My double stud wall assembly
We are in Minnesota (Zone 6a). 2200 sq/ft single level, slab on grade.
Finalizing my 10″ double stud wall assembly details and want to get your suggestions and recommendations for improving the assembly without increasing significantly increasing costs.
-Sheetrock
-Certainteed Membrane 1-10 perm
-Dense pack cellulose
-1/2″ plywood
-Tyvek 58 perm
-furring strips on the (north and east sides of the house)
-LP smartside lap, b&b, & shakes (north and east sides of the house)
-No furring strips, just vinyl siding (south and west sides of the house)
I have selected a “smart membrane” Certainteed Membrane to allow some drying to the interior with not much penalty on my wallet over poly.
I selected Tyvek for the WRB because it’s high perm and honestly just because it’s very familiar. I’ve read a fair amount about these assemblies and I’m not seeing a clear performance WRB standout.
Ok I’ll just ask…will this wall assembly work well?
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Replies
I'd make sure to tape sheathing seams, as well as wrb seams.
For our double stud walls, we applied the membrane to the outside of the inner stud wall. Blew the cellulose through the membrane and taped the holes. All wires and pipes went in the inner stud wall, so we avoided having to air seal electrical boxes and most other penetrations.
We're in zone 6, Maine.
That's an interesting approach Stephen. Did you anticipate that your wall would then dry from the membrane to the interior, and the membrane to the exterior?
Is your recommendation for taping the sheathing on my walls, to help with air sealing?
Scott: we figured the wall can dry in both directions if necessary. The obsessive taping was for air tightness. We probably didn't need to tape everything, but it can't hurt. We hit Passivhaus air tightness. We used a ridiculous amount of tape, mostly 3M 8067 flashing tape, but we used a lot of Siga tapes around windows and since we used a Siga membrane, we used Siga tape to patch the holes after insulation.
I like your approach Stephen. I'll have to bounce the off my framer and insulation.
Anyone else want to chime in on my wall detail?
It sounds like a good assembly to me. I assume you've read some of the threads expressing concern about moisture accumulation in double-stud walls. If not, check out Martin's "Two Views of Double-Stud Walls" on this site. I've concluded that these concerns are largely hypothetical, as long as interior moisture and exterior bulk water are handled correctly.
What is your plan for managing interior humidity?
Thanks Jon. I had not read that one, but have read several others talking about the same/similar concerns. I agree, it seems to me, like the concerns are vary valid, but also very manageable to end up with a low risk wall assembly. My plan for dealing with the potential issues are an HRV and humidity monitoring and careful air sealing. Also, I think the Certainteed Membrane will help increase the wall's ability to dry as long as we keep the interior humidity levels in check. From the exterior, we are using plywood, furring strips for siding ventilation (except where we use vinyl siding), 24" overhangs and the eve and rake and gutters on the north and west sides of the house to help manage bulk water.
Why not Delta SA or Henry's Blueskin - they are both peel & stick WRB- to eliminate the TYVEK and tape? They would eliminate taping the plywood and WRB. They also seal around the nails used for siding.
I haven't looked at them too closely, but I like the self adhered concept. The cost would be a big factor though. Tyvek is about $.12sq/ft and the one price I have found (online) for Delta SA is about $1.40 sq/ft (gotta be cheaper somewhere right?). I know the taping would bring the cost of the Tyvek up a lot, but I'm just not sure it's worth it. Henrys Blueskin seems to be much more cost effective at about $.46 sq/ft, than Delta SA. Might look into that one more. Although the SA product seal better, they have a bit lower perm than rating than tyvek too. Is the performance increase really worth the additional cost, I'm not so sure. Hmm.
I am also building a double-stud wall house in Zone 6 (Montana). While we are finishing up the foundation, so not framing yet. My current wall detail is from inside to outside:
- sheetrock
- 1.5" horizontal batten with R-6 Roxul infill. Receptacles are shallow mounted in this space.
- IntelloPlus air barrier
- 11.25" double stud wall
- 1/2" plywood, seams taped
- Tyvek Drainwrap (Craigslist find)
- 3/4" furring strip
- LP SmartSide or similar.
If I were you, I'd consider IntelloPlus instead of Membrane as it allows you to dense pack before installing sheetrock (high QA/QC). Membrane stretches too much. I would also consider the 1.5" utility cavity to protect the air barrier.