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.
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).
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.
Vinyl frames are primarily made from Polyvinyl Chloride (PVC). Some manufacturers fill the hollow parts of the fame with foam or fiberglass insulation. Data shows that the difference in performance between filled and hollow vinyl frames is small.
Vinyl windows are 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 cheaper construction grade windows that are mixed with recycled plastic rather than pure “virgin Vinyl”.
Pure vinyl frames are resistant to warping, shrinking, and rotting. Pure vinyl window frames are also quite strong and durable and can hold a large expanse of glass.
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.
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.
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.