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Low SGHC windows in a heating climate?

Don Otto | Posted in Green Building Techniques on

The equivalent of low solar heat gain is high infrared reflectance. In the winter season, given that 1. there’s more heat (infrared) inside the house, and 2. there are more hours of dark than daylight, and 3. the heating contractor has to keep you comfortable every hour (not just the seasonal average), does it make sense to have a window that reflects more heat at you in winter? Similar benefit in summer when you’re trying to keep it cool inside.

I live in Iowa where temperature extremes range from -10°F to +100°F, and am fond of using Andersen’s Smart Sun Low-E, SHGC of about 0.20 and Visible Transmittance of about 0.50.

Would that glazing be more comfortable and help the HVAC contractor use a smaller heat source?

Anyone else with experience or calculations?

Thanks,

Don Otto, [email protected]

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Don,
    A good energy modeling program can answer your question. In most cases, you want high SHGC windows on your east and south sides. You may want low SHGC windows on your west side, since it's hard to shade west windows from the low-angle afternoon sun.

    Homeowners who choose low-SHGC windows for all orientations usually pay higher energy bills for the life of the building -- especially in a heating climate like Iowa -- compared to homeowners who choose high-SHGC windows or orientation-specific glazing.

  2. Don Otto | | #2

    Thanks for the reply, Martin. I'm not too concerned about overall heating costs since my homes do a good job on energy conservation and I use geothermal HVAC. For the last house I built, 2,300 conditioned sq ft, Energy Star estimates it will cost less than $150 a year to heat & cool, and has a HERS index of 39.

    I'm concerned about winter night time occupant comfort. When I look at data on the interior glass surface temperature, low SHGC comes out on top, so it seems reasonable that that glazing is preferred.

    I'm putting together bids for a new home that has lots of glass on the south, along with a wood burning stove, and I want them to be comfortable well into the evening.

    Thanks again,

    Don

  3. GBA Editor
    Martin Holladay | | #3

    Don,
    The most important determinant of occupant comfort on winter nights is the U-factor of the window, not the SHGC. Choose window glazing with a low U-factor -- triple glazing is better than double glazing -- and a high SHGC (except on the west side, where a lower SHGC may make sense).

  4. John Brooks | | #4

    Solar Heat Gain...
    I realize that there is more Solar PLUS Conductive heat gain thru West Windows than East Windows....and West windows are overall more evil during the cooling season than East windows.

    But the SOLAR heat gain is virtually the same thru East & West.
    Is it not just as important to shade and/or use low SHGC for East windows as West windows when considering the cooling season?
    http://www.susdesign.com/windowheatgain/index.php

  5. Keith Gustafson | | #5

    Because while the gains from both are much lower than south glass, the gains from east glass help in the winter[giving you little heat in the early am when temps are low] the gain from the west in the summer kills you because it comes exactly when temps are peaking in the late summer afternoon. the lack of this gain in the winter does not hurt much because it comes when daytime temps are the highest.

  6. GBA Editor
    Martin Holladay | | #6

    John,
    I agree with Keith. Of course, climate matters, as well as the characteristics of your site. Are there trees to the east? Are there trees to the west?

    In Texas, I'm fairly sure you want low-SHGC windows on the east side. In Vermont, I'm happy with high-SHGC windows on the east side. In between Vermont and Texas, the usual answer applies: "it depends."

  7. Don Otto | | #7

    Martin, thanks for separating insulation values from radiant transmission. While there's overlapping effect, it's the insulation value that my heating contractor works with and what the occupants feel.

    Don

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