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Community and Q&A

Spray foam under outdoor space. Good or bad idea?

unclejemima | Posted in General Questions on

I’ve got a outdoor patio, about 300sq/ft. Its got a hallway on one side, and covered patio on the other.

The hallway already has sprayfoam on the underside of the roof deck, in the wall cavity and under the slab. Its represented by area in blue (see attached).

The covered patio area currently has nothing for insulation in it. I was planning on doing T&G fir on the underside of the covered patio under the vault.

I’m wondering if there is any benefit to putting spray foam or any insulation under the covered patio even though the area is not heated?

Should I be venting the soffit area? Do I need any roof vents?

It goes from -40C in the winter to +40C in the summer. I’m located in Northern Alberta.

I’m looking to prevent mold or deterioration of the sheathing under the covered deck.


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  1. Expert Member
    Dana Dorsett | | #1

    If the patio area is open to the outdoors the roof deck is inherently vented, and you can finish the underside of the roof deck with whatever is appropriate for outdoor use.

    If it's an enclosed walled but unconditioned space, it isn't vented, but there is still no point to spray-foaming the underside of the roof deck, which only impedes drying.

  2. unclejemima | | #2

    Thanks for the reply.

    If you look at the attachment, the area at the bottom of the blueprint is outside and there is no wall there so I've got nothing to worry about then? I was just thinking that if I put non-vented soffit (ie, the T&G fir) on the ceiling of the covered deck, that heat and condensation would get trapped inside the vaulted space between the ceiling and roof deck area causing potential mold issues down the road.

    Should I at least put some vented slats in the soffit/ceiling space, if even just a few vents to allow airflow to the roof space?

    I recall my dad saying that he did this on commercial applications where a truck would be parked under a covered canopy with 3 wall closed in and 1 wall open and exposed to the elements, they would put insulation and poly in the roof space to prevent the heat and condensation from the trucks exhaust from going up and damaging the bottom of the roof deck...but I must have mis-understood.

    If I don't have to spray foam that area, then I saved myself some money lol :-) I thought it was required.


  3. GBA Editor
    Martin Holladay | | #3

    I don't think that you are anticipating that you will have an unusual moisture source under this roof.

    Dana is right -- the best way to encourage fast drying, and to make sure that the sheathing and ceiling last as long as possible, is to keep the roof assembly uninsulated.

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