Is this an adequate wall design for Zone 6 in New Hampshire?
1. 5/8” sheetrock
2. “smart” vapor retarder
3. Roxul R-23 batts
4. Avantix sheathing (taped)
5. 2” of type X XPS foam (taped)
6. (Hardie wrap?),
7. 3/4″ strapping (?+Coravent SV5 product on top and bottom of walls and windows)
8.Hardie clapboard siding
Three issues we’re still not entirely sure about are (1) we barely meet the green builder recommendation for R11.25 on the outside, and just fall short of the 36% of the total insulation on the outside (we estimate it is about 34% if we give an R=1 for the air trapped behind the Hardie Board, and R=5.5 for the XPS in the cold. (2) the type of “smart” vapor retarder on the inside; (3) where in the wall the Hardie wrap should go (we understand that it goes on the outside of the foamboard if the windows are “outie” and that’s where we’ve put it for the moment); (4) type of XPS – it comes in sheets but also in a finger jointed product from DOW, we’re not sure how these compare in terms of cost, ease of installation and quality of the finished product.
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Replies
Frank: Look into reclaimed foam, either xps or eps, rather than new xps. It's much cheaper and way greener and is readily available in New England. You could do 3 or 4" of reclaimed foam for less than 2" of new.
Instead of skimping on your exterior rigid insulation, can you save money by doing a cheaper batt (fiberglass? cellulose? Not sure about the costs vs mineral wool), then get your full R11.25 (or more) on the outside? 2.5" of XPS or Polyiso or 3" of EPS would get you there.
If you have the full R11.25, then you don't need the smart vapor retarder (unless it is required by your local codes).
General advice is to avoid new XPS if at all possible, as the blowing agents have a much higher global warming potential than other foams. Any recycled foam is great of course, and for new rigid insulation, EPS, polyiso, and rigid mineral wool are all preferable from a global warming standpoint.
You are correct that for outie windows, the weather resistant barrier generally goes on the outside of the foam.
Frank,
I've never heard of Avantix sheathing. By any chance did you mean to write "Advantech"?
I agree with Stephen and Brendan. Ideally, you want to avoid the use of XPS. (For more on this issue, see Choosing Rigid Foam.)
It's also important that you choose rigid foam with an R-value of at least R-11.25. Don't skimp.
If installing exterior foam, you might consider using OSB instead of ZIP panels. I would at least price out the possible cost savings.
As Martin points out, in zone 6A you need at least R11.25 on the exterior of the sheathing to meet IRC prescriptives. With R23s instead of R20s it would be prudent to bump that to at least R12.
There is no advantage to higher density Type-X foam in a wall-sheathing application, but the additional polymer per R makes it quite a bit less green. The labeled R10 will slowly drop to R8.4 as the HFC blowing agents bleed out of the XPS doing their damage.
Going with either 2.5" of polyiso (any density) , or 3" of Type-II (1.5lbs nominal density) EPS would be more likely have sufficient sheathing-R for the R23 cavity fill, and since both are blown with comparatively benign pentane, they have dramatically lower environmental impact.
Any type of reclaimed foam would be greener than any type of virgin-stock goods, since the environmental hit has already been taken, and re-using it is just loading up the "benefit" side of the cost-benefit balance. There are multiple vendors of reclaimed and factory-seconds (blemished) foam board in New England. Reclaimed fiber faced 2" and 3" roofing polyiso is pretty common and cheap, typically less than $20/sheet for 3" x 4' x 8' , under $15 for 2" , for goods in good to excellent shape. A layer of 2" reclaimed goods next to the sheathing with a layer of 1" foil faced stuff over it, staggering the seams of the layers by a stud-bay, and taping the seams of both make it a pretty good secondary air barrier for a lot less money than the inadequate 2" of Type-X XPS.
Sometimes odd-lots from smaller reclaimers show up on craiglist, as well a some of the larger vendors:
https://nh.craigslist.org/search/sss?query=rigid+insulation
Be aware that some roofing foam was manufactured with a taper, intended to promote drainage on flat roofs. In your application you want the flat stuff, within 1/8" of it's nominal thickness dimensions sheet-to-sheet, edge to edge.