Advice about a wall assembly
I’m in the process of adding a second story to an existing one story home in climate zone 6A (southern Wisconsin.) The first step in this project was to strip the exterior of the existing first floor, remove the fiberglass from the walls, and resheathe. We then dense packed the walls with cellulose and are in the process of adding an exterior layer of rigid foam.
The plan for the wall assembly is that (from inside out) the first floor would be 1″ plaster, 2×4 wall with dense pack cellulose, 7/16 OSB sheathing, 1-2″ polyiso, WRB, 3/4 rainscreen, and smartside siding. The second floor would have 1/2 drywall over 2×6 wet blown cellulose walls but everything else would be the same. There is no vapor barrier on the first floor, and the second story will likely have something like membrain installed on the interior for the peace of mind of our building inspector.
My two questions therefore are whether foil faced polyiso would be an issue in this assembly (whether being able to dry to the interior will be enough?) And as a follow-up, I’m not sure what sort of WRB to use in this assembly. The two products I’ve considered most are Blueskin VP100 and traditional tyvek housewrap. The appeal of blueskin being that it could also be used as a rather effective air barrier, but at a higher cost and it would be the installers first project using it. The other consideration is that if we install the foil faced polyiso then the vapor permeability of the WRB would be essentially moot; We would only be using it for a convenient way to tie in our window flashings and potentially as an air barrier.
Edit: One other thought is would it be reasonable to turn the polyiso into our air barrier and WRB by taping all the seams? Additionally, would not covering the foil faced polyiso improve thermal performance? My understanding of radiant barriers is that it would only make a difference on reducing heat gain in the summer rather than the more important reduction of heat loss in the winter. The exterior in that scenario would be sheathing, taped polyiso (probably 2×1″ layer with staggered seams), rainscreen, smartside cladding.
Advice or questions about any aspect of this would be appreciated, and I’ll do my best to respond quickly!
Darron
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Replies
I'd use table 2B here with a conservatively low, cold temp, very long term R value for the polyiso (R4.5/inch?)
That paper seems to say that:
1) Ratio of R-value of exterior insulation (polyiso - R4.5 or R9) to cavity insulation (cellulose - ~R14, ~R20) needs to be .35 with with just latex paint, or .2 with something like membrain.
This seems to indicate that with a class II vapor barrier I would be within their spec to just use 1" of foam, but it's probably a better idea to use two? This is also comes out a bit more lax compared to some of the things I've read on here - I seem to recall ~11 being the recommended minimum exterior R-value for a 2x6 wall?
2) Section 4.2.3 recommends that I include an air barrier layer both externally and internally.
It seems to me this would probably be easiest achieved with the Blueskin VP100 and a careful membrain/ drywall installation.
One other thought is would it be reasonable to turn the polyiso into our air barrier by taping all the seams? Additionally, would not covering the foil faced polyiso improve thermal performance? The exterior in that scenario would be sheathing, taped polyiso (probably 2x1" layer with staggered seams), rainscreen, smartside cladding.