Spray foam and abating moisture issues in Zone 4
I am remodeling a home outside of Seattle, WA. I plan on spray foaming the existing 2×4 exterior walls with closed cell spray foam (looking for R-24) and plan to frame out a new interior wall (probably 2×3).
I’d like to install a 2″ polyisocyanurate board in the gap between my existing exterior wall and new interior wall to add a continuous R-13 across the faces of my exterior wall studs. The polyisocyanurate will be caulked to the studs, to each other, and joints will be taped.
I am concerned about moisture collecting on the interior of my wall. Any tips or suggestions on managing moisture entering the interior of the wall? I am eager to keep my home air tight, insulated, kill sound from a busy nearby street AND keep from growing a nice mold farm inside my wall.
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Matthew,
If you install closed-cell spray foam in the stud bays, and a continuous layer of polyiso on the interior of the studs, your wall sheathing and studs will probably stay dry -- as long as you have good exterior flashing and water-management details. This type of wall can only dry to the exterior, so it would benefit from a ventilated rainscreen gap.
That said, there are lots of ways to build a wall. It sounds like you want to build a double-stud wall -- is that correct? Were you planning to insulate between the studs of the new 2x3 wall? Most double-stud walls are insulated with dense-packed cellulose, not spray foam. (Many green builders try to limit their use of closed-cell spray foam because most brands of closed-cell spray foam use a blowing agent with a high global warming potential.)
For more information, see How to Design a Wall.
Closed cell foam installed in 2x4 framing 16" o.c. is a waste of good foam (at a high envrionmental & financial cost), due to the thermal bridging of the 25% frmaing fraction. Even though you would have a center-cavity R north of 20, after factoring in the thermal bridging it only adds about R1 to the "whole wall-R" perforamance beyond the same wall with ~R13 half-pound density open cell foam that uses 1/4 the polymer, and water as the blowing agent instead of climate-damaging HFC245fa (with a global warming potential of about 1000x CO2).
Adding a mere 1/4" to your polyiso thickness would exceed the performance gains of 2lb foam over 0.5lb foam cavity fill, and at a dramatically lower environmental hit. Using 1lb polyiso (rather than 1.5-2lb roofing polyiso) uses half the polymer per R of 2lb closed cell foam, and it's blown with pentane, which has a GWP of about 7x CO2.
With 2" polyiso (all facer types) in the middle of the assembly, the exterior studs & sheathing will have to dry toward the exterior, and the interior needs to dry toward the interior. The interiror studs will not have an issue- there is PLENTY of exterior R for keeping those materials above the ~45F interior air dew point in that climate. But the siding type , roof overhangs, and flashing details will determine how dry the structural sheathing remains.
In a US climate 4 location it's possible to hit Net Zero Energy with a PV array that fits on the house with whole-wall performace of about R20- R25. See Table 2, p.10 of this document:
https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf
In 2010 when that was written better class ductless heat pumps had an HSPF of about 10- now they're about 12-13, and high-output photovoltaic solar (PV) efficiencies were around 15%, now running about 20%. The price of rooftop PV was also about 2-2.5x what it cost in 2016. A typical 2x4/R13 wall with typical siding & interior finish options comes in at about R10 whole-wall. Adding a continuous R13 (2" of polyiso) brings it well north of R20 even without the interior studwall. Adding an 2x3/R9 studwall inside the 2" polyiso brings it north of R30 whole-wall. At some point it stops making financial sense compared to installing rooftop PV (even in foggy-dew Seattle) or putting the money into other things such as foundation insulation, etc, to bring the house more in line with the zone 3 or 4 prescriptives in Table 2. of that document rather than going for an ever-fatter wall. In that claimte my own inclination would be open cell foam (or 3lbs+ density cellulose) in the 2x4s and 2" of polyiso, gluing & long-screwing the gypsum directly to the polyiso, skipping the interior studwall. YMMV.
In many locations there are vendors of used roofing polyiso (usually 2lb density, R5.5/inch) at a fraction of the cost of virgin stock goods. In my area reclaimed 4x8 sheets of 2" thick fiber-faced roofing iso runs $8-12/sheet. (and apparently about the same per square foot in your area too: https://seattle.craigslist.org/sno/mat/5937828295.html ) Reclaimed/reused foam (any type) is by far greener than any virgin stock, since the environmental hit has already been taken, and re-using it is just piling on to the benefit side of the cost-benefit equation.