GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Spray foam and abating moisture issues in Zone 4

Matthew Wahlrab | Posted in PassivHaus on

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.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    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.

  2. D Dorsett | | #2

    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:

    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: ) 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.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |