Double vapor barrier with rigid foam
I am considering purchasing a house and have a question about the insulation. The outside of the house was sheathed in plywood, then covered in foil faced polysiso foam sheets, then vinyl siding – don;t yet know if housewrap was used as well. Not sure about a vapor barrier on the inside – if it turns out to have both the foil faced on the outside and either poly sheeting or kraft faced fiberglass bats on the inside, is it a deal killer? Heating system is forced hot air and it does have an air exchange system, location is Central Maine.
Also, in the basement, the wooden wall above the foundation, but below the first floor ceiling joists has also been covered on the inside with the foil faced polyscio insulation. Not sure of the correct term for this wall, but it is as tall as 3 feet or so in places, so more than just a band joist. I assume that the outside of the same wall was covered with the foil faced foam as well. Is this an issue since it creates a double vapor barrier? It was done very tightly, taped seams, foamed gaps, etc. The basement is unfinished. House is about 20 years old, and no apparent moisture/mold issues observed or disclosed.
I really apprecite any thoughs you folks might have on the matter. Thanks for your help, I really like the house, but want to make sure it isn’t trouble down the line.
Replies
The "double vapor barrier" thing is greatly overrated. You haven't mentioned anything so far that would specifically concern me (other than the vinyl siding).
well if ya live a climate that has hot summers and cold winters the vapour barrier will be on the right side in summer and winter .
The double VB is not best practice, but neither is it a deal killer.
IF there is sign of moisture having penetrated the wall assembly from either side, there is reason for concern.
The reason that the dual VB is not a good idea, is that IF water gets in, it is almost imposible to get out again without causing growth of destrutive micro-organisms - rot and mold.
So a lot depends on other etails such as how well the flashings are done, whether there is a rainsccreen type wall, whether interior humidity can transmit to dew point within the wall.....
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