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Insulated impact-resistant windows

Pamela G | Posted in Energy Efficiency and Durability on

I have a house in Florida (climate zone 2a) with windows facing west. We replaced all windows and sliding doors with white aluminum frame impact rated, insulated, low e glass with grey tint. The specifics on the windows are: Insulated, large missile, Low-E = Windoor GR2, grey tint, 3/4″
NFRC ratings: U factor =0.61, CR = 16, SHGC = 0.23, VT = 0.29

We have a serious problem with heat gain in the afternoon now. On a recent day where the outside air temperature was 81 degrees, the outside temp of the glass was 123 degrees and the inside glass temperature was 112.4 degrees. It is like turning on an oven in the house which is not what I expected after paying a lot of money for these windows. I thought the insulated, impact and low SHGC rating would help keep heat out, not amplify it! I have tried calling window companies but cannot get an answer as to how much heat should reasonably come through.

Is this normal? Are there alternate products or window that don’t transfer heat this much?

I can provide the spec sheets if needed.

Thank you!

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  1. Greg Smith | | #1

    Hi Pamela,

    Does the IG make up consist of gray tinted glass the outside lite and LowE coating on the inside lite?

    It's not the outside temperature, its the solar gain on west-facing windows that you are dealing with.

    Your best bet might be to find a way to shade them in late afternoon, but not sure how possible that might be.

  2. GBA Editor
    Martin Holladay | | #2

    Assuming that your window manufacturer and your window installation contractor didn't commit fraud, and the windows have the stated SHGC, there is no reason to blame the window manufacturer or the installer.

    If a window has a SHGC of 0.23, that means that 23% of the solar heat that hits your window enters your house. If you have lots of west-facing glass, including sliding glass doors, that's a lot of heat in the late afternoon.

    Green builders in hot climates try to avoid unshaded west-facing glass. (Because the sun is low in the sky in the late afternoon, west-facing windows are often difficult to shade, unless the windows are protected by a very wide porch.) In other words, you may be talking about a basic design error -- one that can't be easily solved by windows that have a SHGC of 0.23.

    Exterior shutters are one option. If you like the view, you may need to solve this problem by talking you your HVAC contractor and improving your air conditioning system.

  3. D Dorsett | | #3

    Not that it's going to fix this exact problem, but a U-factor of U0.61 is WAY above the IRC 2015 max of U0.40 for US zone 2.

    The high surface temperature of glass is almost certainly due to the absorption of the tinted glass. The tinted pane is effectively a solar collector. I'm haven't recently reviewed how SGHC is tested, but I suspect it's a measure of how much of the radiated solar spectrum actually gets through the window, and does not factor in the direct heating effects of the glass due to the energy absorbed by the tinting. A heavy tint can limit how much light energy passes through to be absorbed by objects in the room, but it also makes the glass hotter. Low-E coatings work by reflecting rather than absorbing light energy (but also impedes it's ability to radiate heat away.) Tinting helps cut glare by absorbing some of the visible light spectrum, but it causes the glass to heat up. The 112F window surface with less than R2 of insulated glass is very likely to be the problem.

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