Michigan
State University Extension
Home Maintenance And Repair
06/24/03
Energy Tips - Energy Efficient Windows
Windows
can be one of your home’s most attractive features. Windows provide
outside views, daylight, ventilation, and solar heating in the winter.
When properly selected and installed, windows can minimize a home’s
heating, cooling, and lighting costs. But they also can account for 10
– 25% of your winter heating bill through heat loss.
If your home has single pane windows--as almost half of the U.S. homes
do--you might consider replacing them to conserve energy and save
yourself money. New double pane windows made with high performance
glass are available on the market. In colder climates, select windows
that are gas filled with low emissivity (low-e) coatings on the glass
to reduce heat loss. If you are building a new home, you can offset
some of the cost of installing more efficient windows because you can
install smaller, less expensive heating and cooling equipment. However,
in older homes, it can take 20 years to recoup the cost of installing
new windows through heating and cooling cost savings.
Controlling Air Leaks
Energy is wasted when air leaks in or out around windows, but
you can reduce those air leaks. The least expensive options are
caulking and weatherstripping, adding storm windows, or replacing
window frames.
Caulking and Weather Stripping
Caulks are airtight compounds (usually latex or silicone) that
fill cracks and holes. Before applying new caulk, you should remove old
caulk or paint residue that remains around a window using a putty
knife, stiff brush, or special solvent. After old caulk is removed, new
caulk can 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 45o F. Low humidity
is important to prevent cracks from swelling with moisture. Warm
temperatures are necessary so the caulk will set properly and adhere to
the surface. Weather stripping is a narrow piece of metal, vinyl,
rubber, felt, or foam that seals the contact area between the fixed and
movable sections of a windows 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, ask for
these Energy Facts sheets at your county MSU Extension office.
Storm Windows
Installing exterior or interior storm windows can reduce your
heat loss through the windows by 25 – 50%. Storm windows should be
weather stripped at all moveable joints.
Additional options to reduce heat loss and gain
Moveable 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
purchasing energy efficient replacement windows and these options
should be considered first. For information on energy efficient window
treatment, call MSU Extension in your county and ask for Extension
Bulletin E-888 titled Low Cost, Energy- Efficient Window Treatments.
Buying New Windows
Trying to determine which new, high tech windows to buy can be
daunting. Standardized labels, required in only a few states, are
supposed to make it easier to comparison shop for new windows. The
National Fenestration Rating
Council (NFRC) label may appear on new windows sold in Michigan even
thought it is not required.
At present, only a window’s U-factor must be labeled. Few window
manufacturers participate in the federally sponsored Energy Star
labeling program. The Energy Star labels use the data from the NFRC
label and identify a window as suitable for a specific region of the
country.
Window manufacturers use the term U-factor as a measure of thermal
performance. U-factor describes a window’s ability to conduct heat.
Window ratings also give you the R-value that describes the insulating
value. The higher the R-value (or the lower the U-factor), the more
efficient the window will be in keeping your home cool in the summer
and warm in the winter. Window R-values range from 0.9 to 3.0
(U-factors from 1.1 to 0.35) and some highly energy efficient
exceptions exist on the market. When comparing different windows, you
should ensure that all U or R-values listed by manufacturers are based
on current standards set by the American Society of Heating,
Refrigeration, and Air Conditioning Engineers (ASHRAE). Also be certain
the values are calculated for the entire window including the frame and
not just
for the center of the glass, and that the values represent the same
size and style of window.
The following five factors affect a window’s R-value:
• The type or glazing material (e.g. glass, plastic treated
glass)
• The layers of glass
• The size of the glass
• The thermal resistance or conductivity of the frame and spacer
materials
• The tightness of the installation
For more information on energy efficient windows, visit the National
Wood Window and Door Association web site: http://www.nwwda.org
Types of Glazing Materials
Traditionally, clear glass has been the primary material
available for windowpanes in homes. In recent years, several special
glazing materials (specialty glass types) have appeared on the market
to help control heat loss and condensation. Your selection will be
based on the qualities that are most important to you and your
particular home style and your local climate. Low-emissivity (low-e)
glasshas a special coating to reduce heat transfer through the window.
These coatings reflect from 40 – 70% of the heat that is normally
transmitted out through clear glass back into the house but still allow
the full amount of light and heat from the sun to pass through.
Heat absorbing glasscontains 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
re-radiation.
Reflective glassis 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 heat-absorbing glass, it
reduces solar transmittance. Plastic glazing materials--acrylics,
polycarbonate, polyester, polyvinyl fluorides, and polyethylene--are
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.
Glass Layers and Air Spaces
Standard single pane glass windows have very little insulating
value (approximately R-1) and provide only a thin barrier to the
outside that accounts for considerable heat loss and gain.
Double or triple pane windows have insulated air or gas-filled spaces
between each pane and each layer of glass and air space 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 R-values. Advanced, multipane windows are manufactured with inert
gasses (argon and krypton) in the spaces between the panes since these
gases transfer less heat than air does. One caution: multi-pane windows
are more expensive than single pane windows and can limit framing
options.
Framing Materials
Window frames are available in a variety of materials. Frames
can be composed of a single material or made of a combination of
different materials such as wood clad vinyl or aluminum-clad wood. Each
framing material has its advantages and disadvantages. Though ideal for
strength and customized design, aluminum frames conduct heat and
therefore lose heat faster and promote condensation as well. The
corrosion and electro-galvanic deterioration of aluminum frames can be
improved by placing continuous insulating plastic stripes between the
interior and exterior of the frame or the frames can be anodized or
coated. Wood frames have high R-values, are not affected by
temperatures 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 made primarily from polyvinyl chlorides (PVC) offer
many advantages. Available in a wide range of styles and shapes, vinyl
frames have moderate to high R-values and are easily customized,
competitively priced, and require very low maintenance. While vinyl
frames do not possess the inherent strength of metal or wood, larger
size windows can be strengthened with aluminum or steel reinforcing
bars. Fiberglass frames are relatively new and are not widely available.
Definitions
U-factor - a thermal performance value that describers a
window's ability to conduct heat; a lower value denotes efficiency.
R-values - a thermal performance value that describes a
window's insulating ability - a higher value denotes efficiency.
Glazing materials - clear glass, specialty glass, or
plastic-treated or coated glass; also plastics of various types used in
windows.
Heat loss and gain - heat loss refers to losing heat (energy)
out from the home in the winter; heat gain refers to gaining heat into
the house in the summer from solar heating.
Emissivity - the relative ability of a surface to emit heat by
radiation - low-e windows are those that reduce heat transfer from the
house back outside, thereby losing energy.
References
Energy Tips - Energy Efficient Lighting, Michigan State University
Extension E2791
Consumer
Energy Information: EREC Fact Sheets, Energy-Efficient Windows, U.S.
Department of Energy
Window Shopping: Consumer Report, October 2000, pages 42-45.
Energy Savers - Tips on Saving Energy and Money at Home, U.S.
Department of Energy DOE/GO-10097-431, September 1997.
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