WINDOW SELECTION

Modern windows provide energy savings,

durability, and comfort

BUILDING TECHNOLOGIES PROGRAM

OFFICE OF ENERGY EFFICIENCY AND RENEWABLE ENERGY . U.S. DEPARTMENT OF ENERGY

Buildings for the

21st Century

Buildings that are more

energy efficient, comfortable,

and affordable... that’s the

goal of DOE’s Building

Technologies Program.

To accelerate the development

and wide application of energy

efficiency measures, the

Building Technologies Program:

• Conducts R&D on technologies

and concepts for energy

efficiency, working closely with

the building industry and with

manufacturers of materials,

equipment, and appliances

• Promotes energy/money

saving opportunities to both

builders and buyers of homes

and commercial buildings

• Works with state and local

regulatory groups to improve

building codes, appliance

standards, and guidelines for

efficient energy use

Technology Fact Sheet

WHY ARE WINDOWS IMPORTANT?

Windows bring light, warmth, cooling breezes,

and beauty into homes and give a feeling of

openness and space to living areas. Although

beneficial, windows can negatively impact a

home’s energy efficiency, comfort, indoor air

quality, and durability if not properly selected

and installed.

Windows can be major sources of heat loss in

the winter and heat gain in the summer. In cold

climates, windows can be responsible for 10 to

25 percent of a home’s heat loss during the

winter and heat gain in the summer. While

energy-efficient windows cost more than

standard models, they can cut energy bills

significantly and reduce heating and cooling

loads enough to permit smaller and cheaper

heating and cooling equipment and ductwork.

Energy-efficient windows improve comfort in

the winter by increasing surface temperatures

and by being more airtight, both of which cut

drafts. Increased surface temperatures also

reduce condensation that forms on the inside

frame and glazing during the winter, protecting

building materials and reducing mold growth.

High-performance, energy-efficient windows

usually use multiple glazings and insulated

frames, which reduce unwanted noise from the

outside. Another window technology —

low-emittance coating — blocks up to

98 percent of the ultraviolet radiation from the

sun, reducing fading and wear on curtains,

carpeting, and furniture.

STEPS TO WINDOW SELECTION

When properly selected and installed, windows

can help minimize a home’s heating, cooling,

and lighting costs. Achieving improved window

performance in a house is a three-step process:

• Design — Window sizes, types, placement,

and properties should be determined as an

integral part of the whole house design

because of their impact on heating and

cooling loads, lighting, and interior decor.

Passive solar features and their impact on the

windows should be considered during

design. Window properties that meet the

design goals should be specified using

applicable guidelines.

• Selection — Windows should be selected

that meet the specified design properties after

considering the window technologies that

improve performance and factoring in

appearance, purchase price, cost of

installation, warranty coverage, maintenance,

replacement frequency, need for extra window

treatments (like awnings), and resale value.

• Installation — Even the most energyefficient window must be properly installed

to ensure that its energy performance is

achieved and that the window does not

contribute to building moisture problems.

WINDOW LABELING

Look for these

labels to guide

your window

purchase

decisions.
WINDOW PROPERTIES

Heat is lost and gained through windows by direct conduction

through the glass and frame, by air leakage through and around

the window assembly, and by the radiation of heat into the house

(typically from the sun) and out of the house from roomtemperature objects such as people, furniture, and interior walls.

The National Fenestration Rating Council (NFRC – 8484 Georgia

Avenue, Suite 320, Silver Spring, Maryland 20910, 301-589-1776,

www.nfrc.org) operates a voluntary certification program for

windows that documents three measured criteria corresponding to

each of these heat loss and gain methods: U-factor, air leakage,

and solar heat gain coefficient (SHGC). In addition, NFRC labels the

visible transmittance (VT) of the window.

• U-factor — U-factor is a measure of the rate at which a

window conducts non-solar heat flow, usually expressed in

units of Btu/hr-ft2

-°F. The U-factor may refer to just the glass

alone but, under the NFRC ratings, it represents the

performance of the entire window, including the frame and

spacer materials. A window with a lower U-factor is more

energy-efficient than one with a higher U-factor. Window

U-factors generally range from 0.2 to 1.2.

• SHGC — The SHGC is the fraction of solar radiation admitted

through a window, either transmitted directly and/or absorbed

by a window assembly and subsequently released as heat

inside the home. The lower a window’s SHGC, the less solar

heat it transmits and the greater its shading ability. A window

with a high SHGC is more effective at collecting solar heat gain

during the winter. A window with a low SHGC is more effective

at reducing cooling loads during the summer by blocking heat

gained from the sun. SHGCs range between 0 and 1.

• Air Leakage — Air leakage is a measure of the rate of air

infiltration around a window in the presence of a specific

pressure difference across the window. The air leakage is

expressed in units of cubic feet per minute per square foot of

frame area (cfm/ft2). A window with a low air leakage rating is

tighter than a window with a high air leakage rating. Air

leakages are usually less than 1.

• VT — VT is the fraction of the visible spectrum of sunlight

(380 to 720 nanometers) weighted by the sensitivity of the

human eye that is transmitted through the window’s glazing. A

window with a higher VT transmits more visible light. VT is

expressed as a number between 0 and 1.

Window selection is primarily based on U-factor and SHGC,

although VT and air leakage can be important. Thus, the NFRC

label helps consumers make educated purchasing decisions.

WINDOW DESIGN CONSIDERATIONS

Windows are an important element of passive solar designs,

which can reduce heating, cooling, and lighting needs in a

house. Given the proper building site, virtually any type of

architecture can integrate at least some passive solar design

features. Passive solar design strategies vary by building

location and regional climate, but the basic techniques

involving windows remain the same — select, orient, and size

glass to maximize solar heat gain in winter and minimize it in

summer, with different glazings usually selected for different

sides of the house (exposures).

In heating climates, major glass areas are generally faced south

to collect solar heat during the winter when the sun is low in the

sky. In the summer, when the sun is high overhead, overhangs

or other shading devices (e.g., awnings) prevent excessive heat

gain. To be effective, south-facing windows usually must have a

SHGC of greater than 0.6 to maximize solar heat gain during the

winter, a U-factor of 0.35 or less to reduce conductive heat

transfer, and a high VT for good visible light tranfer.

Windows on east-, west-, and north-facing walls are reduced in

heating climates, while still allowing for adequate daylight.

East- and west-facing windows are limited because it is difficult

to effectively control the heat and penetrating rays of the sun

when it is low in the sky. These windows should have a low

SHGC and/or be shaded. North-facing windows collect little

solar heat, so they are used just to provide useful lighting.

In cooling climates, particularly effective strategies include

preferential use of north-facing windows and generously

shaded south-facing windows.

The type of window impacts the air leakage and effective natural

ventilation that can be achieved. Fixed-pane windows are

air-tight but are not suitable in places where ventilation is

desired. Casement, awning, and hopper windows provide good

ventilation when opened and are moderately air-tight when

closed because they have compression seals. Sliding,

double-hung, and single-hung windows are the most leaky and

provide minimal ventilation when opened.
WINDOW SPECIFICATION GUIDELINES

There are three sources of guidance to help in the specification

of windows. These should be used after considering the

passive solar features that will be included in the house and the

impact these have on the window requirements. Shading to be

provided by landscaping, overhangs, shutters, etc., should also

be considered, especially when focusing on the SHGC needed

for the windows.

2000 I NTERNATIONAL ENERGY CONSERVATION CODE

As a minimum, windows should be selected in accordance with

the 2000 International Energy Conservation Code. The

International Energy Conservation Code can be obtained from

the International Code Council, 703-931-4533,

www.intlcode.org. The U-factor required for a specific climate

may be dependent on the window area being installed or the

efficiency of other elements of the house depending on the

compliance approach chosen. The air leakage rating must be

less than 0.3, and the SHGC must be less than 0.4 if the

heating-degree days are less than 3,500.

ENERGY STAR ®

Energy Star is a federal initiative that helps consumers identify

energy-efficient products, including windows and skylights.

Windows carrying the Energy Star label use less energy than

standard products for three major climate regions across the

country. All Energy Star windows must carry the NFRC

certification label.

Information on the Energy Star windows program can be found

at www.energystar.gov. The following are the Energy Star

window qualification guidelines by climate group.

Climate

Window & Door Skylight

U-factor SHGC U-factor SHGC

Northern

0.35 or

less

None

specified

0.45 or

less

None

specified

Central 0.40 or

less

0.55 or

less

0.45 or

less

0.55 or

less

Southern

0.75 or

less

0.40 or

less

0.75 or

less

0.40 or

less

VT > 0.60 recommended in all climates.

ENERGY STAR® FENESTRATION GUIDELINES

WINDOW TYPES

Energy-efficient windows come in traditional styles.

Awning Hopper Sliding

Fixed Double-hung Casement

TH E EFFICIENT WINDOWS C OLLABORATIVE

The Efficient Windows Collaborative provides information on

energy-efficient windows, including recommendations for

product selection based on climate and state. Its website

(www.efficientwindows.org) includes a regionally-designed

window selection tutorial to guide consumers through NFRC

and Energy Star window criteria. The tutorial includes

comparisons of the annual energy performance of various

window options for an average 2000-ft2 house. Additional

SHGC recommendations are provided compared with

Energy Star criteria, and U-factor criteria are somewhat different

for homes located in California.
WINDOW SELECTION

WINDOW INSTALLATION

Window installation varies depending on the type of house

construction (wood versus masonry), exterior cladding (e.g., wood

siding, stucco, brick), and type of weather-resistive barrier being

installed. Windows should be installed following manufacturing

recommendations and considering the following additional

guidelines (primarily for windows with fin mounting systems).

The window opening must be flashed and integrated into the

home’s weather-resistive barrier so that any potential water

leaks do not cause damage. A weather-resistive barrier is a

drainage plane that allows water that has penetrated past the

siding to drain away from the wall system. The figures illustrate

a procedure when building paper or housewrap is being used as

the weather-resistive barrier. Details will vary with siding and

window type and the installation sequence for the window, trim,

and weather-resistive barrier. It is advisable to install window

head and sill flashing, whether it is metal, plastic, or a selfsticking elastomeric membrane. Avoid relying on tapes or

sealants to provide waterproofing, as these products may fail

over time.

It is common practice and recommended by some manufacturers

to cut an “X” in housewrap placed over window openings, pull

the material inside, and secure it by stapling. Other manufacturers

require alternative methods, such as the modified “I”-cut,

depending on the overall flashing approach. The “I”-cut allows

the vertical leg of the head flashing to be placed under the

weather-resistive barrier and then taped or sealed.

It is best to divert drainage onto the face of the weatherresistive barrier. Do not tape down or seal behind the bottom

nailing flange of the window, as doing so could accidentally

trap in water.

Barrier strips are attached with nails and overlapped with successive barrier strips and weather-resistive

barriers in shingle fashion. Do not depend on tapes or glues, as they may fail over time.

One housewrap technique utilizes a modified “I”-cut in the material over a window opening, which creates

flaps that are then wrapped to the inside of the window frame and fastened. The head flashing and barrier

strip are fastened and “shingled” under a flap that is cut in the housewrap. These seams are then taped.

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FLASHING WINDOW OPENINGS
WINDOW SELECTION

Windows must also be properly air-sealed during installation to

perform correctly. To air seal the window, caulk the backsides of

the window mounting flanges (top and sides only) to the

weather-resistive barrier during installation. The mounting

flange (nailing fin) is an integral part of most window frames

that laps over the conventional stud construction. Nails are

driven through it to secure the frame in place. Also, from inside

the house, seal the gap between the window frame and rough

opening using backer rod and caulk or non-expanding

latex-based spray foams that will not pinch jambs or void

window warranties. Backer rod is a closed-cell foam or rope caulk

that is pressed into cracks or gaps with a screwdriver or putty

knife. Insulation stuffed into this crack does not stop air flow.

WINDOW TECHNOLOGIES

A number of different window technologies improve the energy

efficiency of windows and raise or lower the three primary

window rating criteria: U-factor, SHGC, and VT.

• Frame material: Vinyl, wood, fiberglass, and some composite

frame materials have a higher thermal resistance than metal

window frames, so they lower the U-factor for the window.

Metal window frames should have a thermal break (insulating

plastic strip placed between the inside and outside of the

frame and sash) to reduce heat flow and the window U-factor.

WINDOW TECHNOLOGIES

Energy-efficient window technologies are available to

produce windows with the U-factor, SHGC, and VT

properties needed for any application.

Use backer rod

to fill gaps between the

window and rough opening 

before caulking 

SEAL ROUGH OPENING

Use backer-rod and

caulk or non-expanding

foam sealant to fill the

gaps around the window

and rough opening.
For more information, contact:

Energy Efficiency and

Renewable Energy

Clearinghouse (EREC)

1-800-DOE-3732

www.eere.doe.gov

Or visit the Building

Technologies Program

Web site at

www.buildings.gov

Or refer to the Residential

Windows: A Guide to New

Technology and Energy

Performance, Second Edition,

Efficient Windows

Collaborative

www.efficientwindows.org

Written and prepared for

the U.S. Department of

Energy by:

Southface Energy Institute

404-872-3549

www.southface.org

U.S. Department of Energy’s

Oak Ridge National

Laboratory

Buildings Technology Center

865-574-5206

www.ornl.gov/btc

WINDOW SELECTION

NOTICE: Neither the United

States government nor any

agency thereof, nor any of their

employees, makes any warranty,

express or implied, or assumes

any legal liability or responsibility

for the accuracy, completeness,

or usefulness of any information,

apparatus, product, or process

disclosed. The views and opinions of authors expressed herein

do not necessarily state or reflect

those of the United States government or any agency thereof.

Printed with a renewable-source ink on paper containing at

least 50% wastepaper, including 20% postconsumer waste.

February 2003 DOE/GO-10099-777

• Insulated glazing: Insulated glazing

(also called double glazing or triple glazing)

refers to windows with two or more panes of

glass. The glass panes are spaced apart and

hermetically sealed to form a single glazed

unit with an air space between each pane of

glass. The glass layers and the air spaces

resist heat flow. Insulated glazing’s main

impact is to lower the U-factor, but it also

lowers the SHGC.

• Spacer: A spacer separates two panes of

glass. Some spacers are designed to conduct

heat less readily than others, which lowers

the window U-factor.

• Low-emittance coating: A low-emittance

(or low-e) coating is a microscopically thin,

virtually invisible, metal or metallic oxide layer

deposited directly on the surface of one or

more of the panes of glass in a window with

insulated glazing. The low-e coating reduces

the infrared radiation from a warm pane of

glass to a cooler pane, thereby lowering the

U-factor of the window. Low-e coatings also

reduce the SHGC and VT. To keep the sun’s

heat out of the house (for hot climates, eastand west-facing windows, and unshaded

south-facing windows), the low-e coating

should be applied to the outside pane of

glass. If the windows are designed to provide

heat energy in the winter and keep heat

inside the house (typical of cold climates),

the low-e coating should be applied to the

inside pane of glass.

• Spectrally selective coating: A special

category of low-e coating is spectrally

selective. This coating is optically designed

to reflect particular wavelengths and be

transparent to others. Such a coating is

commonly used to reflect the infrared (heat)

portion of the solar spectrum but admit a

higher portion of visible light, creating a

window with a low U-factor and low SHGC,

but a high VT.

• Inert gas fill: Inert gases are ones that do

not react readily with other substances

(e.g., argon, krypton). Because these gases

have a higher resistance to heat flow than air,

they (rather than air) are sealed between the

window panes of windows with insulated

glazing (and usually a low-e coating) to

decrease a window’s U-factor.

• Heat-absorbing (or tinted) glass:

Heat-absorbing glass contains special tints

that change the color of the glass. Tinted

glass absorbs a large fraction of the

incoming solar radiation, which reduces

SHGC, VT, and glare. Gray and bronze tints

reduce the penetration of both visible light

and heat into buildings in equal amounts

and are the most common colors used. Blueand green-tinted windows offer greater

penetration of visible light (higher VT) but

also heat (higher SHGC). Heat-absorbing

glass reflects only a small percentage of light

and therefore does not have the mirror-like

appearance of reflective glass.

• Reflective glass: Reflective glass is coated

to reflect radiation striking the surface of the

glass. The reflective coating usually consists

of thin metallic layers and comes in various

metallic colors (silver, gold, bronze).

Reflective glass reduces the passage of solar

radiation through the window, generally

blocking more visible light than heat. It

reduces SHGC and greatly reduces VT and

glare. Reflective glass is most useful in hot

climates in which solar control is critical, and

it is usually used just for special applications.

Reduced cooling energy demands may be

offset by the need for additional electrical

lighting.