Friday, December 4, 2009

Solar Cats and Passive Solar Energy Collection

Did you ever wonder what the surface temperature is of your solar kitty?

As you can see, Natasha runs about 112F on a clear, cold winter day in Denver. Note: she is laying in front of a window with an SHGC (solar heat gain coefficient)of 0.56. That's far higher than the average low E window which has an SHGC of about .32. What's that mean to a cat? About 15F. A black surface in front of a conventional low E window under the same conditions is around 97F. Still warm, but that's a significant penalty for not specifying the right windows.

Hurd "hard coat low E" windows from Rocky Mountain Solar Glass has been the only local solution I have found for passive solar. Serious Windows (formerly Alpen) should also be able to supply a high SHGC low E window. Most other window suppliers and manufacturers have no clue about this issue.

Increased solar gain in the summer isn't a concern, because these are south facing windows. In the summer, the sun is high in the sky, and the window is completely shaded by the properly designed overhangs. Even the Energy Star program is silent on this, although their energy modeling computer programs do give credit for high SHGC windows on the south side.


  1. No that's is a solar kitty.
    I cant wait to optimize my windows for my c.1948 navajo deco solar home with your help by exposure, SHGC,
    and insulative and radiant window properties...thanks for the post Kevin!

  2. To Clarify, Please tell me if I understand you correctly. With a south facing window one would want a low 'E' window with hi SHGC and high 'R' value (low U). in the summer though we should install some sort of Awning or cover over the windows, Yes? what should we specify for the north and west facing windows? Our house was built in 1980. The rear of the house is directly south facing and is covered in windows (double pane (not sealed) pella windows. the windows are old and let in alot of cold in the winter evenings. In the day (winter) the rear of the house gets hot. what type of windows should we be looking to buy?

  3. If you are "overglazed" on the south, you may not want the hi SHGC on those south windows, due to uncomfortable overheating in winter.

    Passive solar homes have a natural temperature swing, however. What I would consider normal, would be about 79F max on the sunny afternoons. Your current windows probably have a little higher SHGC than either kind of low E window.

    Regular low E windows are the choice for the East, South and West exposures.

    Since it sounds like you may need expert help, the best vendor is Serious Windows. They aren't the low cost vendor, though.

    Call me if you want, 720 435-5909