Energy Saving Window Treatments

Housing
Fact Sheets

W indows are a major source of heat loss but, of course, they are necessary
for light, ventilation, and view. In a typical single family home, 25% to
35% of the heat is lost through windows.
A single pane window loses 20 times as
much heat as the same area in an adjacent
well insulated wall, and a double-glazed
window loses ten times as much. However,
windows can be a source of solar heat gain if
they are strategically placed and properly
installed and if energy efficient interior
window treatments are used.

The addition of insulating treatments to

windows is very cost-effective and should be
the next step after the following measures
have been taken
• Turning back the thermostat at night from
68° to 60°.
Figure 1 • Adding insulation to un-insulated ceiling
(to R-49) and walls (to R-11 or R-19).
• Adding storm windows to single glazing;
and caulking and weather-stripping all
crevices.
• Keep the heating system properly
maintained
Some of the heat loss through a window forinfiltration
is due to peak efficiency.
of cold air around the
window. This can be reduced by caulking and weather-stripping. These
measures are very cost-effective and will help ensure the success of insulating
interior treatments.

Cornell Department of Design & Cornell University

Cooperative Environmental Analysis Martha Van Rensselaer Hall
Extension 607-255-2144 Ithaca, NY 14853
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Window heat loss also occurs through radiation, convection, and conduction.
Radiation is heat passed from a warm object to a cooler object. Heat always
moves from warm to cool. If you are near a cold window, your body heat will
radiate to the cooler window, making you feel cold.

Convection is heat transfer through a fluid such as air or water. Air infiltration
through cracks around the window increases convective air movement and heat
loss. This explains why caulking and weather-stripping to reduce air movement
also reduce heat loss.

Conduction is heat transfer through solids. Glass is a good conductor of heat

and therefore a poor insulator. Heat readily passes through glass (Figure 1).
Using insulating treatments on windows can reduce all types of heat loss. In
cold climates such as New York State, heat loss in winter is the major concern,
although energy can also be saved by reducing solar heat gain in the summer.

Methods for Reducing Window Heat Loss

After caulking and weather-stripping have been completed, the use of double-
glazed or storm windows should be considered. A double-glazed window loses
only half the heat of a single-glazed window and is cost effective. Triple glazing
reduces the heat loss another 30 percent but usually is not cost-effective. Each
additional layer of glass reduces the solar heat gain. For cold climates, the most
energy-efficient window is double-glazed with an operable R-5 interior
treatment. (R-value represents resistance to heat loss. The higher the R-value,
the better the insulating value of the material.) On south-facing windows the
treatment should be opened during the day for solar heat gain and properly
closed at night. Treatments on north windows may be left closed 24 hours a day
for increased efficiency.

A variety of interior window treatments, if properly installed and used, can be

effective in reducing window heat loss. Included are roller shades, draperies,
insulating Roman shades, venetian blinds, and insulating panels or shutters.
Some are more energy efficient than others. The choice depends on the location
of the window, the use of the room, the aesthetics, the effectiveness and cost, and
thus the payback period.

To be effective, all window treatments must trap and hold air between the
treatment and the window. To accomplish this the treatment must be tightly
fitted at all edges and should be placed about one inch from the glass.

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Roller Shades
The common roller shade that has been in use for many years is a very effective
energy saver, when properly installed. The effectiveness is due to the shade's
ability to block air flow and to form an insulating layer of air.

A conventional roller shade

mounted inside the frame with
no more than 1/4 inch gap at the
sides and touching the sill
(Figure 2) will reduce heat loss
by about 27 percent. By adding
side tracks (Figure 3) the
Figure 3 efficiency may be increased to 45
percent. An inside-mounted
shade with a reflective coating
Figure 4
also may be 45 percent effective.
Figure 2

To be most efficient, a window shade must be:

• Installed with side tracks for a tight fit,
• Placed one inch from the glass (inside mount),
• Covered with a reflective material facing the glass.

A shade meeting these conditions will be approximately 55 percent effective1.

The reflective material made of un-coated aluminum such as Foylon, is more

effective than aluminized polyester film or fabric that has a plastic coating over the
aluminum.

The opaqueness or translucency of the shade can contribute to its efficiency. An

opaque shade will be slightly more efficient than a translucent shade.

1
The percent effectiveness indicates the heat loss reduction achieved when the window
treatment is installed over a single glazed window.

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Shades on a sunny window should be raised in winter to permit solar heat gain in
daytime, then lowered at night to prevent heat loss. Just the opposite is true in
summer (Figure 4).

Draperies
Conventionally hung or unsealed draperies are
not efficient energy savers. They will reduce
window heat loss no more than 10 percent. This
is due primarily to the movement of air by
convection through the “tunnel” between the
drapery and the glass. Warm air enters at the top
of the drapery, moves toward the floor as it is
cooled by the glass, enters the room and is
Figure 5 Figure 6
warmed, then rises and repeats the cycle (Figue
5). To be more efficient, draperies should be
sealed at all four edges.

This may be accomplished by closing the top (Figure 6) to reduce heat loss
by the “tunnel” effect, sealing both sides, permitting the drapery to touch the
sill or floor and having a four inch overlap at the center. This type of
installation will reduce heat loss by convection. Experiments have shown
that lined draperies installed in this manner will be up to 25% effective. The
method of installation is more important in reducing heat loss than the type
of fabric, although the fabric must have low air permeability, meaning that
little or no air could pass through the fabric. A separate lining is
recommended because the incorporated layer of air has some insulating
value.

A closed top cornice installed over the top of the

drapery (Figure 7) is one way to reduce heat loss
by the tunnel effect. The cornice may be covered
with the drapery fabric, with wallpaper, or it may
be stained or painted.

To take advantage of solar heat gain, the drapery

should be opened during the day and closed at
Figure 7 night to prevent heat loss.

An inside-mounted roller shade, installed as previously described, in combination

with sealed draperies, would yield an R-value of 1.6, including a single pane
window. This translates to about 40% effective. Because of their high cost and low
R-value, draperies are not as cost- effective in energy savings as other treatments.

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If a warm air register is below the window, the warm air should be prevented
from entering between the drapery and the glass. A metal or plastic deflector can
be placed over the register to direct the air out into the room. A deflector may
also be used over a radiator, although this heat is not moving as fast as forced air
heat.
Insulating Roman Shades
Insulating Roman shades offer another
choice for energy efficient interior
treatments. These consist of flat pieces
of fabric with rings sewn on the back
and operated with a pull cord causing
the fabric to stack in horizontal folds at
the top of the window when the shade
is open (Figure 8). They consist of a
center layer of insulating material, a
vapor retarder on the room side, a
decorating fabric on the room side
and a lining on the back. The shade
should be sealed at the sides, top, and
Figure 8 bottom.
For greatest efficiency the seals must be continuous - not just sealed at intervals.
A hinged 1" x 2" wood clamp (Figure 8) at the sides makes an effective seal. This
shade can be as high as 60-75 percent effective, with an R-value of approximately
4.

Venetian Blinds
Standard venetian blinds are not effective in preventing heat loss. They do,
however, allow excellent control of daylight and ventilation. Some manufacturers
are making blinds with insulated interlocking slats that are more energy efficient,
but very expensive.

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Insulating Panels and Shutters

Insulating panels and shutters are another

window treatment option. Panels are removed
from the window and stored elsewhere or left in
place over the window continuously for the
heating season (Figure 11). They can be used
where daylight, view, and ventilation are not as
Figure 9
essential as reducing heating loss. Shutters are
attached to each window frame and remain in
place whether opened or closed (Figure 9 & 10).
The hardware and design are variable: hinged
with center opening, hinged with side opening,
hinged with top or bottom opening; bifold; or
Figure 10
sliding. The choice of hardware and design is
determined by the window size, shape, and
location in the house.

Insulating shutter and panel materials vary tremendously. Unlike other

insulating options described previously, they are usually rigid and are often
made of common building materials. Do-it- yourselfers will find that simple
carpentry skills and hand tools are necessary to build this type of window
treatment.

A typical insulating shutter or

panel might consist of a center
insulating core made of rigid
foam boards; a bag of cellulose
or other loose-fill insulation;
or even a series of insulating
materials and air spaces.
Figure 11

This insulating core may be covered on both or one side by a sheathing material
that gives the shutter or panel rigidity and strength. Sheathing materials include
plywood, hardboard, and other thin rigid materials. Next, on the side of the
panel/shutter that faces the room, a vapor retarder is added. Finally a decorative
material is applied.

The insulating panel or shutter materials are usually encased in a framework

made of wood or extruded plastic. This provides strength, rigidity, and a means
for attaching hardware and weather-stripping.

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The R-value of this type of insulating window treatment varies tremendously and
may be as high as R-10, or about 90% effective. A typical R-value is 4.0 which
provides 75% effectiveness. The total R-value of the shutter/panel is found by
adding the separate R-values of materials and incorporated air spaces.

Commercially Available Window Treatments

A variety of energy-saving window treatments is available commercially. In
general they fall into three categories; glazing treatments, operable products, and
non-operable panels.

• Glazing treatments resemble a second layer of glass and have similar a R-value.
Clear polyethylene sheets that are placed over the window opening and sealed
are one example.

• Operable products are those that can be opened and closed daily. An insulated
window shade is an example of a roll-down treatment. Shutters are another type
of operable treatment.

• Non-operable panels are those that stay in place during the heating season.

Before choosing a window treatment consider the type of window, its location, the
aesthetics of the treatment and the initial cost.

The chart on the following page shows a comparison of the efficiency of the
different types of window treatments. Remember this: a window treatment is
effective only when it traps and holds air between itself and the window.

The following instructions for making window treatments may be obtained from
your county Cornell Cooperative Extension office:

Modifying Draperies to Conserve Energy

Installing an Energy Efficient Roller Shade

Making an Insulated Roman Shade

Constructing and Installing Thermal Shutters and Panels

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EFFECTIVENESS OF WINDOW TREATMENT

IN REDUCING HEAT LOSS

(All treatments assume installation over a single glazed window.)

Approximate % Approximate
Type of Window Treatment Reduction in R-value
heat loss
Roller shade with inside mount, no more than 1/4” gap at sides 24-31% 1.32-1.4

Roller shade, reflective coating, inside mount 45% 1.8

Roller shade, side track, and/or 1” from glass (inside mount) 40-45% 1.67-1.8

Roller shade, side track, reflective coating and 1” from glass 55% 2.22
(inside mount)

Drapery, conventionally hung on traverse rod 5-10% 1.05-1.10

Drapery, tightly woven fabric, separate lining, 25% 1.33

sealed at all four edges

Venetian blind 6-7% 1.07

Roman shade, 2 layers fabric, plastic vapor retarder 65-75% 3.0-4.0

and insulating material

Do-it-yourself shutter, insulating core, sheathing material 75-90% 4.0-10.0

vapor retarder and decorative covering

Clip-on 1/2” rigid insulation board with spun glass 75-90% 4.0-10.0
batt insulation between board and window

As a Comparison
Double pane window, bare 50% 2.0
Double pane window, low e-coating 68% 3.3
Wall, 3 1/2” spun glass insulation 94% 16.6
Wall, 6” spun glass insulation 96% 25.0

Thanks to Gwen Cukierski and Regina Rector, former Extension Associates,

for their assistance in preparing this fact sheet.

Reviewed and revised,

October, 20000
Mark Pierce
Extension Associate
Department of Design and Environmental Analysis
Cornell University