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Insulation techinques for a passive solar greenhouse

MarkDSimmons | Posted in General Questions on

Hello I am in the design phase for a passive solar green house in southern Massachusetts, zone 6/5B. I am wondering what would be the best way to insulate a 2×6 wall in a humid interior environment like a greenhouse? I was assuming to use rigid foam on both the exterior and between the studs. But the drying to the interior/exterior has me confused. Which way do I want to dry towards? Do I need a vapor barrier and if so where does it go, between exterior foam and sheathing, between interior wall surface and studs? I obviously do not want to have a mold problem so what is my best option for high R-value year round in a cold climate? Any input would be greatly appreciated.



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  1. GBA Editor
    Martin Holladay | | #1

    I advise you to use the PERSIST approach: that is, install all of your insulation on the exterior side of the sheathing. For more information on PERSIST, see Getting Insulation Out of Your Walls and Ceilings.

    You should choose water-tolerant materials to finish the interior side of your studs. I used cedar tongue-and-groove boards for my greenhouse, and after 37 years, the boards have held up fine.

    -- Martin Holladay

  2. itserich | | #2

    The best book I have found which covers insulated greenhouses is a 1978 book, The Complete Greenhouse Book: Building and Using Greenhouses from Cold-Frames to Solar Structures.

    As to interior humidity, there is also this article by Building Science Corporation on designing interior pools.

    Passive solar greenhouses in cold regions seem common in theory, but I have not heard much in reality beyond salad greens. My goal would be to super insulate with a small electric resistance heater.

  3. MarkDSimmons | | #3

    Thank you for your responses Martin and Erich. Martin I was reading your blog "Reassessing Passive Solar Design Principles" and if I am understanding it properly, it suggests that Thermal Mass is a waste of space. Any suggestions as to how I can maintain above freezing temperatures in my green house without an active heat source? Other articles I have read suggest a minimum of 5 gallons of water as thermal mass for every 1 square foot of glazing - which is a lot of freaking water even for my small design of a 12x24 greenhouse.



  4. GBA Editor
    Martin Holladay | | #4

    In Massachusetts, you aren't going to keep a greenhouse above freezing all winter without heating equipment. It's too cloudy and cold.

    The solution is to let your greenhouse freeze. Grow frost-hardy greens during the winter.

    The basic approach was pioneered (like so many things were) by Helen and Scott Nearing. A disciple of the Nearings, Eliot Coleman, continues the tradition. Buy these two classic books:

    Building and Using Our Sun-Heated Greenhouse by Helen Nearing

    Four-Season Harvest by Eliot Coleman

    -- Martin Holladay

  5. ranson | | #5

    I highly recommend reading this article on greenhouse building. They successfully built a passively heated greenhouse in Boulder, CO, that cooled down to only 50F on the coldest night, when it was -18F out.


  6. GBA Editor
    Martin Holladay | | #6

    I'd like to point out two facts:

    1. The expensive greenhouse discussed in the article you linked to has automated, motorized, insulated shutters that deploy at night.

    2. Boulder, Colorado has more winter sunshine than Massachusetts.

    -- Martin Holladay

  7. ranson | | #7

    Let me say that I wouldn't advocate building their greenhouse. I would advocate learning from it.

    Recirculating hot air through drainage pipes in the soil to simultaneously cool the greenhouse, conserve moisture, and warm the soil. Earthtube cooling is bad for a house, but it seems fairly practical in a greenhouse.

    Maximizing the efficacy and minimizing the area of your glazing.

    Insulating deep into the soil.

    I would also say that shutters don't need to be automated to be put to good effect.


  8. MarkDSimmons | | #8

    I have dug a 7 foot deep 12x24 foot hole and have placed thin wall sewer pipe connected in one continuous snaking line and buried it. Drilling weep holes at the long corners to help re I've condensation. The pipes are spaced 3 feet on center from each other so as not to over whelming the thermal mass. At a starting point of 7 feet I am hoping to get 3 layers. I have also been reading Galean Brown's book one heating with compost, as a possible passive heating source. I had originally planned on an aquaponics greenhouse but it appears it will be to difficult to keep warm. And I not want to risk killing fish.

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