This document was produced for the U.S. Department of Energy (DOE) by the National Renewable Energy Laboratory (NREL), a DOE

national laboratory. The document was produced by the Information Services Program, under the DOE Office of Energy Efficiency and

Renewable Energy. The Energy Efficiency and Renewable Energy Clearinghouse (EREC) is operated by NCI Information Systems, Inc.,

for NREL/DOE. The statements contained herein are based on information known to EREC and NREL at the time of printing. No

recommendation or endorsement of any product or service is implied if mentioned by EREC.

 Printed with a renewable source ink on paper containing at least 50% wastepaper, including 10% postconsumer waste

Heat escapes around vc-cd16-a1514-01

loosely-fitted frames and 

through glass panes in windows.

DOE/CH10093-290

FS 216

October 1994

Energy-Efficient Windows

Windows bring light, warmth, and beauty

into buildings and give a feeling of

openness and space to living areas. They

can also be major sources of heat loss in

the winter and heat gain in the summer.

In 1990 alone, the energy used to offset

unwanted heat losses and gains through

windows in residential and commercial

buildings cost the United States $20

billion (one-fourth of all the energy used

for space heating and cooling). However,

when properly selected and installed,

windows can help minimize a home's

heating, cooling, and lighting costs. This

publication describes one

option—energy-efficient

windows—available for reducing a

home's heating and cooling energy

requirements.

Controlling Air Leaks

When air leaks around windows, energy

is wasted. Energy is also transferred 

through the centers, edges, and frames of

windows. Eliminating or reducing these

paths of heat flow can greatly improve

the energy efficiency of windows and,

ultimately, of homes. Several options are

available to reduce air leaks around

windows; the least expensive options are

caulking and weatherstripping, followed

by replacing window frames. 

Caulking and Weatherstripping

Caulks are airtight compounds (usually

latex or silicone) that fill cracks and

holes. Before applying new caulk, old

caulk or paint residue remaining around

a window should be removed using a

putty knife, stiff brush, or special

solvent. After old caulk is removed, new

caulk can then be applied to all joints in

the window frame and the joint between

the frame and the wall. The best time to

apply caulk is during dry weather when

the outdoor temperature is above

45E Fahrenheit (7.2E Celsius). Low

humidity is important during

application to prevent cracks from

swelling with moisture. Warm temperatures are also necessary so the caulk

will set properly and adhere to the

surface. 

Weatherstripping is a narrow piece of

metal, vinyl, rubber, felt, or foam that

seals the contact area between the fixed

and movable sections of a window joint.

It should be applied between the sash

and the frame, but should not interfere

with the operation of the window. For

more information on caulking and

weatherstripping, contact the Energy

Efficiency and Renewable Energy

Clearinghouse (EREC).
2

Frame types can affect the energy efficiency of a window. vc-cd16-a1514-02

If a window's 

resistance to

heat flow, or 

R-value, is 

high, it will 

lose less heat 

than one 

with a lower

R-value.

Replacing Window Frames

The type and quality of the window

frame usually affect a window's air

infiltration and heat loss characteristics.

Many window frames are available—all

with varying degrees of energy efficiency.

Some of the more common window

frames are fixed-pane, casement, doubleand single-hung, horizontal sliding,

hopper, and awning.

When properly installed, fixed-pane

windows are airtight and inexpensive and

can be custom designed for a wide variety

of applications. But, because they cannot

be opened, fixed-pane windows are

unsuitable in places where ventilation is

required.

Casement, awning, and hopper windows

with compression seals are moderately

airtight and provide good ventilation

when opened. Casement windows open

sideways with hand cranks. Awning

windows are similar to casement

windows except that their hinges are

located at the tops of the windows instead

of at the sides. Hopper windows are inverted versions of

awning windows with their hinges

located at the bottom. Windows with 

compression seals allow about half as

much air leakage as double-hung and

horizontal sliding windows with sliding

seals.

Double-hung windows have top and

bottom sashes (the sliding sections of the

window) and can be opened by pulling

up the lower sashes or pulling down the

upper sash. Although they are among

the most popular type of window,

double-hung windows can be inefficient

because they are often leaky. Singlehung windows are somewhat better

because only one sash moves. Horizontal

sliding windows are like double-hung

windows except that the sashes are

located on the left and right edges rather

than on the tops and bottoms. Horizontal

sliding windows open on the side and

are especially suitable for spaces that

require a long, narrow view. These

windows, however, usually provide

minimal ventilation and, like doublehung windows, can be quite leaky.

Reducing Heat Loss and

Condensation

Manufacturers usually represent the

energy efficiency of windows in terms of

their U-values (conductance of heat) or

their R-values (resistance to heat flow).

If a window's R-value is high, it will lose

less heat than one with a lower R-value.

Conversely, if a window's U-value is

low, it will lose less heat than one with a

higher U-value. In other words, U-values

are the reciprocals of R-values (U-value

= 1/R-value). Most window manufacturers use R-values in rating their

windows.

Usually, window R-values range from

0.9 to 3.0 (U-values range from 1.1 to

0.3), but some highly energy-efficient

exceptions also exist. When comparing

different windows, you should ensure

that all U- or R-values listed by manufacturers: (1) are based on current

standards set by the American Society

of Heating, Refrigeration, and AirConditioning Engineers (ASHRAE),

(2) are calculated for the entire 
3

Traditionally,

clear glass has 

been the primary 

material available

for window panes

in homes.

window, including the frame, and not just

for the center of the glass, and

(3) represent the same size and style of

window.

The following five factors affect the

R-value of a window.

• The type of glazing material (e.g.,

glass, plastic, treated glass)

• The number of layers of glass

• The size of the air space between the

layers of glass

• The thermal resistance or conductance

of the frame and spacer materials

• The "tightness" of the installation (i.e.,

air leaks—see previous discussion).

Types of Glazing Materials

Traditionally, clear glass has been the

primary material available for window

panes in homes. However, in recent years,

the market for glazing—or cutting and

fitting window panes into frames— has

changed significantly. Now several types

of special glazings are available that can

help control heat loss and condensation.

Low-emissivity (low-e) glass has a

special surface coating to reduce heat

transfer back through the window. These

coatings reflect from 40% to 70% of the

heat that is normally transmitted through

clear glass, while allowing the full

amount of light to pass through.

Heat-absorbing glass contains special

tints that allow it to absorb as much as

45% of the incoming solar energy, reducing heat gain. Some of the absorbed heat,

however, passes through the window by

conduction and reradiation. 

Reflective glass has been coated with

a reflective film and is useful in 

controlling solar heat gain during the

summer. It also reduces the passage 

of light all year long, and, like heatabsorbing glass, it reduces solar

transmittance.

Plastic glazing materials—acrylic,

polycarbonate, polyester, polyvinyl

fluoride, and polyethylene—are also

widely available. Plastics can be

stronger, lighter, cheaper, and easier

to cut than glass. Some plastics also have

higher solar transmittance than glass.

However, plastics tend to be less durable

and more susceptible to the effects of

weather than is glass.

Storm windows can increase the efficiency of single-pane windows, the least

energy-efficient type of glazing. The

simplest type of storm window is a

plastic film taped to the inside of the

window frame. These films are usually

available in prepackaged kits. Although

plastic films are easily installed and

removed, they are easily damaged and

may reduce visibility. Rigid or semirigid

plastic sheets such as plexiglass, acrylic,

polycarbonate, or fiber-reinforced

polyester can be fastened directly to the

window frame or mounted in channels

around the frame—usually on the

outside of the building. These more

durable materials are also available in

kits. 

For more information about advanced

types of glazing materials, contact EREC.

Layers of Glass and Air Spaces

Standard single-pane glass has very little

insulating value (approximately R-1). It

provides only a thin barrier to the

outside and can account for considerable

heat loss and gain. Traditionally, the

approach to improve a window's energy

efficiency has been to increase the

number of glass panes in the unit, because multiple layers of glass increase

the window's ability to resist heat flow. 
4

Double-pane windows are usually more efficient than vc-cd16-a1514-04

single-pane or storm windows.

Window frames are

available in a 

variety of materials,

including aluminum,

wood, vinyl, and

fiberglass.

Double- or triple-pane windows have

insulating air- or gas-filled spaces

between each pane. Each layer of glass

and the air spaces resist heat flow. The

width of the air spaces between the panes

is important, because air spaces that are

too wide (more than 5/8 inch or

1.6 centimeters) or too narrow (less than

1/2 inch or 1.3 centimeters) have lower Rvalues (i.e., they allow too much heat

transfer). Advanced, multi-pane windows

are now manufactured with inert gases

(argon or krypton) in the spaces between

the panes because these gases transfer less

heat than does air. 

Multipane windows are considerably

more expensive than single-pane

windows and limit framing options

because of their increased weight.

Frame and Spacer Materials

Window frames are available in a variety

of materials including aluminum, wood,

vinyl, and fiberglass. Frames may be

primarily composed

of one material, or they may be a

combination of different materials

such as wood clad with vinyl or

aluminum-clad wood. Each frame material has its advantages and disadvantages.

Though ideal for strength and customized window design, aluminum

frames conduct heat and therefore lose

heat faster and are prone to condensation. Through anodizing or coating, the

corrosion and electro-galvanic

deterioration of aluminum frames can be

avoided. Additionally, the thermal

resistance of aluminum frames can be

significantly improved by placing

continuous insulating plastic strips

between the interior and exterior of

the frame.

Wood frames have higher R-values, are

not affected by temperature extremes,

and do not generally promote condensation. Wood frames do require

considerable maintenance in the form of

periodic painting or staining. If not

properly protected, wood frames can

swell, which leads to rot, warping, and

sticking.

Vinyl window frames, which are made

primarily from polyvinyl chloride (PVC),

offer many advantages. Available in a

wide range of styles and shapes, vinyl

frames have moderate to high R-values,

are easily customized, are competitively

priced, and require very low

maintenance. While vinyl frames do not

possess the inherent strength of metal or

wood, larger-sized windows are often

strengthened with aluminum or steel

reinforcing bars.

Fiberglass frames are relatively new and

are not yet widely available. With some

of the highest R-values, fiberglass frames

are excellent for insulating and will not

warp, shrink, swell, rot, or corrode.

Unprotected fiberglass does not hold up

to the weather and therefore is always

painted. Some fiberglass frames are

hollow; while others are filled with

fiberglass insulation.
5

Many types of

windows and 

window films are

available that

serve different

purposes.

Spacers are used to separate multiple

panes of glass within the windows.

Although metal (usually aluminum)

spacers are commonly installed to

separate glass in multipane windows,

they conduct heat. During cold weather,

the thermal resistance around the edge of

a window is lower than that in the center;

thus, heat can escape, and condensation

can occur along the edges. To alleviate

these problems, one manufacturer has

developed a multipane window using a

1/8-inch-wide (0.32 centimeters-wide)

PVC foam separator placed along the

edges of the frame. Like other multipane

windows, these use metal spacers for

support, but because the foam separator is

secured on top of the spacer between the

panes, heat loss and condensation are

reduced. Several window manufacturers

now sandwich foam separators, nylon

spacers, and insulation materials such as

polystyrene and rockwool between the glass

inside their windows.

Additional Options for Reducing

Heat Loss and Gain through

Windows

Movable insulation, such as insulating

shades, shutters, and drapes, can be

applied on the inside of windows to

reduce heat loss in the winter and heat

gain in the summer. Shading devices,

such as awnings, exterior shutters, or

screens, can be used to reduce unwanted

heat gain in the summer.

In most cases, these window treatments

are more cost-effective than energyefficient window replacements and

should be considered first. Additional

information on window treatments is

available from EREC.

Conclusion

Reducing heat loss or gain in homes

often includes either improving existing

windows or replacing them. Low-cost

options available for improvement are

caulking, weatherstripping, retrofit

window films, and window treatments.

Replacing windows will involve the purchase of new materials, which should

adhere to certain energy efficiency

standards.

Different combinations of frame style,

frame material, and glazing can yield

very different results when weighing

energy efficiency and cost. For example,

a fixed-pane window is the most air-tight

and the least expensive; a window with a

wood frame is likely to have less

conductive heat loss than one with an

aluminum frame; double-pane, low-e

window units are just as efficient as

triple-pane untreated windows, but cost

and weigh less.

No one window is suitable for every

application. Many types of windows and

window films are available that serve

different purposes. Moreover, you may

discover that you need two types of

windows for your home because of the

directions that your windows face and

your local climate. To make wise

purchases, first examine your heating

and cooling needs and prioritize desired

features such as daylighting, solar

heating, shading, ventilation, and

aesthetic value.
6

Source List

The following resources provide more information on energy-efficient windows.

American Architectural Manufacturers

 Association (AAMA)

2700 River Road, Suite 118

Des Plaines, IL 60018

(708) 202-1350

Developed a testing procedure [AAMA

1503] for measuring the thermal transmission properties of aluminum-, vinyl-, and

wood-framed windows.

American Society of Heating,

 Refrigerating, and Air-Conditioning

 Engineers (ASHRAE)

1791 Tullie Circle, NE

Atlanta, GA 30329 

(404) 636-8400

ASHRAE's Handbook of Fundamentals

contains tables citing heat transfer, light

transmittance, and shading properties for

various window types and materials.

For information about many kinds of energy

efficiency and renewable energy topics, contact:

The Energy Efficiency and Renewable

 Energy Clearinghouse (EREC)

P.O. Box 3048

Merrifield, VA 22116

(800) DOE-EREC (363-3732)

Fax: (703) 893-0400

Email: doe.erec@nciinc.com

EREC provides free general and technical

information to the public on the many topics

and technologies pertaining to energy

efficiency and renewable energy.

National Fenestration Rating

 Council (NFRC)

962 Wayne Avenue, Suite 750

Silver Spring, MD 20910

(301) 589-6372

Developed the Procedure for Determining

Fenestration Product Thermal Properties

(NFRC 100-91). These procedures are now

being used in NFRC's window certification

and efficiency labeling programs, which

have already been adopted by three states.

National Wood Window and Door

 Association 

1400 East Touhy Avenue

Des Plaines, IL 60018-3305

(708) 299-5200

Issues seals of approval for manufacturers of

wood-framed windows.

U.S. Department of Energy (DOE)

Building Systems and Materials Division

EE-421

1000 Independence Avenue, SW

Washington, DC 20585

(202) 586-9214

Developed the WINDOW computer program, which aids window manufacturers

and building designers in optimizing the

thermal and daylighting performance of

window systems. For their certification and

labeling programs, the NFRC uses the

WINDOW computer program and DOEsupported research and testing to determine

the thermal and optical properties of

windows.

Vinyl Window and Door Institute

355 Lexington Avenue

New York, NY 10017

(212) 351-5400

Developed performance standards and

certification program for manufacturers of

vinyl-framed windows.

Reading List

"Low-E Glass—Why the Coating Is Where It

Is," Energy Design Update, pp. 5–7, March

1990.

"No Pane, No Gain (Window Technology:

Part One)," Popular Science, pp. 92–98, June

1993.

"Through the Glass Darkly," Popular Science,

pp. 80–87, July 1993.
7