Choosing a wall assembly: discussion
I really tried to not ask this question since I’m aware that it has been discussed a ton, and I have read a ton on the subject. And even if I’ve accepted the fact that there’s no clear answer, I still have to choose a wall assembly – double wall, 2×6+4″foam, SIP, or some hybrid. Around R40 (I have 12″ to work with)
The top priority here is actually budget and ease of construction/quality control.
I’ve gone through many articles from BSC and GBA amongst many other case studies. This is how my thought process went:
At first most sources seem to suggest the most cost effective way to do it is double stud wall. That works with me too but my exterior wall would be the structural one. Just as I got comfortable with the concept, I start reading about all the “potential” moisture issues. And then I see the “ideal” double stud wall with interior wall sheathing (vapor) AND exterior (structural) wall sheathing. And since I’m framing this outside-in, I didn’t really like the need for the interior OSB layer from a construction sequencing perspective. I then see that people spray CC foam on the inside of the exterior sheathing, but I didn’t like that because it adds a different type of insulation in the form of foam adding complexity and cost, so I might as well go exterior foam sheathing.
And then I see that a lot of people are saying that exterior foam is the best way to go in terms of thermal, air leakage and moisture control anyway, and everything else is inferior in their own way. I was concerned with the extra complexity of windows, doors and siding installations, but maybe I am over thinking it – 2×6+foam seem to be the standard amongst contractors anyway. But I thought it costs more (foam is expensive).
Then I read conflicting reports – “practical residential wall systems – R30 and beyond” (https://c.ymcdn.com/sites/www.nibs.org/resource/resmgr/BEST/BEST2_017_EE6-5.pdf) show a generally higher cost for 2×6+foam compared to double walls (my calculations show 1.2-1.8 $/sqft more than double wall), while Moisture management for high R-value walls” (http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/moisture_management_high_rwalls.pdf) suggest that 2×6+foam is the cheapest way to go. (1.4 $/sqft LESS).
At least one thing they relatively agree on – that SIP incremental cost as compared to their reference wall of 2×6+batts is about the same in both articles.
In terms of cost 12″ SIPs don’t seem like a bad option either compared to the other 2, and the extra speed is money, but then there are horror stories about moisture issues at the seams.
So what am I to think!? Whats the most simple, budget friendly, trouble free way to get close to R40? Of course I take these cost estimates with a lot of salt, but I thought at least they would agree with each other in relative terms and trends. Which doesn’t seem to be the case.
background – House is simple rectangle, 2 floors, 4 corners (65’x26′) supported by 2 concrete basement foundation boxes (2 of 22×22, one box as garage, other is movie room+entrance) so 3 floors total. Mono pitch shed roof. Climate zone 5 @ 6000′. See attached illustration.
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