Double wall assembly
We plan to build a PGH on the coast of Nova Scotia (CZ 6, lots of wind, fog, some hurricanes, but mostly brilliant weather). One section of the house is two storey, and one section is one storey. It all sits on an insulated slab on grade. It will have an ERV, and be heated via air source heat pumps. I plan to use air conditioning in the summer only sparingly; otherwise the windows will be open.
We are aiming for R40 walls, and would like to use double wall construction. There have been myriad posts on double wall construction details, with some consensus, but some topics, like moisture control, seem to prompt contradictory opinions and recommendations. At the risk of travelling over a well trodden path, I am interested in comments on our design, which (currently) comprises:
1. Wood siding (hemlock board and batten)
2. 1 x 3 strapping (horizontal and vertical) for rain screen gap
3. Self adhered WRB (Henry Blueskin V160 or equivalent)
4. 2 x 6 24″ o.c. structural wall with dense pack cellulose insulation
5. 2″ foil faced polyiso, taped, as air and vapour barrier
6. 2 x 4 24″ o.c. interior service cavity with fiberglass batt insulation
7. 1/2″ drywall
We are mulling over several options, including:
1. Insulation. My preference is the above hybrid approach. I like cellulose for its moisture performance, the presence of Borax (speculated to mitigate mold at cold sheathing) and its sustainability. The cellulose sub only needs to visit the site once since our contractor does his own fibreglass batts, which can wait until all of the services are in. Alternatively, our contactor says that he can do the exterior cavity in fibreglass batts as well. He is very diligent, so I know his quality control will be solid. Besides losing the benefits of cellulose, are there any other downsides to having fibreglass in both cavities? Or even having fibreglass in the interior cavity?
2. Cold sheathing. This seems to attract a considerable amount of attention, and some excellent research. The objective of the proposed assembly is to facilitate drying to both the exterior and the interior. I do not intend to try to keep the sheathing warm with extra exterior insulation wrap. Thoughts on ways to improve our strategy to enhance this performance are welcome.
3. Integral rigid insulation. I like the idea of a robust and protected air/vapour barrier on the outside of the interior frame. I question whether I need the foil face, or if I could simply use GPS insulation (whose perm rating seems to place it in the Class III category). Also, should it be adhered and sealed to the outside frame or to the inside frame? Or does it matter from a performance standpoint?
In the end, I’d like a cost effective bomb proof wall that is air and water tight, well insulated with climate friendly insulation, and that won’t keep me up all night visualizing impending rot. Any comments on our plans to reach this nirvana state are welcome.
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Replies
john_ferguson,
It sounds like a very good wall.
1. Having the cellulose in that outer-wall sounds like a benefit to me. Just in terms of scheduling: Is the roof insulated with cellulose too? If so the insulation sub might have to come back twice anyway, as the services won't be in the roof when they do the exterior walls.
2. The sheathing should be fine with a rain-screen, and good air-sealing. It would be even safer if that outer-wall was insulated with cellulose to buffer moisture.
3. Canadian codes typically require a vapour-barrier (less that 1 perm) close enough to the inside to prevent condensation. That seems to preclude using GPS, which isn't a problem as the wall can dry in both directions from that layer of polyiso. Practically, that layer is probably best installed on the inside of the outer-wall. Taping and sealing it depends where your primary air-barrier is. Your stack-up didn't mention exterior sheathing. I'm assuming that was an omission? If the sheathing is taped and sealed, that mid-wall foam doesn't have to be.
Thanks for your comments. And yes, I missed the 1/2” taped plywood sheathing under the Blueskin. Good catch.
If you go with Polyiso in your middle portion, I suppose you get a "free" boost to the full R6-7 of polyiso, since you don't need to worry about the cold impacts on it as much.
With that said - does the use of foam between the walls add complication to the assembly vs cellulose? Used Polyiso is fairly cheap by me at least, could be a good way to go.
I am familiar with this wall assembly and feel that it is a solid solution based on your goals. Foil faced Polyiso or GPS with taped seams is important because this pushes your vapour control layer closer to the inner third of the wall (approx 7” in from exterior of a 10.5” wall). I have a project in the works with a very similar wall assembly but with a 2x8 (R30 BIBS / R25 cellulose) exterior load bearing wall and a smart vapour retarder on the interior. An insulated 2x4 (R14 BIBS / R12 cellulose) service cavity wall with studs offset from the 2x8 wall will take the place of the polyiso in terms of thermal bridge reduction. Depending on the insulation selection, achieving an R37-40 wall is doable without any foam. Both assemblies would work well, we just felt that stapling on a membrane and taping seams, seemed a little easier than tapping all of the seams in the polyiso. Personal preference thing really.
Tim, I agree that the polyiso will stay warm, and therefore retain its design R-value. I envisage that the cellulose gets installed in the exterior frame cavity behind netting, and the polyiso is then adhered to the studs after.
You don't need to tape the sheathing if you are doing self adhered WRB. Taping the seams is only needed for house wrap.
Foil faced polyiso is not cheap, faced EPS/GPS is much cheaper and the slight reduction in R value doesn't matter for a thick wall like that. The GPS stocked at the Canadian box stores is about 0.5 perm which works as a vapor barrier.