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. However, when properly
selected and installed, windows can help minimize a home's heating, cooling,
and lighting costs. This information describes one option-- energy-efficient
windows--available for reducing a home's heating and cooling energy
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 weather stripping, followed by replacing window frames.
Caulking and Weather Stripping
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 45 degrees
Fahrenheit (7.2 degrees 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.
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
window joint. It should be applied between the sash and the frame, but
should not interfere with the operation of the window.
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, double and 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. However, 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.
Single-hung 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 double-hung 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-values = 1/r-value). Most window
manufacturers use r-values in rating their windows.
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 of conductance of the frame and spacer
- The "tightness" of the installation (i.e., air leaks-- see previous
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 glazing 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
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 heat absorbing 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
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 semi-rigid 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.
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.
Double-pane windows are usually more efficient than single- pane or storm
windows. 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) have
lower r-values (i.e., they allow too much heat transfer). Advanced,
multipane 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
and vinyl. Each frame material has its advantages and disadvantages. Though
ideal for customized window design, aluminum frames cause conductive heat
loss (i.e., they have low r-values) and condensation. However, thermal
breaks made of insulating plastic strips placed between the inside and
outside of the frame and sash greatly improve the thermal resistance of
Wood frames have higher r-values, are unaffected by temperature extremes,
and are less prone to condensation, but they require considerable
maintenance in the form of periodic painting. If wood frames are not
properly protected from moisture, they can warp, crack, and stick.
Vinyl window frames, which are made primarily from polyvinyl chloride (pvc),
offer many advantages. They are available in a wide range of styles and
shapes, have moderate to high r-values, are easily customized, are
competitively priced, require low maintenance, and mold easily into almost
any shape. But vinyl frames are not strong or rigid, which limits the weight
of glass that can be used. In addition, vinyl frames can soften, warp,
twist, and bow.
Fiberglass frames are relatively new and are not yet widely available.
They have the highest r-values of all frames; thus, they are excellent for
insulating and will not warp, shrink, swell, rot, or corrode. Fiberglass
frames can be made in a variety of colors and can hold large expanses of
glass. Some fiberglass frames are hollow; others are filled with fiberglass
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.
Many types of windows and window films are available that serve different
purposes. To alleviate these problems, one manufacturer has developed a
mulitipane window using a 1/8-inch-wide (0.32 centimeters- wide) PVC foam
separator place 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 rock wool
between the glass inside their windows.
Additional Options For Reducing Heat Loss
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
energy efficient window replacements and should be considered first.
Reducing heat loss or gain in homes often includes either improving
existing windows or replacing them. Low cost options available for
improvement are caulking, weather stripping, 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 airtight 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 window's, 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 day lighting, solar heating, shading, ventilation,
and aesthetic value.