I am in Climate Zone 4 in Virginia. Currently, my wall assembly will be as follows from the inside out: - drywall - poly vapour barrier - 2x6 16in OC stud frame with Roxul - OSB sheathing - Tyvek house wrap - 1" R5 XPS - SmartSide siding Two questions: 1. Do I need that poly vapor barrier or would it be best to leave it off for better drying capability to the inside? 2. Should I tape the XPS seams? I apologize if this is overly simplistic. I'm building my own house and I'm very new to all of the foam board and drying in/out thinking. Things that seem right to me (vapor barriers behind dry wall) seem to trap moisture in the wall if combined with good water protection on the exterior.

I would imagine you would want the Tyvek house wrap. I cant think of any residential application where it wouldn't be used. Regardless, no house is perfectly sealed. Windows, cracks.. moisture will still escape/enter somewhat. As for the XPS seams, tape is highly controversial. Many say it fails. Many other swear by it. There is a tape called JointSealR which you may want to search. Ive also heard of screwing furring strips to keep the foam in place, but you would need to get confirmation on that first. There are adhesives you can caulk into the joints as well.

I would skip the poly, tape the OSB, use Tyvek Drainwrap and add furring strips.

So, I'm about to make the same decision - with a minor twist.

Climate Zone: 7b

Wall Stackup - on top of ICF wall - Gable ends: LPSmartside, 7/16" Furring Strips, Tyvek, OSB, 2x6 Wall filled with Open Cell, 1" airgap (much of it is filled with open cell overspray), 2x4 wall, Smart Vapor retarder, Gypsum. So, a double wall thermally broken. (All this in MD, yep. I'm nuts)

So, everything is done but the vapor barrier and sheetrock, and I'm not sure I really need the vapor barrier because my wall can dry to the outside with the furring strips.

What do you all think? Thanks for your help.

-C2

Posted By craigtoo on 25 Jan 2017 04:58 PM
So, I'm about to make the same decision - with a minor twist.

Climate Zone: 7b

Wall Stackup - on top of ICF wall - Gable ends: LPSmartside, 7/16" Furring Strips, Tyvek, OSB, 2x6 Wall filled with Open Cell, 1" airgap (much of it is filled with open cell overspray), 2x4 wall, Smart Vapor retarder, Gypsum. So, a double wall thermally broken. (All this in MD, yep. I'm nuts)

So, everything is done but the vapor barrier and sheetrock, and I'm not sure I really need the vapor barrier because my wall can dry to the outside with the furring strips.

What do you all think? Thanks for your help.

-C2

There are no location in MD that is in US climate zone 7B. It's all zone 4A, with the exception of the western-most county, which is 5A.

The IRC does not demand an interior vapor retarder tighter than latex paint in zone 4A, and with a vented air space between the OSB and siding it doesn't demand it in zone 5A either.

But in zone 7B the air gap isn't sufficient protection, since the OSB itself is too vapor retardent for the length & depth of the wintertime moisture drive. It can dry toward the exterior, but not fast enough. It runs less than 1 perm when dry (a class-II vapor retarder), but even at mold-inducing moisture levels it's still in the same permeance range as interior latex paint.

At high R the OSB runs a bit cooler on average than it would with mere 2x6/R20, which increases it's peak moisture accumulation from interior side moisture drives. If the open cell foam is thick enough it can be sufficiently vapor retardent to protect the OSB, but the foam itself would be at risk of accumulating enough moisture to suffer freeze/thaw degradation in a zone 7B location.

Using let-in bracing for the structual capacity and asphalted fiberboard as the exterior sheathing is less risky, since the fiberboard is highly vapor permeable, and not easily damaged by moisture.

But if you're in MD, lose the polyethylene, even the smart vapor retarder- it isn't buying you anything.

A real problem with the stackup is the 1" gap between the 2x6 and 2x4 sections, which is a fire-spread path and a thermal bypass. It's better to spray the 2x6 wall first (5.5")in one pass, wait for it to cool, then spray the next 4.5" to fill up the gap and the 2x4 framing, completely filling up the gap between the two walls.

Excellent.
Thanks Dana1.

Sorry about the Climate Zone mess up.