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Interior Greenhouse – wall and ceiling assembly concerns

Homesteader1 | Posted in Energy Efficiency and Durability on

I’m designing and detailing a greenhouse/garage with bedrooms above. It will be built in two phases; first the garage and greenhouse with a temporary shed truss roof with vented attic space, and a final phase with bedrooms above the garage under a gabled roof. The garage and greenhouse share the same roof and exterior walls (not an add on greenhouse). The greenhouse will have a wall separating it from the garage with a door and a 5′ x 5′ window. Think of it as a 32′ deep by 64′ long garage with a 10′ by 32′ greenhouse on one end. For a variety of reasons, the second level framing and plywd. floor will be built in the first phase. The greenhouse will have lots of windows. The project is in southern North Dakota.

Proposed greenhouse wall assembly:
We’re planning on exterior siding on 3/4″ air space, on 3″ rigid foam sheathing on 6 mil vapor barrier (or peel and stick membrane), on 1/2″ plywood. (not OSB), on 2×6 studs with 2″ high density closed cell spray foam at stud cavities and 5/8″ drywall. There will be mechanical ventilation in the space. I want to prevent mold and rotting problems. Does the spray foam make sense and help the assembly or am I creating a potential problem?

Proposed greenhouse ceiling assembly:
Similar to wall assembly but we have a vented attic space above. We are thinking about 3″ foam or batts at plywd. floor of attic on 16″d floor trusses with 2″ high density closed cell spray foam and drywall. does the spray foam cause issues? Where does the vapor barrier go? Does the insulation at the attic floor make sense?

Our goals are to control moisture in the greenhouse so it doesn’t damage the structure, create mold, or create a problem for the future upper level bedrooms. I know the humidity will not be as great as with a spa or interior swimming pool but I want to avoid any future issues.

Thanks for your feedback.


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  1. GBA Editor
    Martin Holladay | | #1

    You're on the right track. Either rigid foam insulation or closed-cell spray polyurethane insulation is a good choice for a greenhouse. However, I wouldn't choose the assembly you have settled on.

    One possible approach is a PERSIST wall. (For more information on PERSIST, see Getting Insulation Out of Your Walls and Ceilings.) If you go this route, you want your wall sheathing to be able to dry to the interior in case the sheathing ever gets wet due to a flashing defect. You are halfway there; however, you made the questionable decision to fill your stud cavities with a vapor-impermeable insulation instead of leaving them empty.

    If you want to have a PERSIST wall with insulated stud bays, here's what you can do:
    (1) make sure that 2/3 of your R-value is on the exterior side of your vapor barrier (the peel-and-stick);
    (2) choose a vapor-permeable insulation (cellulose or open-cell spray foam) to install between the studs.

    If you insist on filling your stud bays with closed-cell spray foam, then you should skip the peel-and-stick and the exterior rigid foam.

  2. Homesteader1 | | #2

    Martin, Thanks for your response. We'll leave out the closed cell spray foam at the studs and add a little more foam at the exterior. At the ceiling, I think we'll add peel and stick to the attic floor and then foam board insulation to mimic the exterior wall assembly. In the future, when we add the second level, I assume we'll be OK because the peel and stick will prevent moisture penetration to the upper level. Thanks again, Rick

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