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How do I insulate the ceiling of a room that is located under a porch which is exposed to outside air?

V8ZrUnCZZW | Posted in Energy Efficiency and Durability on

The room is located under a covered porch that is enclosed on three sides. Do I need to leave any room for air movement? Should I have a vapor barrier toward the living area even though it is a ceiling?

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  1. Expert Member
    CARL SEVILLE | | #1

    First you need to make sure that the porch floor is perfectly waterproofed and are sure that no water will get through it into the room. I would insulate it like a flat roof without ventilation, use either closed cell or open cell spray foam on the underside of the porch floor, making sure to seal the entire perimeter as well as any gaps between framing members where the floor meets the walls. If you eliminate air movement and have enough insulation for your climate zone you shouldn't need a vapor retarder - if you use air conditioning the vapor retarder will probably cause more problems than it will solve.

  2. GBA Editor
    Martin Holladay | | #2

    1. Where are you located?

    2. Will the room under the porch be heated?

    3. What is the current construction of the ceiling of the room in question? (Does the room have a framed ceiling with joists? Or is there a concrete ceiling?)

  3. Foamer | | #3


    A flat roof like yours can't be effectively ventilated. Therefore your best choice is a non-air permeable insulation system. I would use closed cell spray foam if you are up North where the deck can get really cold and potentially cause condensation. If you are in a warmer climate, open cell foam will work well also. Either way, you should not install a separate vapor barrier at the ceiling level. The roof material is a vapor barrier and vapor barriers on both sides of a cavity is really bad news since any moisture that gets in will be trapped.

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