Wall insulation Oakland county Michigan
My wife and I are planning to build a new house in Oakland County Michigan. It’s going to be a walkout ranch with a Superior Walls XI foundation and 2×6 framing. We’re trying to decide on the best way to insulate. After reading a lot of articles and blog posts, we’re really intrigued by the Roxul mineral wool for exterior insulation – the comfortboard IS. Our original plan was to do 2-3 inches of closed cell spray foam on the interior of the OSB sheathing (we like the the fact this also serves as an air barrier) and for thermal bridging control, use the exterior Roxul rigid insulation – preferably two staggered layers of 2 in mineral wool.
How does this setup sound to you Martin? Has anyone else used a similar setup? Our main questions stem from a few issues 1.) how important is it to keep the exterior sheathing warm and is 4 in of the Roxul enough to do this? ; 2.) is it more critical to stem the thermal bridging? ; 3.) is the closed cell spray foam worth it as far as being the best way to air seal? ( we’ve read about the air tight drywall approach, and Zip system and other similar ways to seal the outside sheathing with tape, which we are weary of. The liquid WRBs sounded very intriguing if we went in that direction.
We’ve also read the articles about minimum thicknesses of exterior insulation, and about the various perfect wall setups with all the insulation placed outside of the sheathing. We’re just confused about what our best approach should be to tackle our air sealing, thermal bridging, and overall insulation as economically as possible. It seems like the setup we’re proposing creates a sandwich with our OSB sheathing keeping it at a midrange temperature between the outside and inside.
Any thoughts on these conundrums would be much appreciated. Thank you in advance.
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Replies
Brian,
Usually, walls with insulation on the exterior side of the wall sheathing are designed to dry to the interior. So your suggested approach is unusual.
It's usually a waste of money to install spray foam between studs, because the thermal bridging through the studs undermines the thermal advantages of the spray foam. But I see no building science reason why you couldn't do as you propose. (However, there are environmental reasons to drop your plan.)
If you decide to create an air barrier at the sheathing layer (by taping your wall sheathing), and then you install mineral wool insulation on the exterior side of the sheathing, there shouldn't be any need for spray foam between the studs. Dense-packed cellulose would work fine; it would be cheaper and more environmentally friendly, and it would allow the wall sheathing to dry to the interior.
Thanks for getting back so quickly Martin. One of the reasons why I liked the exterior mineral wool was that it could dry to the exterior - being more permeable than standard rigid foam (am I correct on that?).
Is there anything you like better about the method described in "Breaking the Thermal Bridge" by Stephen Bonfiglioli on Fine Homebulding, compared to what you suggested with the dense packed cellulose, taped sheathing and exterior mineral wool? His method describes using polystyrene strips on the interior of the studs as a thermal break.
Also, are the durability and sealing longevity issues of the sheathing tapes (Zip tape, Sig Wigluv) valid concerns? Would you prefer something like a Carlisle's CCW peel and stick, or a liquid applied WRB as opposed to taping? Or is something like the Knauf Ecoseal a better option. for getting the best airseal? Thanks again.
Brian,
Q. "Is there anything you like better about the method described in 'Breaking the Thermal Bridge' by Stephen Bonfiglioli on Fine Homebulding?"
A. No.
Q. "Are the durability and sealing longevity issues of the sheathing tapes (Zip tape, Sig Wigluv) valid concerns?"
A. Yes.
Q. "Would you prefer something like a Carlisle's CCW peel and stick, or a liquid-applied WRB as opposed to taping? Or is something like the Knauf Ecoseal a better option for getting the best air seal?"
A. The durability of all of the products you mention -- Zip tape, Siga Wigluv tape, peel-and-stick products, a liquid-applied WRBs, and Knauf Ecoseal -- are uncertain. We can make a guess about their longevity, but we won't really know how they perform over the long term until 50 or 60 years from now.
Researchers who have re-tested well-sealed homes from the 1980s have discovered that the air barriers of the 1980s (often interior polyethylene sealed with Tremco acoustical sealant) have held up very well. That's reassuring.
My own advice: choose a product you like, do a careful job of air sealing, and perform a blower-door test. Then enjoy your house and let your children or grandchildren worry about future air leaks.