Tyvek WRB over exterior insulation?
Islander6
| Posted in General Questions on
Building a home, with no sheathing. All tension strapping. 4 inch rigid rockwool comfortboard on the exterior. Also going on steel studs. Is it ok to fasten my air barrier to the comfortboard instead of under? if so how do i fasten to roxul
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Replies
Kirk,
That's really tricky assembly to build. I don't think I've ever heard of anyone applying exterior mineral wool without sheathing - especially on steel studs.
The location of the WRB (Tyvek) will determine how you flash openings. How easy it is to integrate the WRB with the openings is also very heavily determined by where the windows and doors are in the wall assembly.
Before going any further I'd suggest doing a full wall section showing the base, an opening and the intersection with the soffits. I don't want to be discouraging, but that's a build I would decline to bid on if I was asked.
Hi Malcom,
I might have to go the polyisco route for a rigid board as its very stiff and not much flex unlike the rockwool, as the no sheathing could be tricky to keep flat with no sheathing. Maybe an interior air barrier and taped rigid board acting as a double air barrier would suffice and no need for tyvek. However i agree flashing windows and doors will be tricky to intergrate
Kirk,
One problem that remains is that the exterior insulation and strapping need to be fastened to the framing with screws, and if it is also steel, you are going to have some pretty big thermal bridges from outside to in. The fastener schedules I've seen for that deep exterior insulation also assume a certain depth of penetration into the wood framing to counteract sagging. Something that won't be present in steel studs.
Malcom,
Screws will be stainless so thermal bridging is very low and negligible, expecially through such thick insulation.
I can understand the sagging, however cladding is standing seam metal and a little 3/4 horizontal cedar, so siding is very lightweight compared to a hardie board or plank.
Kirk,
Switching to foam from mineral wool will alleviate some of the concerns about bearing, but I'd urge you to look at this guide to see the forces at work on the screws.
The relevant pages are from #11 on: https://www.victoria.ca/assets/Departments/Planning~Development/Permits~Inspections/Example~Plans/Illustrated-Guide-R22-Effective-Walls-In-Wood-Frame-Construction.pdf