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Community and Q&A

Daylight Wanted

dsmcn | Posted in GBA Pro Help on

I’m building a new house in Knoxville, TN (zone 4). This is a wet climate and often dark—enough to require artificial lighting during the daytime in our current house. My wife & I are willing to sacrifice some U value for brighter daylighting, because the resulting personal productivity warrants it (we both feel more innervated on dark days). One of the greatest pleasures in life is to see sunlight streaming in through windows, the way interiors are always photographed in those upscale magazines!

My design provides for eaves >= 3’, such that direct sunlight will not hit glass during the warmer months.

My questions:
Does low-e contribute to insulation value when there is no direct sunlight (e.g. on the north side, or at night)?
At what value of VT does the window begin to look tinted?

The last time I installed new windows, I was quite surprised how tinted the “clear glass” was! But this was in L.A. and an advantage because it reduced glare. In TN I want no tint at all. But I also want an energy-efficient house, and I want to be a responsible member of a society heading quickly toward catastrophic global warming.

My research indicates spectrally selective glass will provide the best alternative. I welcome any recommendations for minimum acceptable VT value, minimum U value, and how to get there (i.e. window manufacturer recommendations).

Thanks to all: this is one of the biggest design decisions, and cost decisions as well!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    David,
    Here's an article you might want to read: All About Glazing Options.

    Q. "Does low-e contribute to insulation value when there is no direct sunlight (e.g. on the north side, or at night)?"

    A. Yes. The effect of the low-e coating is to lower a window's U-factor (that is, raise its R-value).

    Q. "At what value of VT does the window begin to look tinted?"

    A. Different people provide different answers to this question. I would certainly advise you to choose glazing with a VT above 0.40. According to Robert Clarke, former president of Alpen Windows, any window with a VT below 0.40 “would not be ethical to sell as clear glass.”

    Q. "In TN I want no tint at all."

    A. Perhaps you should consider buying clear triple glazing, without any low-e coatings.

    Q. "My research indicates spectrally selective glass will provide the best alternative."

    A. Spectrally selective glazing has a low SHGC -- a good choice for east and west windows, but probably a poor choice for south windows.

  2. user-659915 | | #2

    Bear in mind the factors other than just the light transmission of the glass. What happens to the light after it gets inside is just as important to the proper daylighting of your home, probably even more so. First golden rule - daytime rooms should have windows on two adjacent sides (not facing each other, and not in one wall only). Second golden rule - the walls facing the windows should have high reflectance, i.e. painted light colors, no paneling. Light finishes on floors, countertops, cabinetry etc. will also help distribute light and minimize contrast glare effects. Third golden rule - avoid installing solatubes and the like in a desperate attempt to compensate for having ignored rules 1 & 2. This strategy will fail and can even make the situation worse. Proper skylights (roof windows) can sometimes help in a remedial situation but they should be deployed sparing and located with great care.

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