Energy-Efficient Windows

When purchasing a new home, homeowners want
their home to be correct in every detail while costing
as little as possible. However, savings to a homeown-
er can also occur even after the home is purchased,
by using energy-efficient components as the home is
being constructed. One example involves a home-
owner's choice of energy-efficient windows which
have a tremendous impact on energy performance
(and costs!) of heating and cooling a home, as well as
the seasonal comfort level experienced by the home's
occupants. While energy-efficient windows will cost
more initially, they often return their additional cost
investment through substantial savings to the home-
owner on monthly energy bills.

Installation of energy-efficient windows is just one
area where homeowners can decrease their energy
usage, save money, and help the environment all at
the same time. Other areas range from having the
correct (cost-effective) level of insulation in a home's
walls, ceilings, and floors to simple, periodic mainte-
nance of the furnace and air-conditioning system.
Investing in energy-efficient options provides a con-
tinued payback not only in dollars and cents, but
also in a more enjoyable and comfortable living envi-
ronment for your family for many years.

This fact sheet will help new-home buyers make
informed decisions about selecting energy-efficient
windows and will assist owners of existing homes
understand the benefits of energy-efficient windows.

Windows provide less resistance to heat flow than
walls, ceilings, and floors of your home. Even when
windows comprise a small area of a home, they are
the area of greatest heat loss and gain, and air leak-
age. Windows can account for as much as 25-30%
of the heat loss in a home. This increases energy use
and costs, and decreases your comfort.

The performance of windows, walls, ceilings, and
other building components determine the monthly
energy cost as well as the required size of your heat-
ing and cooling equipment. The installation of ener-
gy-efficient windows (as well as other aspects of a
home's construction) reduces not only your monthly
energy use (and costs) but also means that a smaller,
less expensive furnace and air-conditioning system
will be required. Therefore, while energy-efficient
windows will cost more initially, the monthly savings
on your energy bills coupled with a reduction in the
purchase price of the heating and cooling system, can
more than offset the higher initial cost.

Energy-Efficient
Windows

Engineering Extension

January 2000

Introduction

The impact of windows on the overall
energy consumption of a home

Purpose of this fact sheet

A window's ability to resist heat flow, its insulating
value, is defined by its R-value or U-factor. The U-
factor is the reciprocal of the R-value (R=1/U). The
larger the U-factor the greater the heat flow through
the window. Glass by itself is an extremely poor
insulator. A single-pane window of clear glass has an
R-value of around 1 (U=1/R ( U-value =1) while
double-glazed units have R-values of 2 (U-fac-
tor=0.5).

To help alleviate confusion associated with window
energy performance and ratings, the National
Fenestration Rating Council (NFRC) has developed
a window rating system based on whole window per-
formance. The NFRC rating accurately accounts for
all product components and presents window infor-
mation in a concise and easy-to-understand format.

The NFRC energy ratings take into account the win-
dow’s resistance to heat flow (U-factor), the amount
of heat admitted as radiant energy (solar heat gain
coefficient, SHGC), and the amount of visible trans-
mittance (VT). In the future, NFRC ratings will
include ratings for air infiltration (leakage), heating
and cooling performance, condensation, and long-
term energy performance. A typical NFRC rating
label for an example window is presented below
along with the new NFRC rating logo.

Windows have two U-factor ratings: one for the cen-
ter of the glass and one for the total window assem-
bly which includes edge-of-glass and frame effects
(overall U-factor). Center-of-glass U-factor is the U-
factor for the glass alone and the overall U-factor
accounts for heat flow through the glass itself, the
edges that occur in the unit, and the window frame
and sash. NFRC rating is for the whole window.
Windows should be compared with an overall U-fac-
tor. In addition, the NFRC label includes ratings for
both residential and non-residential windows. Non-
residential windows generally have larger areas where
the edge effects are less pronounced than residential
windows. Overall, or whole-window, U-factors for
most commercially available windows range from 0.3
for a double-glazed, low-e window assembly to 1.00
to 1.30 for a single-glazed clear window assembly.

The solar heat gain coefficient (SHGC) refers to the
fraction of solar radiation that passes through a win-
dow assembly and warms the interior living spaces of
a home. The SHGC is expressed as a number
between 0 and 1 with a low SHGC meaning a lower
amount of solar heat is transmitted. The higher the
SHGC, the greater the amount of passive solar gain
by the interior of the home, which is important in
heating dominated climates (cold weather locations).
For warm climate locations, a SHGC of 0.4 or less is
recommended. Double-glazed windows can have a
SHGC from 0.3 to 0.75, while standard single-pane

Energy performance characteristics of
windows

Frame area

Center-of-glass area

Edge-of-glass area

windows generally have values greater than 0.8.
Visible transmittance (VT) is an optical property of
the window pane defined as the fraction of visible
light transmitted through the glazing material. It is
influenced by the glazing type, number of layers, and
any coatings applied to the glazing. VT does not
affect the heating and cooling loads. The higher the
VT, the more daylight transmitted through the
pane(s). A high VT is desirable for locations in
which daylighting is important. VT ratings based on
the total window assembly range from 0 to 1, with
single-glazed clear glass having a rating around 0.69
and double-glazed units having VT's from 0.51 to
0.62.

Heat gain and loss can occur not only from the
transfer of thermal radiation through a window, but
also from the leakage of air around the edges and
spacings of the window frame. The infiltration of air
around a window pane and frame is indicated by an
air leakage rating (AL) expressed in cubic feet of air
per square foot of window area (ft

/ft

). The lower

the AL rating, the lower the air infiltration around
the window. An AL of 0.30 or less is recommended.
Air infiltration is a function of the style and sealing
characteristics of the window assembly. AL ratings
vary from 0.98 for single-glazing clear windows to
0.10 for double-glazed units.

sion. Because single-glazed units are the least energy-
efficient of all window types, they are used as the
base case for comparisons to other more efficient
units.

Tinted glazing

Tinted glass is made by altering the chemical compo-
sition of the glass with chemical additives. Primary
uses for tinted glass are to reduce glare and solar heat
gain. Glass that is tinted also reduces visible light
transmittance. Some tints, referred to as spectrally
selective, allow a greater amount of daylight to pass
through the glazing while at the same time decrease
the heat gain from sunlight. Applications for tinted
glass are almost entirely in commercial buildings and
primarily in warmer climates where reduction of
solar heat gain is a concern.

Multiple-pane glazing

In double-glazing window units, two layers of glass
are separated by a spacer. This provides increased
thermal resistance to winter heat loss and summer
solar heat gain. With double glazing, the visible light
transmittance is only slightly diminished. Tests show
that the best thermal performance occurs when the
spacing between the glazings is about 1/2 inch.

Another way of improving thermal performance of
multiple-glazed units is to fill the space between the
panes with an argon or krypton gas, both of which
are much less conducting than air. Both gases are
inert, non-toxic, odorless, and clear. Krypton has
better thermal performance properties, but does cost
more. Specially designed spacers help reduce heat
loss through the window edge. Sealing of double
glazings to maintain the argon or krypton is a well-
proven technology and should last for several
decades.

One advantage of using gas fills versus plastic films
for multiple pane units is that the visible light trans-
mittance is not affected. Because sealing of windows
is not perfect, an expected loss of some of the gas
over several decades is to be expected, but will only
result in a few percentage points decrease in the over-
all U-factor.

Types and characteristics of
energy-efficient windows

Glazing is defined as the glass or plastic panes in a
window assembly and most windows in U.S. resi-
dences contain glass. Technological advances in
recent years have improved the energy performance,
and mechanical and chemical characteristics of glaz-
ing materials. Plastic glazing is used primarily in spe-
cialized applications.

Single clear glazing

A single-glazed clear window is the most common
window type in existing residences. Of all the glaz-
ing options, a single-glazed unit allows the greatest
heat loss in the winter and the most heat gain in the
summer. It also allows the most daylight transmis-

Addition of a second pane to a window assembly
doubles the thermal resistance. If a third or fourth
pane is added, the thermal resistance also increases,
but with a diminishing effect. The trade-off with
multiple panes is that while thermal resistance is
increased with each pane, visible light transmittance
decreases as well as the solar heat gain in the winter
months. In addition, more panes also add to the ini-
tial cost which may not be recovered over the life of
the window.

Low-emissivity (low-e) coatings

Low-emissivity or low-e coatings refer to a micro-
scopically thin, transparent layer of metal or metal
oxide applied to a window glazing to reduce the

transfer of heat through window glazings while
allowing the full amount of sunlight to pass through.
Emissivity is defined as the window glazing's ability
to radiate energy or heat. Emissivities of window
glazings range from 0 to 1. The lower the emissivity
of a window glazing, the lower the amount of heat
radiated to another surface.

Standard clear glazings have an emissivity value of
about 0.85, meaning that 85% of the heat absorbed
by the window pane will be radiated through it either
to the interior of the home (summertime) or to the
outside (wintertime). By contrast, double-glazed
windows with low-e coatings have emissivity value of
between 0.05 to 0.20, implying that as much as 95%
of the heat will be reflected from the window surface.
In wintertime, low-e coatings are used to reflect heat
back to the interior living spaces of your home, while
in the summertime they reflect heat back to the out-
side. Low-e coated windows generally cost 10 to
20% more than standard clear-glazed ones, but can
save 30 to 50% on energy costs associated with the
window assembly throughout their lifetime.

Thermally improved edge spacers

In multiple-pane units, the glazing layers must be
held apart by edge spacers. These spacers must
accommodate expansion and contraction of the glaz-
ing layers due to seasonal temperature differences,
provide a moisture barrier and a gas-tight seal, and
create an insulating barrier to prevent unwanted heat
gain or loss.

Double glazing

Argon- or krypton-

filled space

between glazings

Low-emittance

coating

In the past, most edge spacers were manufactured
from aluminum, but since aluminum has such a high
thermal conductivity, edge losses tended to offset the
benefits of multiple glazings. In addition, colder alu-
minum edge spacers were more prone to condensa-
tion. Thermally improved edge spacers incorporate
new materials and designs to improve performance.
Stainless steel spacers are now used because they are
less conducting than aluminum. All new edge spacer
technologies are designed to interrupt the heat trans-
fer between the glazing edge and the glazing itself.
These improved edge spacers help maintain a higher
temperature at the edge of the window unit, thereby
decreasing the potential for condensation. The most
common new design is to use a spacer, sealer, and a
desiccant in a single-tape element. Other new,
improved edge spacer technologies include butyl tape
or silicone foam, and aluminum spacers with thermal
breaks.

shade architecturally. Landscaping and use of over-
hangs can influence the total energy performance
and consumption of your home.

Planting broad-leafed trees in locations that will
shade your east and west windows during the day in
the summer months will reduce solar heat gain but
still allow sunlight through the windows during win-
ter. Also, use of shrubs and other foliage as wind-
breaks will help reduce wind speeds and unwanted
cold air infiltration into your home.

On southern exposures, one of the best ways to
decrease summer solar heat gain is with overhangs.
There are two different types of overhangs: fixed and
moveable. Fixed overhangs refer to the portion of
your roof that provides shade to the window. The
length of the overhang required to provide full shade
in the summertime can be determined and built to
these specifications during construction. Moveable
overhangs can be temporarily attached above or to
the side of the window to block high-angle summer
sunlight.

Energy-efficient windows will typically cost more
than conventional windows (clear-glazed, aluminum
frame) and will be financed with the home's mort-
gage or some other sort of long-term financing that
distributes the payments over an extended period of
time (15 to 30 years). However, monthly savings
that result from installation of energy-efficient win-
dows (and other energy-savings measures) may offset
all or a significant fraction of the homeowner's addi-
tional monthly payment.

Energy-efficient mortgages (EEM) allow homeown-
ers to qualify for a larger mortgage because the com-
bination of their monthly mortgage payment plus
their monthly utility bill will be less than their pay-
ments for a non energy-efficient home. To qualify,
the home must be rated a 4 Star Home with the
Home Energy Rating System (HERS) or meet the
1993 Model Energy Code (MEC). An EEM also
allows homeowners to finance the purchase of ener-
gy-efficient improvements as part of their first

Energy-efficient windows can help control condensa-
tion because they keep interior glass surfaces and
window frames warmer. This in turn helps the home
retain more of the warmth provided by the furnace,
which will keep the walls and windows of the home
warmer, thereby reducing the potential for condensa-
tion.

As an example of the ability of energy-efficient win-
dows to control condensation, when the outdoor
temperature is 10 degrees Fahrenheit, condensation
will occur on a single-glazed clear glass window when
the relative humidity of the home is as low as 15%.
If a double-glazed energy-efficient window is
installed, the relative humidity could be 3 times that
amount before condensation would occur and nearly
4 times if double-glazed, low-emissivity, argon gas-
filled windows are used.

While the selection of energy-efficient windows can
help lower monthly energy bills, other energy-effi-
ciency considerations need be taken into account
when planning a new home. Some homes, due to
the placement of their windows, may be difficult to

Financing energy-efficient windows and
energy-efficient mortgages

Condensation

Site planning and overhangs

mortgage. Contact your state energy office for more
information or visit this web-site at
www.pueblo.gsa.gov/press/eemguide.htm.

Climate, the total window area of your home, land-
scaping, the type of heating and cooling system, and
the level of insulation in your home all play a role in
determining the type of energy-efficient window that
is most cost-effective. Just because one energy-effi-
cient window has better thermal/energy performance
properties, such as lower U-factors or SHGC's than
another one, does not mean that it will be the most
cost-effective window to install. When selecting
energy-efficient windows, start with the following:

Look for the Energy Star label

This label signifies and insures the window is energy
efficient and has met certain U.S. Department of
Energy and U.S. Environmental Protection Agency
energy performance criteria.

Look for the NFRC label

This label can be used to make comparisons between
windows. The NFRC label is the only reliable and
proven way to determine the basic energy-related
properties of the window unit and compare them.

Following is a simple example associated with deter-
mining the most cost-effective window to install in a
new two-story home in Kansas City, given three differ-
ent commercially available common window choices.

The windows considered were as follows:
1. Aluminum frame, single-glazed, clear (window #1)
2. Aluminum frame, double-glazed, clear (window #2)
3. Wood/Vinyl frame, double-glazed, low-e, argon-

filled (window #3)

The home has ten windows and the home's annual
heating and cooling costs have been estimated to be
$978 (window #1), $828 (window #2), and $659
(window #3), respectively, for the three window
types which cost $100, $150, and $175 per window
installed, respectively.

On first glance, it would appear that window #3 is
the best choice since its installation provides the low-
est annual heating and cooling costs. However, other
considerations must be taken into account when
determining which window is actually the most cost-
effective. These include the initial cost associated
with each window choice, cost of maintenance (if
any), and expected life of the window.

Most energy-efficient window purchases on new
homes are financed through the homeowner's mort-
gage payment at a particular interest rate, which is
assumed to be 8% in this example, and all three win-
dows are expected to last 20 years with negligible
maintenance. A simple way to determine which of
the three windows is most cost-effective is to calcu-
late the additional cost to the homeowner’s mortgage
payment associated with installing one window
choice versus another and compare this cost to the
annual energy savings attained by that particular
window. This is demonstrated as follows:

The annual energy savings using window #2 versus
window #1 is $150 ($978 minus $828) and the
increase in cost is $500 between the two windows
($150 per window for window #2 versus $100 per
window for window #1 and there are ten windows).
The $500 increase financed in the homeowner’s
mortgage payment at 8% equates to an additional
$47 per year. Therefore, since the additional annual
mortgage payment ($47) is less than the annual ener-
gy savings ($150) derived by installing window #2,
window #2 is more cost-effective than window #1.

Selecting an energy-efficient window for
your home

Determining the most cost-effective,
energy-efficient window

Using the same logic, window #3 can be compared
to window number #2. The annual energy savings is
$200 ($894 minus $694) and the total increase in
cost between the two windows is $250 ($175 per
window for window #3 versus $150 per window for
window #2 and ten windows). Using these values,
there is roughly a $24 increase in the homeowner’s
annual mortgage payment in order to achieve $169
in energy savings. Since window #3 is more cost-
effective than window #2 and window #2 was better
than window #1, window #3 is the most cost-effec-
tive window of the three.

Estimated annual energy costs for five
different residential window types

The following table presents estimated annual heat-
ing and cooling costs for both one- and two-story
homes, three different heating and cooling cost sce-
narios, and five different residential window types
located in each of three distinct Kansas zones (1, 2,
or 3) shown on the map. The windows considered in
this analysis are a single-glazed, clear and four other
commercially available energy-efficient residential
window types with aluminum (Al) or wood/vinyl

(W/V) frames. (Each is listed at the top of the table.)
Assumptions used to obtain these estimates are listed
at the bottom of the table. Use the low energy prices
if you have natural gas heating and conventional air
conditioning, an air-source heat pump with electric
costs below $.05 per kWh, or a water-source heat
pump with electric costs below $.08 per kWh. Use
the medium energy prices if you use propane in com-
bination with a conventional air conditioner, an air-
source heat pump with electric costs below $.07 per
kWh, or a water-source heat pump with electric costs
below $.12 per kWh. Use the high energy prices if
electric costs exceed those listed above or if you use
electric-resistance heating.

Kansas climate zones

Estimated average annual heating and cooling costs for five window types

Al, single- Al, double- Al, double-

W/V,

W/V, double- Al, single- Al, double- Al, double-

W/V,

W/V, double-

glazed,

double-

glazed,

double-

glazed,

clear

clear,

glazed,

low-e,

clear

clear,

glazed,

low-e,

thermal

clear

argon-filled

thermal

clear

argon-filled

break

$514

$429

$401

$371

$345

$737

$608

$566

$521

$478

$481

$413

$388

$362

$331

$696

$594

$556

$516

$468

$420

$359

$337

$314

$287

$613

$522

$488

$453

$410

$770

$639

$598

$554

$519

$1,094

$897

$834

$767

$713

$686

$583

$547

$509

$472

$978

$828

$775

$717

$659

$599

$505

$474

$440

$407

$868

$728

$680

$628

$576

$1,398

$1,174

$1,102

$1,026

$959

$1,997

$1,654

$1,544

$1,427

$1,323

$1,156

$1,001

$948

$890

$828

$1,655

$1,421

$1,338

$1,251

$1,155

$1,016

$869

$819

$765

$708

$1,477

$1,253

$1,176

$1,094

$1,005

Assumptions: one-story house 1,500 ft

floor area, two-story house 2,250 ft

; for each, window area 15% of floor area,

ceiling insulation R-30, wall and floor insulation R-19, heating system efficiency 78%, and a 10 SEER.

Estimates in this table were made using the RESFEN (Residential Fenestration) energy-efficient windows computer simulation
model available at www.windows.lbl.gov/software/resfen_getcopy_31.htm.

Low Energy Prices

Medium Energy Prices

High Energy Prices

General

Window

Description

Zone 1
Zone 2
Zone 3

One-Story

Two-Story

Zone 1

Zone 2

Zone 3

Using these total energy estimates, locally obtained
installed-window costs, and the guidelines and
methodology outlined in the example in the previous
section, you should be able to determine the most
cost-effective, energy-efficient window for the zone
in which you live.

Summary

Further information

Windows have a tremendous effect on the heating
and cooling requirements and costs of a home, as
well as the comfort level experienced by its occu-
pants. Windows are by far the weakest link in a
home's building envelope for heat gain in the sum-
mertime and heat loss in the wintertime. Glazing
type, number of glazings, window frame materials
and design, interior and exterior shading, and win-
dow orientation all contribute to the energy perfor-
mance of your home and to your choice of energy-
efficient window. Therefore, it is extremely impor-
tant to carefully consider all options when determin-
ing which energy-efficient window is best and most
cost-effective for your needs both now and in the
future. It does make a difference to use energy-effi-
cient windows–in cost savings to you, the homeown-
er, in energy savings to our country, and as a benefit
to our environment.

Notice of Nondiscrimination

Kansas State University is committed to a policy of nondiscrimination on the basis of race, sex, national origin, disability, religion, age, sexual orientation, or other nonmerit reasons, in admissions,
educational programs or activities, and employment (including employment of disabled veterans and veterans of the Vietnam Era), all as required by applicable laws and regulations. Responsibility
for coordination of compliance efforts and receipt of inquiries, including those concerning Title IX of the Education Amendments of 1972, Section 504 of the Rehabilitation Act of 1973, and the
Americans with Disabilities Act, has been delegated to Jane D. Rowlett, Ph.D., Director of Unclassified Affairs and University Compliance, Kansas State University, 225 Anderson Hall, Manhattan,
KS 66506–0124 (785-532-4392).

“This material was prepared with the support of the U. S. Department of Energy (DOE) Grant No. DE-FG48-

97R802102. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the
author(s) and do not necessarily reflect the views of DOE.”

Printed on recycled paper

The following Web sites offer additional facts and
information on energy-efficient residential windows:

■

www.efficientwindows.org

■

www.nfrc.org

■

www.eren.doe.gov/erec/factsheets/eewindows.html

Also, an excellent book on energy-efficient residential
windows is Residential Windows by John Carmody,
Stephen Selkowitz, and Lisa Hescong published by
Norton.

For questions regarding this fact sheet or further
information on energy-efficient windows, please
contact Engineering Extension Programs at
785-532-6026. This fact sheet is posted on the
Kansas State University Engineering Extension Web
page at www.oznet.ksu.edu/dp_nrgy/ees. Other KSU
Engineering Extension Fact Sheets posted at this site
include the following:

■

Tips for Purchasing an Energy-Efficient Home

■

Foundation Insulation

■

Selecting a Home Heating System

■

Selecting a Home Cooling System

■

Energy-Efficient Mortgages

■

Residential Insulation

■

Air Sealing Your Home