Out of all items I have researched, looking into windows has taken by far the most time. And I am not talking about hours. I have spent days on figuring out what to do.
Why? Windows are a critical component in the energy efficient building envelope we would like to create. The objective is to have the window efficiency somewhat correspond to the R-value of the walls, which would make each investment worthwhile. To get there, we have to look at triple pane windows.
But they are a very big, if not the biggest, investment component in the building shell. Initial prices ranged from $400 to $1,300 for a 100 UI window. I suspected that the $450 must be a “too good to be true” case, while a commitment to the $1,300 window requires a level of obsession that I am lacking.
UI = unit inches (window width + window height = unit inches)
Looking at the energy performance helped me to narrow the field and get products organized. Four performance metrics of the NFRC ratings (National Fenestration Rating Council) help in that process.
U-value
The U-value is an expression of the heat transfer coefficient (or insulation value) and is determined according to the NFRC 100 test.
I initially used a U-value of 0.25 for the energy model, but hope to get it below 0.20, which would correspond to an R-value of 5 or greater. A window with a U-value of 0.20 or less would feel comfortable even during a cold winter day, as long as it is air tight (see also below).
Solar heat gain coefficient (SHGC)
SHGC reflects the heat gain in a space through a window and is determined according to the NFRC 200 test.
Solar gain can be a double sided sword. It could be desired in the wintertime for passive solar heat gain, but not so during summer. We would prefer passive solar heat gain through the east and south windows (SHGC of around 0.5), which we can shade during the summer months to prevent overheating. All westward facing windows should have a low SHGC, such as 0.25 or less.
Visible transmittance (VT)
VT is the amount of visible light that passes through the window and is determined according to the NFRC 200 test.
Some light will be blocked by the window frame and by various glass coatings. We would like to keep the VT at or above 0.4 to allow enough daylight into the building.
Air leakage (AL)
AL is a measure of air infiltration through and around a window (in cubic feet per minute per square foot [cfm/sf]) and is determined according to the AAMA/WDMA/CSA test.
Air infiltration is often overlooked but is rather critical to the window performance. What is the point of having a window with good U-value (say 0.20) if plenty of cold (or hot) air constantly leaks through the window.
The Efficient Window Collaborative recommends windows with an AL of 0.3 cfm/sf or less. That said, I have heard a number of times that 0.3 cfm/sf can feel uncomfortably drafty on a cold winter day. We would prefer an AL less than 0.05 cfm/sf.
See also:
Existing Homes: Selecting Energy Efficient Windows in Illinois
Questions About Replacement Windows & Energy?
Styles and materials
I learned that different styles of windows have different energy performance characteristics, although it really comes down to air leakage.
Double hung windows, the most common style, and sliders typically have a greater air leakage when compared to single hung, casement, awning and picture windows.
Because double hung and slider windows have two operable sections, outside wind pressure may push air in between the two sashes. Casement and awning windows are pressed into the jamb gasket when subjected to outside wind pressure, resulting in an even tighter seal.
Picture windows are non-operable and as such should not have an air leakage issue, unless they have a shoddy frame.
An item that influences the pricing of windows is the frame material. Vinyl is the most common and least expensive, but has an unfavorable environmental footprint and suffers from expansion and contraction with temperature change.
Aluminum frames need a really good thermal break to prevent heat loss or gain. Another option is a wood frame with aluminum cladding on the outside. These are very nice looking windows in my humble opinion.
Composite material frames (mixture of wood and plastics) are another higher end option and are sold as being virtually maintenance free. So are fiberglass frames, which have the reputation to last a long time.
Who sells high efficiency windows?
Good question. There are European models and a number of Canadian manufactures. I searched a lot for more local manufactures, with limited success. I did build a small product list, bit by bit, through recommendations, web searches and the reading of other green blogs.
Triple pane windows with a U-value of 0.20 or less seem a rarity in the U.S. market. The ones I found (U.S and Canada) were available with vinyl or fiberglass frames.
Return on investment (ROI)
I needed help to put the price of a window into the context of its energy efficiency and probable energy savings. The Efficient Window Collaborative has a very helpful web site that explains the basics and provides a window selection tool.
http://www.efficientwindows.org/selection.cfm
I found additional help in a simple to use energy model called RESFEN. It allowed me to input the performance metrics of the windows around the building and calculated the estimated energy costs or savings.
I now have accumulated a pretty good knowledge base, have a number of products, have an idea about the potential ROI, but still have to make a decision.
