Both wood and aluminum are poor insulators.
- Even with double or triple glazing, the amount of energy loss through the frame is enormous.
- That being said wood is better than aluminum, and you should definitely get multiple glazing.
For maintenance purposes, your best option is a clad window. That is, a wood window with an exterior cladding of aluminum, fiberglass or vinyl.
- Marvin's Integrity window (a very inexpensive production window) uses fiberglass with some recycled content on the exterior.
- Loewen Windows, a much higher quality, higher performing window, offers Forest Stewardship Council certfied wood in some of their products. Marvin also offers FSC wood in their higher end products.
- Both of these are wood windows clad with aluminum. Although I don't know of aluminum windows using recycled aluminum, the aluminum clad wood windows are probably the most durable of the clad choices, so the least likely to go into the land fill in your lifetime.
Windows lose energy not only through their glass and frame, but also through their assembly and because of the way they are installed.
- The space needed to shim or level the window, is a space that can be a weak point for the infiltration of air and should always be filled with insulation after the window is installed.
- Most windows made in the US also have large gaps between the components that make up the frame and sash, causing another place for energy loss.
- If you find a brand of window with an insulated frame, you will have much higher quality construction and lower heat loss.
Window ratings: U-value and SHGC
All windows come with two important ratings for glass: U-value and Solar Heat Gain Coefficient (SHGC.)
U-value. This is the inverse of R-value, which is the rating used in standard forms of wall insulation. So while you want your wall insulation to have a high R-Value, you want your window to have a low U-Value.If you can find a window that has a framing or installation U-value, you will also be able to know how much energy is seeping out around the glass and frame.
- The National Fenestration Rating Council (NRFC) recommends a U-vale of .30 for upstate New York, but
- the Passive House Institute (passivehouse.us) which is a very strict system with the ultimate goal of using almost no heating or cooling, would want something nearer to .15, for window and frame.
SHGC. The solar heat gain coefficient measures the heat gain in the summer through the window and/or assembly. It is a number between 0 and 1. If you are more concerned with heating than cooling, use a high SHGC; if your problem is mainly air conditioning choose a low number.
For more information:
The National Fenestration Rating Council is a nonprofit that has developed these fenestration energy ratings. Look for windows that have their label and it will give you the information on U-value, SHGC, as well as visable trasmittance, air leakage and condensation resistance.
The Efficient Windows Collaborative is a good website for explaining the NFRC ratings.
Also, read "Should I choose vinyl or non-vinyl replacement windows?" a Q&A answered by Susan Davis.