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Best glazing option

kaf2303 | Posted in Energy Efficiency and Durability on

I am building a greenhouse that will track the sun in Vermont. Glazed on the one side that faces the sun. It will have an R20 insulated door that will cover all glass at night
What is the best glazing option for balance of heat gain/loss on cloudy days ?
Will south facing single pane glass loose heat on cloudy days?
Thanks for any thoughts,Kurt

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  1. GBA Editor
    Martin Holladay | | #1

    The easiest question to answer is the last one:
    Q. "Will south facing single pane glass loose heat on cloudy days?"

    A. Yes, unless you limit your definition of "days" to mean "the part of the day between 10:00 am and 2:00 pm" and unless you limit your definition of "cloudy" to mean "with only a very thin layer of translucent clouds."

    Q. "What is the best glazing option for balance of heat gain/loss on cloudy days?"

    A. There is no easy answer to your question. You will need to compromise. Glass with a low U-factor (for example, triple glazing with two low-e coatings and argon gas fill) will have less heat loss on cloudy days, but most types of low-U-factor glass also have a lower SHGC. High-SHGC glass (a single layer of low-iron glass) has a high U-factor. There is no magic glass with a very high SHGC and a very low U-factor.

    For more information on this issue, see All About Glazing Options.

  2. kaf2303 | | #2

    Thanks for the quick reply Martin,
    Since It will need light for the plants and heat for the fish I'm probably better off with 2 or 3 layers.
    Any suggestions for sourcing glass units in the 6'to 6-6 tall range?
    Thanks again, Kurt

  3. GBA Editor
    Martin Holladay | | #3

    Back in the 1970s, we were all using IGUs sold as "patio door replacement glass" -- the size was standardized and the price was cheap -- to build passive solar homes and greenhouses. You could ask local glass suppliers if they have any low-cost units.

  4. GBA Editor
    Martin Holladay | | #4

    I used to buy these IGUs in boxes of 10 -- they came in a wooden crate. I still have a few in my basement -- just went down to measure them. They are 34"x76".

    I'd say "Make me an offer," but I really don't think you want them.

  5. Stephen Sheehy | | #5

    Kurt: If you plan on building a greenhouse from scratch, I'd look into polycarbonate panels.

  6. kaf2303 | | #6

    Thanks guys,
    I will look into those options .
    Martin how many do you have? Are they 70s vintage?location? I'm near Burlington,VT.
    Stephen what is the life cycle of the pc. Insulated panels

  7. GBA Editor
    Martin Holladay | | #7

    My IGUs aren't for sale. They've been in storage for 40 years, and you don't want them. Get something manufactured more recently -- ideally, some IGUs with a low-e coating and argon gas fill.

  8. Stephen Sheehy | | #8

    Kurt: I don't know how long polycarbonate panels last, but they are quite a bit cheaper than insulated glass. Since they are also much lighter, building a greenhouse should be easier. I'd go visit a local commercial greenhouse and see what the pros say about polycarbonate. The panels are available in a variety of sizes and thicknesses.

  9. kaf2303 | | #9

    Thanks a lot for the advice Martin & Stephen.
    Best, Kurt

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