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I'm re-insulating a room in my house from the inside. I've read that polyisocyanurate (polyiso) rigid foam insulation is the best R-value, so I plan on using that.

Details: The house was built in 1955 in Camrose, Alberta, Canada. And the previous wall was insulated with fibreglass batting with a type of tar paper as a barrier. On top of that (on the interior) was 2 sheets of plaster - pretty impermeable to air flow I'm guessing.

Question: Do I still install vapour barrier? I'm almost certain I should, but I've heard it could cause moisture trapping problems when using a faced insulation like polyiso.

Edit: I should mention that I'm planning on putting this insulation between the studs, not completely over the studs from the outside. I'm not taking down the outside siding, I'm doing this all from the inside.

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You definitely don't want double barriers. Also, many believe that older houses 'breath' a lot naturally and don't necessarily need the VB at all. (VB's are always a slightly controversial topic in the trades anyways...) – DA01 Jul 3 '12 at 20:35
I've heard that too, and I've also seen comments that say "breathing" is nonsense. I'd tend to believe VB is necessary as otherwise - what's to stop air flow? Thanks for the comment! – Pickle Jul 3 '12 at 20:38
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Airflow is really about the house wrap. You don't want outside air coming in. The VB is more about moisture, the idea that you don't want moist interior air condensing in the colder wall cavity. A good place to read up on VBs is Building Science's web site: buildingscience.com/documents/digests/… – DA01 Jul 3 '12 at 20:41
Sorry, completely missed the fact that you are using foam insulation. Foam boards are typically considered a form of vapor barrier (especially with the foil face) by default. So, as long as you have a tight seal, you shouldn't need any other barrier. – DA01 Jul 3 '12 at 20:49
Ya, that's what I've seen in places too. However, I'd need to have an airtight seal for the VB to be superfluous. Since I'm attaching the foam to whatever siding is there, (I haven't ripped out the old insulation yet to take a look), I don't know if I'll be able to create an airtight seal. Your comment still stands though - there should only be 1 airtight seal. Thanks. – Pickle Jul 3 '12 at 20:53

1 Answer

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Rigid foam insulation is usually a qualified vapor barrier. However, an installation between the studs can pose a challenge. To complete the vapor barrier you would want to foam-seal or tape all the sides where the rigid meets timber, because these gaps will facilitate vapor flow.

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