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Vapour barrier for odd insulation order

user-7058482 | Posted in General Questions on

Hello, first time poster.

I insulated my 16×24 workshop with recycled 4″ XPS(r20 theoretically) in the 3.5″ stud bays + left over foam from a 2 part kit for the gaps/stud faces.  This has been fine, but I want to actually finish the interior wall as it’s just the exposed XPS at the moment.  My messy spraying would make it difficult to finish so I was going to add an interior stud wall and some extra Fiberglass insulation I had lying around (r14). 

Would a vapour barrier/retarder in the form of 6mm poly be needed?  I’m going to finish with t&g 5/8th plywood.

Also, I stood the 4″XPS off the exterior siding(no sheathing, just the wood siding) with some 3/4″ strips of wood, but there’s no specific air flow in that cavity.

Also, live near Ottawa.  Zone 5 I believe.


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  1. charlie_sullivan | | #1

    From a building science point of view, it's not needed, unless this is a workshop for something that involves boiling cauldrons of herbs or some other unusually intense source of moisture.

    From a code point of view, it might depend on your local inspector but you can probably argue that the XPS is your vapor barrier.

    1. user-7058482 | | #2

      If I try and keep the hammer away from my thumbs it should keep the tears from condensing on the back of the XPS.

      So just so I understand properly, the interior 'warm' surface of the XPS will be warm enough to not become a condensing surface even though it's going to be somewhat insulated from the hear source by the new Fiberglass batts? And the RH is probably fairly low because it's a workshop?

      Thank you,

      1. charlie_sullivan | | #3

        Yes, you understood correctly regarding why the XPS won't be a condensing surface and also understood correctly about keeping your thumbs safe.

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