirculation must be provided through all spaces. 

Gable; 

over 

3 in 12 Total net area of at least two louvers on opposite

sides located near the ridge to be 1/300th plus

a vapor barrier in the top story ceiling. 

Hip Total net area of ventilation should be 1/300th

with 1/600th distributed uniformly at the eaves 

and 1/600th located at the ridge with all spaces

interconnected. A vapor barrier should also be 

used in the top story ceiling. 

Gable or 

hip4/ Total net area of ventilation should be 1/300th

with 1/600th distributed uniformly at the eaves 

and 1/600th located at the ridge with all spaces

interconnnected. A vapor barrier should also be 

used on the warm side of the top full story

ceiling, the dwarf walls, the sloping part of 

the roof, and the attic story ceiling. 

Same as zone I Same as zone I 

Same as zone I Same ventilation as zone II. 

A vapor barrier is not 

considered necessary. 

Same as zone I Same ventilation as zone II. 

A vapor barrier is not 

considered necessary. 

Same as zone I Same as zone I except that 

a vapor barrier is not 

considered necessary if 

insulation is omitted. 

1/ In many areas increased ventilation may be desirable for summer comfort. For winter comfort, insulation is 

recommended between a living space and a loft or attic ventilated at these rates. 

2/ The zone numbers refer to fig. 8. 

3/ Total net area refers to area enclosed within building lines at eave level. 

4/ With occupancy contemplated. 
Figure 9.–Relation between computed conductivity and moisture content for wood having different specific gravity values. (Specific gravity based on volume at current moisture content 

and weight when ovendry. Conductivity computed from formula K = S [1.39 + 0.028M] + 

0.165.) (M 140 415) 

33 
Table 5.--Average specific gravity, thermal conductivity, and resistance values of 

various species of hardwoods and softwoods 

Specific ConducResistivity or Specific ConducResistivity or resistance resistance 

Species, hardwoods grav- tivity 

1 2/3/4-

Species, softwoods grav- tivity 

1 2/3/4­ ity1/ k 

k in. ity 1/ k 

k in. 

Ash: Bald cypress

Black Cedar: 

Commercial white Alaska 

Aspen: Atlantic white 

Big tooth Northern white 

Quaking Port Orford 

Basswood, American Western redcedar 

Birch, American Douglas-fir: 

Birch: Coast-type

Sweet Intermediate 

Yellow Rocky Mountain 

Cherry, black Fir: 

Chestnut, American Balsam 

Cottonwood: Commercial white 

Black Hemlock: 

Eastern Eastern 

Elm: Western 

American Larch, western 

Rock Pine: 

Slippery Eastern white 

Hackberry Loblolly

Hickory: Lodgepole

Pecan Longleaf

True Ponderosa 

Magnolia, southern Red 

Maple: Shortleaf 

Black Slash 

Red Sugar

Silver Western white 

Sugar Redwood: 

Oak: Virgin growth

Commercial red Second growth (openly grown)

Commercial white Second growth (closely grown)

Sweetgum Spruce:

Sycamore, American Engelmann

Tupelo: Red 

Black Sitka 

Water White 

Yellow- poplar 

1/ Average specific gravity based on volume and weight of ovendry specimens.

2/ The thickness of nominal 1-in. boards is 3/4-in. 
Reflectance and Surface Effects 

Air Films and Spaces 

In addition to the insulation provided by 

the construction materials, each face of a 

component of construction has an air film 

that provides some resistance to heat flow. 

Where the air is still, the conductance varies 

with the position of the surface, the direction 

of heat flow, and the surface emissivity. 

Where there is movement of air over the surface, the above variables do not apply. Conductance values for winter conditions are 

usually based on 15-mile-per-hour wind. For 

summer, 71⁄2-mile-per-hour wind speed is 

used. The conductivity values for air films are 

given in table 6. 

The transfer of heat across an air space 

is accomplished by a combination of radiation, convection, and conduction. The radiation component is affected by the temperature of the two boundary surfaces and their 

surface emissivities. The other elements of 

heat flow are affected by the temperature difference, the thickness of the space, the 

orientation (horizontal or vertical), and the 

direction of heat flow. Thermal conductance 

values for air spaces are given in table 7. The 

transfer of heat through an air space cannot 

be determined by extrapolation, so exact 

values are given for 1⁄2, 3⁄4, 11⁄2, and 31⁄2 

inches. 

Reflective Insulation 

High thermal resistance is obtained by 

multiple air layers bounded by reflective surfaces. The total resistance is the sum of the 

resistance values across each air space. One 

or both sides of the air space may have reflective surfaces; however, there is little 

gained by adding a second reflective surface 

to the same air spaces except for thick, horizontal air spaces with heat flow down. It 

makes no appreciable difference in resistance to heat transfer on which side the 

reflective surface is placed; however, it 

should usually be placed on the warm side of 

the air space because moisture usually will 

not condense on the warm surface. Changes 

in the reflective surface which may reduce its 

reflectivity are chemical action, dust accumulations, and the presence of condensation or frost. 

Thermal conductance values for air 

spaces with reflective surfaces are given in 

table 7. 

Heating and Cooling Load 

Computations 

The thermal load on a building is made 

up of numerous elements. A major factor is 

the area of the building envelope-walls, 

ceilings, and floor. The heat conducted 

through these construction assemblies is 

directly proportional to their area, their thermal conductivity, and the temperature difference between the inside and outside of the 

building. 

Infiltration or air leakage through openings or cracks also contributes to thermal 

load. Air also passes directly through some 

construction assemblies such as walls. Infiltration is quite dependent upon wind 

speed and direction, so that heating requirements increase with wind speed. Also, 

the higher the rate of infiltration, the greater 

the effect of wind speed on the thermal load. 

It is possible that a building might require 

more heat on a windy day with a moderately 

low outdoor temperature than on a still day 

with a much lower outdoor temperature. The 

most common wind speed for making heating load computations is 15 miles per hour; 

however, local experience may have established other wind speeds as appropriate for a 

particular location. The wind pressure affects the surface heat loss coefficient as 

well as infiltration. 

Design Temperature 

The ideal heating system would be 

designed for maximum output exactly equal 

to the heating load under the most severe 

weather conditions. In reality, the median of 

extreme temperatures is usually selected as 

the outdoor design temperature. For a moderate heat capacity where there is some 

internal load and daytime occupancy, 99 percent of the median of extreme temperatures 

is a reasonable choice. Massive, institutional 

buildings that have little glass can usually be 

35 
Table 6.--Surface conductances and resistances for air (ASHRAE Handbook, 1977)1/,2/ 

Surface Emittance 

Position of Direction Wind speed Reflective 

surface of heat 

flow Summer Winter Nonreflective ε = 0.90 ε = 0.20 ε = 0.05 

hi R ho 

R hi R hi R 

Still air

Horizontal Upward 

45° slope Upward 

Vertical Horizontal 

45° slope Downward 

Horizontal Downward 

Moving air 

Any Any

Any Any 

1/ All conductance values expressed in British thermal units per hour per square foot per degree

Fahreheit. A surface cannot take credit for both an air space resistance value and a surface resistance value. No credit for an air space value can be taken for any surface facing an air space of less 

than 0.5 in. 

2/ Conductances are for surfaces of the stated emittance facing virtual blackbody surroundings at 

the same temperature as the ambient air. Values are based on a surface-air temperature difference of 

10° F and for surface temperature of 70° F. 
Table 6A.--Reflectivity and emittance values of various surfaces 

and effective emittances of air spaces 

Effective Emittance E 

of air space 

Average One Both Surface Reflect- Emittance surface tivity surfaces ε emittance emittances ε; the 

ε other 0.90 

Pct 

Aluminum 

Foil, bright

Sheet 

Coated paper, polished 

Steel, galvanized, bright 

Aluminum paint 

Building materials 

Wood, paper, masonry,

nonmetallic paints 

Regular glass 

designed for an outdoor temperature 971⁄2 

percent of the median extreme. 

Indoor design temperatures may vary 

with the type of occupancy, but 72° F is commonly used for winter and 78° F for summer. 

Attic Temperature 

A common expedient in calculating heat 

loss through ceilings is to neglect the roof 

and assume the attic temperature equals the 

outdoor temperature. Another expedient is to 

assume the attic temperature equals the 

average between the indoor and outdoor temperature. The actual attic temperature is usually somewhere between these two values. A 

more accurate estimate of the attic temperature can be calculated from the following 

equation (3): 

where 

ta is attic temperature, 

tc is indoor temperature near top floor 

ceiIing, 

to is outdoor temperature, 

Ac is area of ceiling, square feet, 

Ar is area of roof, square feet, 

Aw is area of net vertical attic wall space, 

square feet, 

Ag is area of attic glass, square feet, 

Uc is coefficient of transmission of ceiling, based on surface conductance of 

2.20; 2.28 = reciprocal of one-half the 

air space resistance, 

Ur is coefficient of transmission of roof, 

based on surface conductance of 

2.20, 

U w is coefficient of transmission of vertical wall surface, and 

Ug is coefficient of transmission of 

glass. 

37 
Table 7.--Thermal resistances of plane air spaces

1-4/ 

Position Direction Air space 

5/ Value of E5/,6/ 

of air of heat Mean Temperature space flow 0.03 0.05 0.2 0.5 0.82 temperature difference 

°F °F 

Horizontal Up 

45° Slope Up 

Vertical Horizontal 

45° Slope 

0.5-INCH AIR SPACE 2/ 

Horizontal Down 

38 
Table 7.--Thermal resistances of plane air spaces

1-4/--con. 

Position Direction Air space5/ Value of E5/,6/ 

of air of heat 

Mean Temperature space flow 0.03 0.05 0.2 0.5 0.82

temperature difference 

°F °F 

0.75-INCH AIR SPACE 2/ 

Horizontal Up 

45° Slope Up 

Vertical Horizontal 

45° Slope Down 

Horizontal Down 

39 
Table 7.--Thermal resistances of plane air spaces

1-4/

--con. 

Position Direction Air space

5/ 

of air of heat Mean Temperature space 0.03 0.05 0.2 0.5 0.82 temperature difference 

Value of E 5/,6 / 

°F °F 

Horizontal Up 

45°Slope Up 

Vertical Horizontal 

45°Slope Down 

Horizontal Down 

1.5-INCH AIR SPACE2/ 

40 
Table 7.--Thermal resistances of plane air spaces 

1- 4/

--con. 

Position Direction Air space 5/ Value of E5/,6/ 

of air of heat Mean Temperature space flow 0.03 0.05 0.2 0.5 0.82 temperature difference 

°F °F 

Horizontal Up 

45° Slope Up 

Vertical Horizontal 

45° Slope Down 

Horizontal Down 

3.5-INCH AIR SPACE

2/ 

(Footnotes on p. 42) 

41 
Table 7--concl. 

1/ All resistance values expressed in British thermal units per hour per 

square foot per degree Fahrenheit temperature difference. Values apply only

to air spaces of uniform thickness bounded by plane, smooth, parallel surfaces 

with no leakage of air to or from the space. Thermal resistance values for 

multiple air spaces must be based on careful estimates of mean temperature

differences for each air space.

2/ Credit for an air space resistance value cannot be taken more than once 

and only for the boundary conditions established. 

3/ Resistances of horizontal spaces with heat flow downward are substantially independent of temperature difference. 

4/ Thermal resistance values were determined from the relation R = 1/C,

where C = hc + Ehr, hc is the conduction-convection coefficient, Ehr is the 

radiation coefficient ≅ 0.00686E[(460 + tm)/100]3 , andtm is the mean 

temperature of the air space. For interpretation from table 7 to air space

thicknesses less than 0.5 in. (as in insulating window glass), assume 

hc = 0.795(1 + 0.0016) and compute R values from the above relations for 

an air space thickness of 0.2 in. 

5/ Interpolation is permissible for other values of mean temperature,

temperature differences, and effective emittance E. Interpolation and moderate 

extrapolation for air spaces greater than 3.5 in. are also permissible.

6/ Effective emittance of the space E is given by 1/E = l/el + l/e2 - 1, 

where e

1 

and e

2 

are the emittances of the surfaces of the air space (see 

table 6A). 

The attic temperature calculated from 

the above equation neglects the effect of 

ventilation. Where the usual ventilation requirements are met, the ventilation rate is 

about 0.5 cubic foot per minute per square 

foot of ceiling area. Such ventilation reduces 

the difference between attic air temperature 

and outdoor temperature by approximately 

50 percent. For other ventilation rates down 

to zero cubic foot per minute, the reduction 

is linear. For example, a ventilation rate of 0.1 

cubic foot per minute reduces the temperature difference by about 10 percent. 

Ground Temperatures 

The ground temperature under basement floor slabs is about the same as water 

temperature at depths of 20 to 60 feet. This is 

not influenced by atmospheric conditions. 

Temperature of soil adjacent to basement 

walls is affected by outdoor temperature at 

locations near the surface. Frost penetrates 

to a depth of over 4 feet in some localities if 

the ground is not protected by snow; however, the ground temperature adjacent to the 

walls of a heated basement is greatly affected by the heat gain from the basement. 

The rate of heat flow through a basement 

wall decreases with depth because the total 

insulation provided by the soil depends on 

the distance to the surface. A precise calculation would be extremely difficult, and 

would involve such variables as the type of 

soil, but an approximate method of calculation has been developed by the National Research Council of Canada (5). Tables 8 and 9 

present conductivity values that were experimentally derived. These values can simply be 

multiplied by the area of wall or floor and the 

temperature difference between indoors and 

outdoors. The effects of 1, 2, and 3 inches of 

insulation added to the basement walls are 

also shown. Windows and basement walls 

above ground should have heat loss calculated the same as other construction components exposed to outdoor temperatures. 

Concrete slabs on grade are usually 

42 
units per hour per degree Fahrenheit per 

square foot (from National Research Council 

of Canada, 1973) 

Table 8.--Heat loss below grade in British thermal units 

per hour per degree Fahrenheit per square

foot (from National Research Council of 

Canada,1973)l/ 

Heat loss 

Path 

length Depth through UninsuInsulation 

soil lated 1 2 3

inch inches inches 

Ft Ft 

1/ Insulation with k = 0.24 Btu/(h)(ft2)(°F/in.)

= 0.02 Btu/(h)(ft)(°F)

Soil k = 0.8 Btu/(h)(ft)(°F) 

Table 9.--Mean basement floor heat loss in British thermal 

units per hour per degree Fahrenheit per

square foot (from National Research Council 

of Canada, 1973) 

Depth of

foundation 

wall below 

grade 

20 

feet 

Width of house 

24 

feet 

28 

feet 

32 

feet 

Ft 

43 
placed on a gravel fill 4 inches thick or more 

which helps insulate the floor from the earth 

and retards the rise of ground water by capillarity. A waterproof membrane should also 

be placed over the gravel fill. It is important 

that the floor be several inches above grade 

and have effective subsoil drainage. A slab 

that is soaked by rain or melting snow will 

have excessive heat loss. For design purposes the total heat loss for a concrete slab 

is dependent upon the length of exposed 

edge rather than the area of the slab. For 

small floor areas with no insulation, 0.81 Btu 

per hour may be used times the length of exposed edge in feet times the difference between indoor and outdoor temperature. 

Heat Loss Computations 

Total heat loss from a building can be 

divided into two major categories: (1) Transmission of heat through building elements 

and (2) infiltration of cold air that must be 

heated. 

Transmission Heat Loss 

Heat loss through a building element is 

calculated by multiplying the coefficient of 

transmission (U value) times the area of the 

element (wall, ceiling, floor, or window) times 

the difference between indoor and outdoor 

temperatures. The U value is the reciprocal of 

the total resistance of a combination of materials. First total the resistance of each 

material plus the resistance of air films on 

each face. Then divide this total into 1. Thermal resistance of most materials of construction is given in table 1, and factors for air 

spaces and air films are given in tables 6 and 

7. For example, the U value for a typical wall 

section is calculated as follows: 

Resistance 

value 

Interior surface resistance 

Wallboard (1⁄2-in. gypsum) 

31⁄2-inch blanket insulation 

(glass fiber) 

Insulating board (1⁄2-in. regular 

density sheathing) 

Wood bevel siding (1⁄2 by 8 in.) 

Exterior surface resistance 

(15-mph wind speed) 

Total resistance 

In many elements of construction, framing members (studs or joists) reduce overall 

thermal efficiency. The effect of framing 

members is often neglected, but for accurate 

heat loss computations framing must be considered. The adjustment factor for the wall 

shown above is calculated as follows: 

where 

Uav = Average U value for building section. 

Ui = U value for area between framing 

members. 

Us = U value for area backed by framing 

members. 

S = Percentage of area backed by framing members. 

From the above example, 

total resistance is 

Subtract resistance of 

insulation 

Add resistance of 31⁄2-inch 

softwood 

Total resistance at wood 

stud 

From previous example, Ui = 0.070 

If the wall backed by framing is 10 percent, 

44 
Infiltration Heat Loss 

InfiItration heat loss includes both sensible and latent losses. The sensible heat 

loss is the heat required to warm the outdoor 

air entering by infiltration. Latent heat loss is 

the heat equivalent of any moisture which 

must be added to replace moisture being lost 

through infiltration. 

Heat required to raise the temperature 

of air 1° F is the specific heat of air (0.240) 

times the density of air at outdoor temperature (assume “standard air” at 0.075 Ib/ft3

) 

times the volume of air in cubic feet per hour. 

This reduces to 0.018 times the volume of air. 

For the total sensible heat requirement, multiply 0.018 times the volume of air times the 

difference between indoor and outdoor temperature. When the volume is infiltration per 

hour, the heat load will be in Btu’s per hour. 

A certain amount of judgment is required regarding quality of construction and 

other factors in estimating infiltration by any 

method, so that infiltration is often based 

upon an estimated number of air changes per 

hour. One to one and one-half changes per 

hour is usually assumed for residences; 

however, this can be reduced by high quality 

construction. This is a total air change, including such sources as opening doors. The 

rate of infiltration may be computed by the 

crack-length method in which the rate of infiltration for a certain type of crack is 

multiplied by the length of the crack. 

Latent heat is required to evaporate 

water when water vapor must be added for 

winter comfort conditions. Total heat needed 

is based on the volume of infiltration and the 

amount of water vapor per unit volume required to achieve the desired humidity. The 

equation for calculating the latent heat is (3): 

where 

Hi is heat required to increase moisture 

content of air leaking into building 

from Wo to Wi Btu per hour, 

Q is volume of outdoor air entering the 

building, cubic feet per hour, 

p is density of air at temperature ti , 

pounds per cubic feet, 

Wi is humidity ratio of indoor air, pounds 

per pound of dry air, 

Wo is humidity ratio of outdoor air, 

pounds per pound of dry air, 

and 

hfg is latent heat of vapor at Wi, Btu per 

pound. 

If the latent heat of vapor (hfg) is assumed to be 1,060 Btu per pound and the air 

density is 0.075 pound per cubic foot, the 

equation reduces to: 

Total Heat Loss 

Total rate of heat loss is the sum of 

hourly rates of transmission and infiltration 

losses. Transmission loss for each element 

of the exterior envelope is the average “U” 

times the area of that element. Rate of infiltration heat loss is 0.018 times the volume 

of air change per hour. Latent heat loss is 

minor in most cases, and therefore, is usually disregarded in this type of calculation. 

Total seasonal loss in Btu’s is the total rate 

of heat loss times 24 hours times adjusted 

heating degree days for a particular geographic location. Heating degree days are 

available from the local weather station and 

from utility and fuel companies. The degree 

day is a unit based upon temperature difference and time. For any one day, when the 

temperature is less than 65° F, there exist as 

many degree days as there are Fahrenheit 

degrees difference between the mean temperature for the day and 65° F. The base of 

65° F to define the degree day was selected 

in the 1930’s because the internal gain from 

such things as appliances plus the solar gain 

made up the difference between 65° F and 

the design indoor temperature. With better 

insulated houses and increased use of electrical appliances, the internal and solar gains 

now provide a larger part of heat required. 

Since the degree day base of 65° F is well 

established, a modification factor is used for 

arriving at an adjusted degree day. The factors for specific winter design temperatures 

are: 

45 
Outdoor design 

temp. F 

Modification 

factor 

Multiply the appropriate factor times the 

degree days to establish the adjusted degree 

days. 

Seasonal fuel usage can be estimated 

by using values in table 10. Divide the total 

seasonal heat loss by the Btu’s shown in 

table 10 for the type of fuel used to arrive at 

the units of fuel required for a heating 

season. Typical efficiencies are shown for 

heating equipment. The higher efficiency is 

for equipment in good operating condition. 

The lower rating is for equipment in need of 

maintenance or repairs. 

Heat Gain Computations 

Rate of heat gain through the building 

envelope is computed in a similar manner to 

heat loss calculations. Thermal resistance 

values are the same except that different 

values are given for heat flow down than for 

heat flow up, as indicated in table 7. These 

differences are particularly significant where 

reflective surfaces are used in conjunction 

with air spaces. Another difference in heat 

gain computations is the significant effect of 

solar heat gain on the walls. The absorption 

Table 10.--Calorific values for fuel and combustion efficiency of 

small heating plants 

Type of Approximate

Fuel Unit firing calorific 

value 

Probable 

Combustion 

efficiency 

Btu Pct 

Coal Pound Hand fired 

Coal Pound Stoker 

Oil Gallon Conversion 

burner 

Oil Gallon Oil design

unit 

Manufactured Cubic foot Conversion 

gas burner 

Manufactured Cubic foot Gas designed

gas unit 

Natural gas Cubic foot Conversion 

burner 

Natural gas Cubic foot Gas designed

unit 

46 
Table 11. --Design equivalent temperature differences (adapted from ASHRAE Handbook, 1977) 

Daily temperature range1/ and design temperature 

Low Medium High 

85 90 95 85 90 95 100 90 95 100 105 110 

°F °F °F °F °F °F °F °F °F °F °F °F 

WALLS AND DOORS 

Frame and veneer-on-frame 

Masonry walls, 8-in. block or 

brick 

Partitions 

Frame 

Masonry

Wood doors 

CEILINGS AND ROOFS2/ 

Ceilings under naturally vented 

attic or vented flat roof 

Dark 

Light

Built-up roof, no ceiling

Dark 

Light

Ceilings under unconditioned 

rooms 

FLOORS 

Over unconditioned rooms 

Over basement, enclosed crawl 

space, or concrete slab-on­ ground

Over open crawl space 

1/ Daily temperature range calculation value: Low--12° F, applicable range is less than 15° F; medium--20° F,

applicable range is 15° to 25° F; high--30° F, applicable range is more than 25° F. 

2/ For roofs in shade, 18-h average = 11° F temperature differential. At 90° F design and medium daily range,

equivalent temperature differential for light-colored roof equals 11 + (0.71)(39 - 11) = 31° F. 
of solar heat has the effect of changing the 

temperature differential between indoors 

and outdoors. This temperature differential 

varies with the time of day, sun angle, and 

wall color. To compensate for these variations, equivalent temperature differentials 

for various materials and exposures were 

developed from actual temperature measurements (table 11). Equivalent temperature differentials are merely used in place of the difference between indoor and outdoor air 

temperature for calculating heat transfer. 

The heat transfer methods that have 

been presented were based on the steadystate heat flow concept which has served 

well for many years. These concepts are 

relatively uncomplicated to apply and result 

in a good degree of accuracy; however, they 

do not account for the heat storage effects of 

the building structure. This heat storage 

capacity has a greater effect on heat gain to 

air-conditioned spaces than on heat loss. 

More sophisticated methods of calculating 

heating and cooling loads have been developed; one of them, the response factor 

method, involves complex mathematical 

treatment. Computer programs have been 

developed and are available at both the National Research Council and the National 

Bureau of Standards. 

Thermal Performance of 

Assemblies 

Information has been presented for 

calculating heat flow through various combinations of materials. For convenience of 

the designer, thermal coefficients of some 

commonly used assemblies are presented in 

tables 12 through 15. In these tables, U. 

values are given for several typical frame 

construction assemblies. Where an air space 

is included in the table, the U value is based 

on the assumption that the air space is 

empty and that its surfaces are of ordinary 

building materials of low thermal reflectivity. 

Variations in conditions, materials, or 

workmanship may cause variations in the U 

value greater than variations resulting from 

mean temperature differences, so temperature differences are not reflected in these 

tables. 

To calculate thermal coefficients for 

various amounts of insulation added to the 

air spaces of these assemblies, simply subtract the resistance of the air space from the 

total resistance of the assembly and add the 

resistance of the insulating material. Thermal coefficients for heat flow through walls 

are the same for winter and summer; however, for ceilings, different coefficients are 

used for heat flow up and heat flow down. 

Enhancing Thermal Performance 

Thermal performance of ceilings can 

usually be enhanced by simply adding to the 

thickness of insulation. Where structural 

spaces restrict the insulation thickness, 

such as in a wall cavity, methods of adding 

insulation are more complex. Some possibilities include compressing fibrous insulation 

into a smaller cavity; making a larger cavity, 

such as by using 6-inch wall studs (77); or 

adding a rigid foam to one side of the construction assembly to supplement normal 

stud space insulation. 

Fibrous insulation in batt or blanket 

form is usually manufactured in an efficient 

density. Compressing these insulations into 

lesser thicknesses reduces their thermal 

resistance. The amount of reduction is 

shown in figure 10. Even with the reduced 

thermal resistance, the compressed insulation may have a higher resistance than the 

blanket manufactured to fill a cavity; however, the added resistance may not be adequate to justify the differences in cost between the two thicknesses. 

The wall cavity is conventional wood. 

frame construction is 31⁄2 inches thick 

because 2 by 4 framing members are used. 

Increasing the framing size to 2 by 6’s would 

provide space for an additional 2 inches of insulation. This system would permit the use 

of R-19 insulation, although if the thickness 

is 6 inches, compressing it into a 51⁄2-inch 

cavity would reduce its value to R-18. The use 

of 1⁄2-inch fiberboard sheathing would bring 

the total back to R-19 without covering materials. Advocates of this system point out 

that 2 by 6 studs permit 24-inch spacing, as 

opposed to the commonly used 16-inch spacing with 2 by 4’s. (The 24-inch spacing cuts 

48 
Table 12.--Coefficient of transmission (U) of frame walls--replace air space with 

3.5-inch R-11 blanket insulation (adapted from ASHRAE Handbook, 1977) 

Resistance (R) 

Construction Construction 

Construction NO. 11/ NO. 22/ 

Between At Between At 

framing framing framing framing

(Ri) (R

s

) (Ri) (Rs) 

1. Outside surface (15 mph wind)

2. Siding, wood, 0.5 x 8 in. 

lapped (average)

3. Sheathing, 0.5-in. asphalt

impregnated

4. Nonreflective air space,

3.5 in. (50° F mean; 10° F 

temperature difference)

5. Nominal 2- x 4-in. wood stud 

6. Gypsum wallboard, 0.5 in. 

7. Inside surface (still air) 

Total thermal resistance (R) 

1/Ui = 1/4.44=0.225; U

s = 1/7.81 =0.128. With 20 pct framing (typical of 2- x 4-in. studs 

16 in. on center), Uav 

= 0.8(0.225) + 0.2(0.128) = 0.206 (see eq. 9).

2/ Ui = 1/14.43 = 0.069; U

s 

= 0.128. With framing unchanged, U

av 

= 0.8(0.069) + 0.2(0.128) = 0.081. 
Table 13.--Coefficient of transmission (U) of solid masonry walls--replacing furring strips

and air space with l-inch extruded polystyrene(adapted from ASHRAE Handbook, 1977) 

Resistance (R)

for construction 

Construction 

Construction 

1/ NO. 1

NO. 2 

Between At

furring furring 

(Ri) 

(Ri) (Rs) 

2/ 

1. Outside surface (15 mph wind) 

2. Common brick, 8 in. 

3. Nominal 1- x 3-in. vertical

furring 

4. Nonreflectiveairspace,

0.75 in. (50°F mean;

10°F temperature

difference) 

5. Gypsumwallboard, 0.5 in. 

6. Inside surface (still air) 

Total thermal resistance (R) 

1/Ui= 1/3.91 =0.256; Us = 1/3.84=0.260. With 20 pct framing (typical of 1- x 3-in. vertical 

furring on masonry 16 in. on center), Uav = 0.8(0.256) + 0.2(0.260) = 0.257. 

2/ Ui = Us = Uav = 1/7.90 = 0.127. 
Table 14.--Coefficients of transmission (U) of wood construction flat roofs and 

1 ceilings--winter conditions, upward flow (adapted from ASHRAE Handbook, 1977) 

Resistance (R) 

Construction Construction 

Construction (heat flow up) NO. 1 2/ NO. 23/ 

Between At Between At 

joists joists joists joists

(Ri) (Rs) (Ri) (Rs) 

1. Inside surface (still air)

2. Acoustical tile, fiberboard,

glued, 0.5 in. 

3. Gypsum wallboard, 0.5 in. 

4. Nominal 2- x 8-in. ceiling

joists

5. Nonreflective air space,

7.25 inches (50° F mean;

10° F temperature

difference)

6. Plywood deck, 0.625 in. 

7. Rigid roof deck insulation,

c = 0.72 (R = 1/C)

8. Built-up roof 

9. Outside surface (15 mph wind) 

Total thermal resistance (R) 

1/ Coefficients are expressed in British thermal units per hour per square foot per degree Fahrenheit difference in 

temperature between the air on the two sides, and are based upon an outside wind velocity of 15 mph.

2/ Construction No. 1: Ui = 1/5.91 = 0.169; Us = 1/14.04 = 0.071. With 10 pct framing (typical of 2-in. joists 

16 in. on center), Uav = 0.9(0.169) + 0.1(0.071) = 0.159. 

3/ Construction No. 2: Roof deck insulation and 7.25-in. air space with 6-in. R-19 blanket insulation and 1.25-in. 

air space. Ui = 1/23.64 = 0.042; Us = 1/12.65 = 0.079. With framing unchanged, Uav = 0.9(0.042) + 0.1(0.079) = 0.046. 
Table 15 .--Coefficients of transmission (U) of pitched roofs (adapted from ASHRAE Handbook, 1977) 

Construction Construction 

Construction No. 1--heat NO. 12/ NO. 23/ 

flow up, reflective air Between At Between At

space rafters rafters rafters rafters 

(Ri) (Rs) (Ri) (Rs) 

1. Inside surface (still

air)

2. Gypsum wallboard,

0.5 in., foil backed 

3. Nominal 2- x 4-in. 

ceiling rafter 

4. 45° slope reflective 

air space, 3.5 in. 

(50° F mean, 30° F 

temperature difference)

5. Plywood sheathing,

0.625 in. 

6. Felt building membrane 

7. Asphalt shingle roofing

8. Outside surface (15 mph

wind) 

Total thermal resistance 

(R) . . . . . . . . . . 

1/ Coefficients are expressed in British thermal units per hour per square foot per degree Fahrenheit 

difference in temperature between the air on the two sides, and are based on an outside wind velocity of 

15 mph for heat flow upward and 7.5 mph for heat flow downward. 

2/ Ui = 1/4.69 = 0.213; Us = 1/6.90 = 0.145. With 10 pct framing (typical of 2-in. rafters 16 in. 

on center), Uav = 0.9(0.213) + 0.1(0.145) = 0.206. 

3/ Uav for construction 2 with heat flow down (summer conditions). Ui = 1/7.07 = 0.141; 

Us = 1/7.12 = 0.140. With framing unchanged, Uav = 0.9(0.141) + 0.1(0.140) = 0.141. 
Table 15A.--Coefficients of transmission (U) of pitched roofs (adapted from ASHRAE Handbook. 1977) 

Construction Construction 

Construction No. 3--heat NO. 32/ NO. 43/ 

flow up, nonreflective Between At Between At air space rafters rafters rafters rafters 

1. Inside surface (still air)

2. Gypsum wallboard, 0.5 in. 

3. Nominal 2- x 4-in. ceiling

rafter 

4. 45° slope, nonreflective 

air space, 3.5 in. 

(50° F mean; 10° F 

temperature difference)

5. Plywood sheathing,

0.625 in. 

6. Felt building membrane 

7. Asphalt shingle roofing

8. Outside surface (15 mph

wind) 

Total thermal resistance 

(R) . . . . . . . . . . 

1/ Coefficients are expressed in British thermal units per hour per square foot per degree Fahrenheit 

difference in temperature between the air on the two sides, and are based on an outside wind velocity of 

15 mph for heat flow upward and 7.5 mph for heat flow downward. 

2/ Ui = 1/3.48 = 0.287; Us = 1/6.90 = 0.145. With 10 pct framing (typical of 2-in. rafters 16 in. 

on center), Uav = 0.9(0.287) + 0.1(0.145) = 0.273. 

3/ Uav for construction 4 with heat flow down (summer conditions). Ui = 1/3.64 = 0.275; 

Us = 1/7.12 = 0.140. With framing unchanged, Uav = 0.9(0.275) + 0.1(0.140) = 0.262. 
Figure 10.–Reduction in thermal resistance of fibrous insulation due to compression to reduce 

thickness. (From HUD Mobile Homes Federal Construction and Safety Standards, 1976). 

(M 146 478) 

down on the area of framing in the wall.) 

However, major model building codes as well 

as HUD Minimum Property Standards for 

One- and Two-Family Dwellings (15) permit 

2 by 4 studs at 24-inch spacing, so the reduction in area of framing by wider spacing of 

studs could be used to advantage regardless 

of whether 2 by 4’s or 2 by 6’s are used. 

The addition of rigid foam to a construction assembly could add materially to the 

thermal resistance. This could be in the form 

of sheathing applied to the outside face of 

studs. Such an application has the advantage 

of being continuous over the whole wall including the framing. Thus it creates a barrier 

at the stud which is otherwise a continuous 

thermal bridge. The insulating value of a 

polystyrene foam panel having a resistance 

of 5.26 added to R-13 batt insulation results 

in a total R of 18.26. This R is comparable 

to R-19 insulation used in wall sections 

employing 6-inch studs. The closed cell, 

rigid foams are somewhat resistant to 

transmission of water vapor, so in cold 

climates a good vapor barrier on the warm 

face of the wall is critical when foam is 

used on the cold side of the wall. Rigid insulation must be continuous over the entire 

face with all holes patched for optimum effectiveness. 

Code Requirements 

Historically, building codes have been 

safety oriented, and so have not been concerned with energy conservation. HUD Minimum Property Standards (15) has included insulation requirements because of concerns 

for resale value, but prior to 1974 these requirements were not adequate for energy 

conservation. Since then a more stringent 

standard has been developed–ASHRAE 

Standard 90-75 (2) which is intended to serve 

as a national energy standard for new buildings. It is applicable to all houses insured via 

government loan programs and is recognized 

by many local code authorities. As this standard is adopted by local code authorities, it 

is likely to be modified, so there will be many 

variations. Therefore, the final design of a 

building remains governed by the applicable 

local code. 

The energy standards are stated in 

terms of maximum “U” values for the particular element of construction for specific 

degree-day regions. This value is the recipro54 
cal of the sum of resistances including surface air films (U = 1 ). These maximum 

R 

values are given in tables 16 and 17. Requirements for slab edge insulation are given in 

table 18. Variations from these values are 

permitted by most standards. The following 

example is given in paragraph 607-3.2(b) of 

HUD Minimum Property Standards: “Where 

the stated U value of any one component of 

roof deck, ceiling, wall, or floor cannot be 

practically obtained, such U value may be increased to the maximum figure attainable 

and the U value for other components 

decreased until the overall heat gain or 

heat loss does not exceed the total resulting from conformance to the stated U 

values.” 

SUMMARY 

This paper has surveyed energy efficiency in light-frame construction. An 

understanding of the principles presented 

should help the designer to plan an energyefficient house. 

The increased use of insulation in recent years has resulted in energy savings, 

but there are practical limits to savings 

from insulation alone. One of the largest 

sources of heat loss is often air leakage, or 

infiltration. Air leakage can be reduced by 

good quality windows and doors and generally tight construction. Additional savings 

are possible by incorporating natural 

climate control which includes shape and 

Table 16.--Maximum "U" values for ceilings, walls,

and openings (from HUD Minimum 

Property Standards. 1977) 

Winter degree-days (65° F base) 

Building component 

Roof deck1/ 

Ceiling 

Masonry walls 

Frame walls 

Doors and windows3/ 

1/ Roof/ceiling assemblies, in which the finished 

ceiling surface is the underside of the roof deck. 

2/ When mechanical cooling is proposed, use 0.08. 

3/ Maximum glass area shall not exceed 15 pct of the 

gross-area of all exterior walls enclosing heated spaces,

except when demonstrated that the winter daily solar heat 

gain exceeds the 24-h heat loss. 

55 
orientation of the house, size and placement of windows, shading, and wind patterns. 

Environmental design requirements 

must be known for effective planning. 

These are presented both in terms of 

climate zones and indoor human comfort 

levels. 

Properties of insulating materials and 

the effect of environmental conditions on 

these properties are essential for determining optimum methods of construction. This 

information is given and followed by instructions for making heating and cooling 

load calculations. Finally, the thermal performance of some typical frame assemblies 

is presented, and methods of enhancing 

thermal performance of assemblies are 

given. 

Table 17.--Maximum "U" values for floors over unheated spaces, or 

for foundation wall sections of heated underfloor 

spaces (from HUD Minimum Property Standards, 1977) 

Winter degree-days Maximum "U" 

1/ value (65° F base) 

MAXIMUM "U" VALUE OF FLOOR 

SECTIONS OVER UNHEATED BASEMENTS,

UNHEATED GARAGES, OR CRAWL 

SPACES2/ 

Winter degree-days Maximum "U" 

1/ value (65° F base) 

MAXIMUM "U" VALUES OF THE 

FOUNDATION WALL SECTIONS OF 

HEATED BASEMENT OR HEATED 

CRAWL SPACE 

1/ For increments between degree-days shown,

interpolated, or the values shown in fig. 5 of ASHRAE 90-75 may be 

"U" values may be 

substituted. 

temperature 

2/ A basement, crawl space, or garage shall be considered unheated 

unless it is provided with a positive heat supply to maintain a minimum 

of 50° F. Positive heat supply is defined by ASHRAE as "heat 

supplied to a space by design or by heat losses occurring from energyconsuming systems or components associated with that space." 

56 
Table 18.--Minimum "R" values of perimeter insulation for slabson-grade (from HUD Minimum Property Standards, 1977) 

Minimum "R" 

1/ degree-days values 

Winter 

(65° F 

base) Heated Unheated 

slab slab 

Minimum "R" 

1/ degree-days values 

Winter 

(65° F 

base) Heated Unheated 

slab slab 

1/ For increments between degree-days shown,

be interpolated, or the values shown in fig. 2 of ASHRAE 90-75 

"U"values may 

may be substituted. 

LITERATURE CITED 

1. Anderson, B., and M. Riordan. 

1976. The solar home book. Cheshire 

Books, Harrisville, N.H. 

2. American Society of Heating, Refrigerating, and Air-conditioning Engineers. 

1975. Energy conservation in new 

building design. ASHRAE Standard 

90-75. New York, N.Y. 

3. American Society of Heating, Refrigerating, and Air-conditioning Engineers. 

1977. Handbook and product directory. 

New York, N.Y. 

4. Kusuda, T., and K. Ishii. 

1977. Hourly solar radiation data for 

vertical and horizontal surfaces on 

average days in the United States 

and Canada. U.S. Dep. Comm., Nat. 

Bur. Stand., Building Science Series 

No. 96. Washington, D.C. 

5. Latta, J. K. 

1973. Walls, windows, and roofs for the 

Canadian climate. National Research 

Council, Canada. 87 p. 

6. Mattingly, G. E., and E. F. Peters. 

1975. Wind and trees–air infiltration 

effects on energy in housing. Center 

for Environmental Studies Rep. No. 

20, Princeton University, Princeton, 

N.J. 101 p. 

7. National Forest Products Association. 

1975. All-weather wood foundation system. NFPA, Washington, D.C. 

8. Neubauer, L. W., and Richard D. Cramer. 

1968. Effect of shape of building on interior air temperature. Transactions 

of the ASAE. p. 537-539. 

57 
9. Olgyay, Victor. 

1963. Design with climate. Princeton 

Univ. Press, Princeton, N.J. 190 p. IIIus. 

10. Oviatt, A. E. 

1975. Optimum insulation thickness in 

wood-framed walls. USDA Forest 

Sew. Gen. Tech. Rep. PNW-32. 37 p. 

11. Owens-Corning Fiberglass Corporation. 

1976. The Arkansas story. Toledo, Ohio. 

12. Peavy, B. A., D. M. Burch, F. J. Powell, 

and C. M. Hunt. 

1975. Comparison of measured and 

computer-predicted thermal performance of a four bedroom woodframe townhouse. U.S. Dep. Comm., 

Nat. Bur. Stand., Building Science 

Series No. 57. Washington, D.C. 

13. Total Environment Action. 

1975. Solar energy home design in four 

climates. Harrisville, N.H. 198 p. IIIus. 

14. U.S. Department of Housing and Urban 

Development. 

1976. Solar dwelling design concepts. 

[Prepared by AIA Research Corporation.] Washington, D.C. 146 p. 

15. U.S. Department of Housing and Urban 

DeveIopment. 

1973 (with subsequent amendments). 

Minimum property standards, oneand two-family dwellings. Washington, D.C. 

16. Watson, D. 

1977. Designing and building a solar 

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