Exterior foam and Roxul batt on a reno
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Have a client who has an older house he just purchased . Scarsdale NY . He intends to put 1″ rigid foam outboard of the sheathing with Tyvek behind and 1x strapping to attach siding .
Interior first floor walls are 2 x 6 , second and third floor are 2 x 4 . Question 1 is what R value Roxul should he use in the 2 different wall assemblies ?
Second question is , is Tyvek the right product to use as the only barrier between the sheathing and exterior ?
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Replies
Richard,
Scarsdale is in Climate Zone 4A. In Climate Zone 4A, there are no limitations to the thickness of exterior rigid foam on walls. Any thickness will work. (For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.)
If your client wants to install Roxul ComfortBatt insulation between the studs, the most likely products to choose would be 5.5-inch-thick batts for the 2x6 walls -- these are rated at R-23 -- and 3.5-inch-thick batts for the 2x4 walls -- these are rated at R-15.
Tyvek is a water-resistive barrier, and it will work fine for this purpose. Every wall also needs an air barrier, and Tyvek isn't a great air barrier (because of the possibility that it will be damaged during insulation by ladders). If this house has plywood or OSB wall sheathing, you might consider taping the seams of the plywood or OSB with a high-quality European tape; that's a good way to create an air barrier.
-- Martin Holladay
Thanks Martin . Is there are better product than Tyvek to use that may serve the dual purpose ? You actually got right to my concern for this gentleman . Air barrier also for a better installation , or will just taping the seams of the sheathing suffice ?
Richard,
If you intend to tape the sheathing seams, you'll want to read these articles:
Backyard Tape Test
Return to the Backyard Tape Test
Two Wingnuts Describe Their Backyard Tape Tests
If you want a housewrap that is more durable than Tyvek -- one that may be tough enough to act as an air barrier (if it is treated gently until the siding goes on) -- you might want to choose Solitex Mento 1000, Tyvek CommercialWrap (which is heavier than ordinary Tyvek), or one of the tough housewraps from Cosella-Dörken (Vent S, Delta-Foxx, Delta-Maxx, or Fassade S). Remember, you have to tape the seams and detail the penetrations if you want this layer to be an air barrier.
-- Martin Holladay