Adding another Vapor Barrier or Not?
Ari1
| Posted in General Questions on
Hello,
I live in zone 6. My new construction house has 2 inches of foam on the outside. On the inside of the wood sheathing + house wrap, is blown in Havelock sheep’s wool insulation. I plan to plaster over lath and / or shiplap/ beadboard wainscoting here and there. Do I really need an interior smart vapor barrier installed such as intello?
Thanks!!
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Replies
As long as you have enough exterior R value in proportion to interior R value, you can get by without an interior side vapor retarder -- although an interior side vapor retarder would still provide additional insurance against any moisture problems in the future. The ratios are explained here:
https://www.greenbuildingadvisor.com/article/rethinking-the-rules-on-minimum-foam-thickness
Personally, I always use an interior side smart vapor retarder (typically Certainteed's MemBrain, which is probably the cheapest of these options), just for extra insurance. I still use exterior rigid foam in R value ratios exceeding those shown in the tables (i.e. I like to use more exterior R value for a given wall type than the tables show).
Bill
Thanks Bill. I found this German wall system calculator website: https://www.ubakus.de/en/r-value-calculator/? –– I plugged in the values to include the smart vapor wrap on the interior verse not and it seemed to make a small difference to include one. It does seem like I might be fine without, too. Thanks for chiming in.