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federer 05-31-2011 11:47 PM

Vapor Barrier Debate (in my head)
So after having talked to the contractor and reading threads on this forum I am conflicted about this topic. Is it better to install a vapor barrier with open cell spray foam or no? The contractor says no but I am reading here that it's a good idea! I am probably overthinking it but I can't decide.

rodeo 06-02-2011 07:46 AM

Your roof is unvented - if I recall.
That being so, then dont VB over the foam.
You will be creating a double vapor barrier
which will eventually rot the woodwork. Especially if its OSB decking.

Some moisture does migrate through open cell.
It may/ may not be OK. It depends in part on the moisture load you are throwing at it. If you can keep your winter humidity levels low like around 35%, you'll be fine. But many times humidity is higher and can be difficult/expensive to lower enough.

Closed cell might be the better option. It will create a double vapor barrier which means any moisture that finds its way through can rot the decking but its much less of a problem. Even if your decking rots, the closed cell foam is rigid enough to essentially become the structure on its own.

federer 06-02-2011 11:34 PM

thanks for the input. yes I am sorry my roof is unvented. that was what I was considering-doing closed cell. except that if the roof does leak I will have no way of knowing until the wood is rotted away. hence why I am using open cell....but you are saying the risk of that is not that great then?

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