Tuesday, January 29, 2008

Windows

Anyone interested in passive solar design has to read up on windows. Most modern windows focus on energy efficiency in terms of heat retention. You can buy an energy star rated window for a northern climate like mine; but, mysteriously, the window performs poorly in terms of solar gain because the coating blocks much the of transmission of energy through the glass. So when it's sunny and 10 degrees in the middle of January, you'd have plenty of light but little in the way of solar gain. When designing a house that takes advantage of heat gained from south facing windows, it would be absolutely foolish to install a standard low-e window. On the other hand, double pane windows without a coating perform very well in terms of solar gain but don't do well with heat retention when the sun goes down. Thermotech Windows' web site is a good source of information on passive solar glazing. (Their triple paned windows sound top notch for a passive solar house.)
There are other glazing options out there. You just have to ask the manufacturer what's available. Marvin, for example, offers a glazing by Cardinal Glass called Low-e 178 that would work well with passive solar design, but it's not listed in their catalog and the local dealer in Blue Hill, Maine, had never heard of it. Andersen, however, does not have a glazing that fits passive solar requirements. (Unless, you buy standard double paned, uncoated windows and then go through the process of insulating them at night.) Bonneville Windows offers an AFG glazing called Comfort Ti-PS with a suitably high solar heat gain coefficient.
So what you look for in a passive solar window are two factors. The first is U-value which should be around .3 or less; this is the number relating to how well the window performs against heat loss. The lower the number, the better the performance. The second is the solar heat gain coefficient (SHGC) and this should be around .6 or higher. The higher the SHGC the more solar energy is allowed to pass through the glass to heat the space.
At this point we're leaning toward Marvin for our windows, the jury's still out.

1 comment:

Timothy said...

Mike,

So far, the Blog has been fantastic.I was looking for a mention of cost for windows in the last segment BUT alas! You may have been too busy making out with U-values to post dollars and cents. I am just curious as a cat regarding the Marvin super window price. Thanks for the edumacation on windiz. Now, I am pretty sure our U-values are around 3000 in our house. YT