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U.S.
Department of Energy
Office of
Energy Efficiency and Renewable Energy |
Consumer Energy
Information: EREC Reference Briefs
Window Frames
For very old windows the
frame and glazing had almost the same R-value (about R-1). Modern glass
technology has improved the insulating performance (R-value) of the glazing
itself (Typically R-3 to 8). What the frame of the window is made of has come to
dominate how well the window performs overall. Modern window frames are
constructed from a variety of materials including steel, aluminum, wood, vinyl
and composites such as fiberglass and epoxy resin. Each material has advantages
and disadvantages.
Metals
Metal window frames are
ideal for openings that require lightweight and strength. However, all metals
are very good conductors of heat (R values hundreds of times less than 1). Even
metal framed windows with plastic “Thermal breaks” (a plastic strip that
separates the exterior side of the frame from the room side) only improve the
R-value slightly. There are no metal-framed windows currently available that are
comparable in thermal performance to any of the other choices in window frames.
Also, unless protected by special finishes, metal frames can oxidize (rust).
Wood
Wooden window frames are
better insulating than metal frames (about R-1.4 per inch of thickness) and help
to prevent cold weather condensation problems inside the house. However wood
does require maintenance (i.e. painting). If they aren’t protected from
moisture, they can stick, crack, and rot.
Aluminum-clad and
Vinyl-clad Wood
Some manufacturers protect
their wood frames by wrapping them with vinyl or aluminum. This helps keep wood
window maintenance low. However, if the cladding should come loose and expose
the wood to water, it can still rot. The thermal performance is about the same
as an ordinary wood frame.
Wood-Plastic composites
Some companies make frames
out of a mixture of wood, vinyl and an epoxy adhesive. Such frames are very
strong, insulate well, and resist rot even when exposed to water frequently. The
insulation value is about the same as an ordinary wood frame.
Vinyl
Vinyl frames are primarily
made from Polyvinyl Chloride (PVC). Some manufacturers fill the hollow parts of
the fame with foam or fiberglass insulation too. Data shows that the difference
in performance between filled and hollow vinyl frames is small. Generally vinyl
frames of all types are slightly less of a thermal insulator than wood.
Vinyl frames are also
available in many styles and are generally considered low maintenance and they
never need painting. However, the
ultraviolet (UV) light can fade colors other than white and cause yellowing and
“crazing” in the plastic’s surface. Vinyl frames are also not very rigid. Vinyl
windows with large openings usually requite an internal metal extrusion to make
the frame stiffer. This can lower the frame’s R-value significantly. Vinyl
window frames can also soften, warp, and twist if heat builds up within the
fame. In very hot sunny climates direct exposure to sunlight is not recommended.
Fiberglass
Fiberglass frames are made
of a fiberglass and resin composite. Some frames are hollow; others are filled
with fiberglass insulation. Most are pre-finished with a polyurethane coating.
Such frames are about R-3.
Fiberglass frames are
resistant to warping, shrinking, and rotting. Fiberglass window frames are also
relatively strong and durable and can hold a large expanse of glass. You can
specify almost any color you desire. You can also paint them.
However, they may not be
widely available, as there are few companies manufacturing them currently, and
the long-term performance is still unknown. Fiberglass window frames also cost
more than the other types of window frames.
Frame Construction
Some windows are divided
into small sections of glazing or “divided lights”. A divided light window is
considerably less energy efficient than other types because of the large number
of glazing edges. However, modern multi-pane windows are often whole sheets of
glass with optional plastic or wood grilles that attach to the glazing to give
the appearance of a divided light window.
The insulation value of a
double-pane or triple-pane window is primarily a product of the air space
between the panes of glass. Spacers separate the panes at the edges. Until
recently, most edge spacers were made of metal, which means the edge of the
window has little or no insulating value. Many manufacturers now use improved
edge seals with much better thermal performance.
Metal and vinyl frame
corners may be welded or screwed together. Either one is acceptable. However,
welding a corner can often distort the weather-stripping groove. Always check
how well the weather-stripping meets at the corners. There must be no gaps. Wood
frames are often “finger-joined” and glued to make a strong corner.
Window frames are rated A,
B, or C according to their performance in an American Society of Testing and
Materials (ASTM) “aging” test that involves exposure to UV light, moisture, and
extreme temperatures. “A” rated windows are the best performers.
Summary
Improving the comfort and
energy use of a building are the primary factors in deciding to replace windows.
Carefully consider your climate, budget, and availability, maintenance, and
style of window. You should also check for durability, quality, and warranties.
Check the local building department’s requirements regarding egress, safety
glass, and window grade before buying. The purchase of the right windows can go
far in turning a drafty home into a comfortable and more energy-efficient one.
Bibliography
The following articles
contain information on window frames. This bibliography was reviewed in November
2002.
“Are Wood Windows on Their
Way Out?” A. Wilson, Progressive Architecture, pp. 112-14, June 1994.
“Consumer Guide to
Energy-Saving Windows” J. Warner, Home Energy, (7:4) pp. 17-22,
July/August 1990.
“Energy-Efficient Window
Retrofits: Install with Care” J. O’Bannon and A. Grieco, Home Energy,
(14:1) pp. 35-42, January/February 1997.
“Energy Ratings Given for
Windows, Doors” Professional Builder, (60:6) pp 66, April 1995.
“How to Avoid Window
Condensation” J. Warner, Home Energy, (8:5) pp 27-29,
September/October 1991.
“More Than One Way to Case
a Wiindow,” J. Beals, Fine Homebuilding, (98) pp. 54-59,
October/November 1995
“Predicting Window
Condensation Potential” A. McGowan, ASHRAE Journal, (98) pp 24-29,
July 1995
“Replacement Windows”
Consumer Reports, (58:10) pp 664-67, October 1993.
“Selecting Windows for
Energy Efficiency” J. Warner, Home Energy, (12:4) pp 11-17,
July/August 1995.
“Shopping for Wood
Windows” C. Wardell, Journal of Light Construction, (12:9) pp
27-34, June 1994.
“Taking a Look at Windows”
J. Kolle, Fine Homebuilding, (No. 97) pp 56-61, August/September
1995.
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