Should I install a vapour barrier if I'm using rigid foam insulation?

Question

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.

Edit Please note the posting time of this thread. I asked this question in summer of 2012. While I appreciate the enthusiasm, the question has been answered, so please refrain from answering again.

Answer 1

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.

Answer 2

There is more to it than vapour barrier. The rigid board most likely meets all the required vapour barrier but does it create enough thermal break? Warm air can still hit the foam board and if the board is cool enough warm air can condense on it. Most rigid board is R-5 per inch so you probably need 2 inches to create the thermal break necessary. On dow's website http://building.dow.com/na/en/tools/installations/batt.htm they mention using a vapour barrier on top of the rigid board, but they don't specify what the board thickness was. Even if you create a vapour trap between the board and poly vapour barrier I think you would be OK as it is the continual influx of vapour that causes problems and you would have stopped that. If you are using Roxul you have nothing to worry about. If you also have BluWood studs even less to worry about. The small amount of vapour you could trap can migrate out through the wood-studded wall as wood naturally dries (probably at the top and bottom plate).

There are so many topics online about vapour barrier and rigid foam but thermal break isn't talked about that much and that is pretty much all that matters when you put rigid board up against an exterior basement wall and stud in front of it. If you create enough thermal break than no barrier is needed. If you do not, then you need additional poly vapour barrier. Keep in mind that even if you used 1 inch rigid board, you have still done wonders for your basement by blocking the wet air from coming into your basement via the walls.

Answer 3

In my experience, you should vapor barrier these even if the material claims to be a good vapor barrier. Especially if you are going to be inspected later. Any airflow will degrade the effectiveness of insulation.

Answer 4

You need 2" of continuous closed cell foam to eliminated the need of a vapour barrier otherwise don't bother with foam. Anything else like too little or gapped open cell foam and other semi solutions will be unpredictable, making a plastic vapour barrier unpredictable.

Answer 5

Any rigid foam board below grade must have 6-mil poly slip sheet covering the entire exterior surface as it is not qualified to be considered self-draining insulation. The only exterior below grade insulation that I have found to be self draining is the mineral fiberglass insulation. Please do your homework because if I am coming to do your inspection and the below grade foam insulation doesn't have a slip sheet, then you'll be mad at me because you'll be digging up the back fill. I am a certified and licensed LBO for the state of MN.