Replies

1. GBA Editor
   Martin Holladay | Jan 03, 2015 05:12pm | #1

   Roy,
   You can either go with a double-stud wall (if you do, choose plywood sheathing, not OSB, and include a ventilated rainscreen gap) or a 2x4 or 2x6 wall with thick exterior rigid foam over the OSB or plywood sheathing.

   If you choose the latter option, follow the recommendations in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

   By the way, GBA readers will have an easier time offering you advice if you tell us what your climate zone is.

2. 
   roygoodwin | Jan 03, 2015 06:59pm | #2

   Ooops, meant to include that. NC Mountains climate zone 4

3. 
   Dana1 | Jan 04, 2015 11:31am | #3

   Putting 5.5" of cc foam between studs is a waste of good foam, since it adds no more than R2 "whole-wall" performance over what you would get with R21 fiberglass or R23 rock wool or open cell foam, due to the severe thermal bridging of the R6 ish framing over 20-25% of the total wall area. It's better to save the high-R/inch foam budget for continuous insulation that thermally breaks the framing fraction as well as the center-cavity, and use cheaper stuff as cavity fill. But the R3.5/inch stuff is open cell, not closed cell. Closed cell polyurethane is typically R6-R7/inch.

   In practical terms you can't really install more than about 5" of closed cell foam in a 2x6 cavity anyway, since it's not readily trimmable where it projects proud of the stud edges, whereas with open cell you can trim it flush. There are a few foam-Michaelangelos who might be able to deliver 5.25" average, but those folks are rare. The lower depth also increases the thermal bridging, due to the shorter path through the framing.

   In performance terms you're looking at whole-wall performance of about R17 for ccSPF studwall (if perfectly 5.5"), plus R6 for the ZIP-R, for an optimistic total of R23 for the whole assembly.

   With R3.5/inch open cell it would be about R20-R21. With the open-cell cavity fill you add 2" of rigid rock wool to the exterior (which would still allow the ZIP-R plenty of drying capacity) you'd be somewhere between R28-R30 whole-wall performance, and the total cost would probably come in well under a closed cell foam non-solution sans-outsulation. At 30-40 perms the vapor permeance of rigid rock wool is higher than most housewraps, and will not create a moisture trap for the OSB the way exterior foam might.

   https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/installing-roxul-mineral-wool-exterior-walls

   A 2" layer of UNFACED Type-I EPS (1lb per cubic foot nominal density) would offer similar thermal performance without severely restricting outward drying, but low-density EPS will shrink over time creating gaps. With Type-I EPS @ 2" you'd be looking at a vapor permeance between 2-2.5 perms, which is about half as permeable as interior latex paint- not super-permeable, but permeable enough that it's not a moisture trap (but still not as resilient as a rigid rock wool solution.) Other rigid foams or higher density EPS will be much lower permeance, and not recommended. With Type-I EPS you can go with as much as 3" (R11.5-R12) if you like, but not much beyond that.

4. 
   lipton_7 | Jan 23, 2022 03:25pm | #4

   Hey Martin,

   I know this is a 7 year old question, however, I already have purchased materials (this is before I consulted with GBA), I live in midlands, South Carolina - I have 0.5” thick Zip, I have a ton of XPS 1-1/2” thick rigid foam board and even more R-16 rockwool. I’m trying to avoid a rowlock of brick for an addition (the rest of the house has no rowlock) and getting outside of the 3.5” of exterior brick with my siding. My current cake layer solution based on what I have purchased and believe I saw in a document on GBA (but maybe I misinterpreted) was:
   2 x 4 framing
   0.5” Zip tape and liquid application
   1-1/2” poly XPS rigid foam
   1-1/2” furring strips
   cement siding

   What would be your advise here?

   Thanks