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Community and Q&A

Sunroom/Greenhouse on the exterior wall?

Hawnes | Posted in General Questions on


I was wondering if it was possible to build a sunroom or greenhouse against a wall assembly such as the picture below?

Would this cause issues for the wall?
My hunch would be a greenhouse is not okay. But what about a sunroom.

Would I have to make sure the humidity stays low?


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  1. walta100 | | #1

    First understand if we define a sunroom as a room addition with 3 mostly glass walls that is mostly open to home on the other side any such room will have a negative impact on your energy bills in any climate.

    The people I know that have sun rooms generally don’t use them much.

    I don’t think you need to worry about excess humidity from the sun room in colder climates as the large amounts of cold glass will often fall below the dew point and lower the humidity in the house.

    If you really want a green house complete with hundreds of plants build it just like the commercial ones. Detached, concrete floors with drains, everything rot proof and a huge furnace to keep it warm.


  2. Expert Member
    BILL WICHERS | | #2

    You can build lean-to style greenhouses against exterior walls, but you have to be very careful about sealing the wall against moisture drive *from the greenhouse*. Your assembly as-is would be a problem here, since you have an interior side poly vapor barrier, and a very vapor open exterior. Greenhouses tend to run very high average humidity levels, and the moisture drive will always be towards the house from the greenhouse as a result. If you put up an exterior side vapor barrier, and ideally use a smart vapor RETARDER (not BARRIER) on the INTERIOR), you'd probably be fine with a greenhouse here. Be sure to seal up any soffit, ideally using PVC trim board which is impervious to moisture.


  3. StephenSheehy | | #3

    On the south side of our house, we have a pretty well insulated room with decent (U .28) casement windows. With the exception of one on the east side, all the windows are on the south side and they pretty much cover the entire south wall from about 30" above the floor to the ceiling. The west side and north side walls are against a very well insulated, very tight double stud wall.
    We use the room a lot, even in winter on sunny days. We don't ever heat the room, but the sun often heats the space enough in winter that we open the door to the house. It doesn't get oppressive in mid summer. We use the space in spring for seedlings and leave several plants in it for the winter. It never freezes.
    I agree that a greenhouse could be a humidity problem.

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