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Community and Q&A

Insulating floor of addition

Ne66ryFunF | Posted in Building Code Questions on

We are adding a 2nd story addition to the back of our house in Portland, Oregon. The addition is supported on columns so the first story is open creating a covered porch. I am trying to decide how to insulate the floor since it is open to below and will have plumbing in the floor.

My thought was to use R-30 unfaced fiberglass batts in the floor, a layer of 2″ XPS fastened to the bottom of the floor joists, 2×4 flat sleepers, and then 1×6 T&G.

However, then I read the Oregon Residential Building Code and it says that I must have a 1 perm vapor retarder on the warm in winter side. The only exception the code allows is for existing construction or slab on grade which I don’t have. I am concerned that faced insulation and the XPS would create a double vapor barrier. Any ideas on how to avoid this?


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  1. Expert Member

    Your first stop should be your building inspector to see what the local practice in such a common situation is, and what they will accept as alternatives if local practice isn't to your liking. It might turn out not to be an issue at all.

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