Do I need Tyvek?
I’m retrofitting a 1952 ranch stye house in Connecticut. The wall sheathing is 1×6 tongue and groove. I’m going to put two layers of 1 3/8″ polyiso on the exterior.
Sheathing is now covered with tar paper. I’m thinking of taking of the tar paper (its damaged on a lot of areas) then install polyiso over the sheathing without any Tyvek – Each sheet of polyiso will be caulked around the perimeter. Interior walls ares stripped and will be dense packed with cellulose.
Can I forgo the Tyvek? Seems the aluminum skin on the polyiso is a good air seal. I’ll be installing the insulation and windows and then do blower door test, so i can always have the unemployed college kids go crazy with caulk if I need to. Thanks.
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While foil taped iso is good air or vapor barrier when protected, I have concerns that under siding it will not provide a long term drainage line for rain water. The foil is just so easily damages. Housewrap also replaces the rain screen that roofing felt used to provide (until it dried out). Make sure you run house wrap up the gables.
The code (and common sense) requires every wall to have a water-resistive barrier (WRB). Your WRB must be integrated with your window flashing, door flashing, and penetration flashing.
While many people use housewrap as a WRB, it's also possible (but a little tricky) to use rigid foam as a WRB. If you want to use rigid foam as a WRB, read this: Using Rigid Foam As a Water-Resistive Barrier.