Friday, December 23, 2011

All about low-e windows and SHGC

Low-e glass, like that used in the Andersen windows of the i-house, is manufactured with a microscopically thin and transparent layer of metal or metal oxide that reflects infrared “heat” energy back into the home, greatly enhancing the thermal performance of the window.

There are two kinds of low-e glass, hard coat (pryolytic)  and soft coat (sputtered), and they fall into three levels:

1. Low-e glass with a low Solar Heat Gain Coefficient (SHGC, 27% of solar heat transmitted, 64% of visible light transmission which is abbreviated VT on window specs) reflects and keeps much of the sun’s heat energy out, but not light, out of the home. This is the best choice in warmer climates dominated by cooling.

2. Low-e glass with a medium Solar Heat Gain Coefficient (39% of solar heat, 70% of light), for climates that use both heating and cooling equally.

3. Low-e glass with a high Solar Heat Gain Coefficient (71% of solar heat, 75% of light) that allows the sun’s heat energy into the home. This is for climates that require very little cooling, and most of the energy cost  is for heating.

A single pane of regular window glass has an R-value (insulation value) of only .85 whereas a low-e insulated window will have 3.5.  That is a significant difference, in a window's ability to hold the heat in, especially in a house with as much window area as the i-house.

For example, here in Santa Fe, the highest summer temperatures are around 90 degrees, and few people have air conditioning. There have been summers where I didn't even use a fan once. So, I would want the #3 Low-e with HIGH Solar Heat Gain Coefficient. It keeps the warmth in during the winter, but lets in most of the hot sunshine.

Low-e coatings weren’t offered until 1979, so the main window in my living room, for example, is completely un-coated, which lets in 100% of solar heat.  It is very good for solar gain in the winter, but doesn’t keep the room heat in well, so I cover it with a thick blanket at night. I have queried Clayton and will update this post when I get an answer on the values of their windows, and if they offer a high solar gain window for people in colder climates who want to take advantage of passive solar heating from the Sun.

This information on SGHC of low-e windows might be a thing an individual dealer might say “doesn’t really matter.” Yet, an architect doing solar design, would definitely incorporate the best kind of window for a specific purpose and location. I hope you find it helpful no matter what home you’re looking to buy, or especially if you are shopping for replacement windows.

Here's a video of a rep explaining low-e glass. He doesn't even get into the Solar Heat Gain Coefficient issue:

Here is a brief explanation of SHGC and when he gives a value of .4 that translates to 40%, which is a SHGC suitable for warmer climates. The lower the SHGC value, the more heat from sunshine the window will block.

No news on the debut of the i-house II, but in the meantime, why not check out my most recent post on the little solar home by Cavco, in my other blog, Greenotter's Manufactured Home Reviews.

For those you who wanted a separate guest bedroom with bath, but can't afford the i-House, check out my post on Palm Harbor's Avanti III. It is a clean modern design with a beautiful interior, 2 bedrooms and 1-3/4 baths all for $70,000. There's one for sale at a dealer in Oregon.

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