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Does it make sense to choose glass that is slightly less insulative but has a higher SHGC and VT?

Martin Fahrney | Posted in Energy Efficiency and Durability on

I live in a heating-dominated climate with many cloudy days. Taking all window buying considerations into play I have found the replacement window decision-making process tricky.

I seem to have found a pretty good window sold through the local lumber yard here in Missoula, MT. It is a Plygem vinyl triple-pane marketed as R-5 glass. The marketing is slick but the real values as rated by the NFRC back it up. Or seem to. Does anyone have feedback about this Plygem product?

A operable casement has a R-4.76, U-0.22, SHGC of 0.18 and VT of 0.32. It is not my dream window for a few reasons but the low U-value, multipoint locking and seemingly good sealing for half the price of the nicer fiberglass triple-pane window has me very interested. If I could change the low SHGC and low VT as well as the PVC construction.

Only one of nine windows is facing south. Then there are three east, four west, plus one north. I would prefer casement but the sliders have R-4.55, U-0.22, SHGC 0.22 and a VT of 0.41.

Shouldn’t the glass in the marketed R-5 package be the same across the window line regardless of operation? What am I missing? And if it is possible does it make sense to choose glass that is slightly less insulative but has a higher SHGC and VT despite orientation?

Thanks in advance for any feedback.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Martin,
    Q. "Does it make sense to choose glass that is slightly less insulative but has a higher SHGC and VT?"

    A. All glazing choices involve compromises. In your climate, a higher SHGC would definitely be desirable, especially on the south side, but better glazing would probably cost you more. It can't hurt to contact Plygem and ask if they offer a better glazing option. If you really want to know the effects of glazing specification options, you'll need to use a good energy modeling program. For more information, see All About Glazing Options.

    Q. "Shouldn't the glass in the marketed R-5 package be the same across the window line regardless of operation?"

    A. Two windows can have the same glazing specifications but still end up with different U-factors or SHGC ratings, because the NFRC rating method takes the entire window into account, including the frame. A wider frame will lower the SHGC compared to a narrow frame. Some framing materials reduce a window's U-factor, while other materials increase it.

    Q. "If it is possible does it make sense to choose glass that is slightly less insulative but has a higher SHGC and VT despite orientation?"

    A. In your climate, a higher SHGC would be preferable. A higher VT is always preferable. It's impossible to determine the best glazing option with the information you have given, because we don't know what you mean by "slightly less insulative," and because your window sizes and orientations all affect thermal performance. You can either make your best judgment, based on product availability and price, or you can hire an energy consultant to run the numbers for you with an energy modeling program.

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