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


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.

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.

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.

Best Answer

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.