I want to passively heat my home in the winter with solar, and have been reading up on glass and windows to make the best choice. There is a really strong push in the form of government subsidy for low-E glass since it decreases heat loss from the home to outside in the winter, and decreases heat gain into the home in the summer. My problem with the low-E coatings is that they block a lot of the radiation I want coming into the home in the winter. After some googling and scribbling, I calculate that for my use 0.1 (SHG) = 0.2 U This is based on 5 hours of 900 watt/m^2 light a day 40F difference across the window No interior blinds (for a worse case) Sound about right ?
Passive solar design requires having at least two different glazings available: low-U, low SHGC for cooling dominated locations, and low-U, high-SHGC for heating dominated locations. Both types exist, but for some reason only the former is US EnergyStar listed. Canadians list both. When I replaced a patio door earlier this year, on a south wall with an overhang that blocks our very brief hot summer sun, I gave up a tax credit in order to get glazing that has the higher SHGC to improve winter solar capture. I didn't lose much credit, as previous work had come close to the cap.
Are you in a heating or a cooling climate? If the later, it probably makes sense to go with low E and not worry too much about passive heating. You need to be heating up a large object anyways (lots of thermal mass). Perhaps you could use the sun to heat water jugs or something similar during the day and bring them in as the sun starts to loose its effectiveness.
Hey guys, My high desert New Mexico climate is a bit unusual, and really perfect for passive heating and cooling. This table shows heating degree days by month: So overall this is a northern climate (And by the way Fuzzy, DOE now recognizes the need for high solar heat gain glass in the rebate formula for norther climates.) My glass preferences are somewhat different than most people's, because I intend to have 100% external solar shading for the summer, I use blinds in the winter at night, and I only cool the home in the summer with ventilation. So I am not interested in a glass that optimizes year round temperatures assuming no homeowner interaction and AC -- which are the base assumptions of DOE. If I can find unglazed clear glass with a U of about 0.5 I'll be delighted. I'll have 0.9+ heat gain, and good frame build since the low U is not from glazing.
I can do the calculations for your particular case if you like, contact me off list. There are a couple of different types of lo-e coating, you want hard coat (aka pyrolytic) for high SHGC, at the expense of a tiny bit of U-value. If you have existing windows is your home, changing them is usually NOT cost effective. If you need better insulation from your windows you might think about interior storm windows. You can see my design here: Green Fret Consulting - Interior Storm Window They can be easily made for $1.25 per square foot, as opposed to the $50 or so per square foot you will be paying for new windows.
How about a window that switches? Switchable Glazing Windows I've seen some in action before. Power-wise, they're like the screens of e-readers: it takes a little bit of electricity to switch them, but they hold onto their light/dark setting after that without requiring continuous power.
