Concept

There are several factors that must be considered when building a 100% solar home, including:

•  U-Value – this is the measure of how fast heat leaves the building (it is the inverse of R-Value), the lower the better, but certainly it should be less than .25.
•  SHGC (Solar Heat Gain Coefficient) – this is the measure of how much solar heating energy can pass though the window unit. The higher the better on the south side – over .40 is good. On the west side, generally the lower the better to minimize overheating in the summer. On the east and north sides, SHGC is less relevant (emphasize U-value on these sides).

Manufacturers can improve U-Value by coating different surfaces of the glass or by adding a third pane; however, both strategies lower SHGC. For solar homes, this is the glazing conundrum – how to improve U-Value while also improving SHGC? I expect this is an area where more selective coatings will help in the future.

•  Infiltration Rate is the third performance aspect of windows; it is the measure of how much air will seep through a closed window. Lower the better.

Application

Oddly, most American manufacturers provide windows with the same U-Value and SHGC for all sides of the house. Unfortunately, most builders and homeowners accept this poor service and install them. This won’t work for the 100% solar home.

The old adage is "when you buy windows, buy the best you can afford." After all, hopefully, you will not be replacing them again! This is doubly true for a solar house; the windows not only keep the heat in but are also a significant factor in providing that heat in the first place.

We bought excellent windows (excellent properties, fair price) from Accurate Dorwin, a Canadian window manufacturer for over 50 years. They built the first fiberglass window in 1983. Fiberglass is the best material for windows because it is stronger than wood or vinyl and it has an expansion rate equal to glass, so the seals work better and last longer.

U-Value and SHGC are both measured for the entire window unit; therefore, windows of different sizes will have slightly different performance parameters even though they are manufactured the same way and have the same glazing – the frame to glazing ratios are slightly different. Our south facing windows have U-Values ranging from .18 to .20 and SHGCs ranging from .52 to .44.

Accurate Dorwin also provides windows with different glazing for different sides of the house, i.e. higher U-Value for the north side, where the lower SHGC doesn’t matter as much. Our east, west, and north windows have a U-Value of .l7 and SHGC of .24.

Not every window needs to open. On the south side I installed just about as much solar glazing as I could (the windows are custom manufactured to the biggest size I could use) – 101 square feet of glazing. I installed two large, "picture" windows to maximize U-Value and SHGC in the Active Area. I also installed casements, which Accurate Dorwin provides with triple seals and double latches to minimize infiltration.

I minimized windows on the north, east and west. I have sufficient windows in those directions to see out and for natural ventilation, but they are smaller than are typically installed by builders today. Remember form follows function – windows are a thermal hole; don’t install them where you don’t need them. I am also fortunate (I’d say lucky if I didn’t plan it) that my best scene is to the south and I have little need or desire to gaze much in the other directions. This is an important aspect of planning street layouts, to better accommodate solar homes – most people would not want to be exposed, with all their south facing windows toward the street.

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