Hi, We're designing a detached wood working shop and I'm trying to figure out some details regarding the walls and insulation. We're in western Washington, Marine 4 climate zone. Our desire is to use only rigid foam for insulation, either all EPS, or a combination of EPS and PolyISO. My research shows that rigid foam is best used outside the wall, which seems like it would be much easier to construct, as well, important since we're doing most of the work ourselves. So, my current plan is this, inside to out: -Drywall -2x6 stud walls 24" O.C. -5/8" Plywood sheathing -House wrap -2" PolyISO -2" EPS (seams staggered) -2x4 Vertical Furring strips (2X needed for wood siding nails) - 1x6 Horizontal Tongue and groove cedar siding This seems fairly simple and straightforward, with excellent air sealing, and just squeaks by on R-value requirements. I have a couple concerns: 1. I've read in several places, that you can't have open stud walls like that - you need to fill the cavities completely with insulation, or use 'fire blocking' (I'm still not clear on exactly what that looks like, structurally). However, I also read about numerous people that have completely open stud bays in their garage - no drywall or sheathing on the interior at all, with or without insulation.... confusing. If the wall has sheathing on one side, drywall on the other, and top and bottom plates, doesn't that keep fire from getting in and out of each cavity? 2. I've also read the same thing with regards to the rain screen I've described for the siding - that you cannot have an open air space behind your siding like that, but that it needs to be fire blocked. If the rain screen needs to be closed up - ie a block of wood across the entire bottom of it, does that not prevent water from leaving, and significantly restrict ventilation, thus defeating the purpose of the rain screen? Is this something that differs widely from one jurisdiction to the next? I am having a really hard time finding anyone who has built using only rigid foam on the outside of the wall - only those who want to ADD it to some kind of fiber insulation between the studs. Thanks in advance!

Yes. This sort of thing is possible to do.

Here's such an example, done in Austin, TX.
https://youtu.be/hTwq-qUnr9I

And yes, you STILL need fire blocking in walls between floors (otherwise your stud bays become chimneys in a fire).

Not sure who says you can't have an airgap behind your siding. That's incorrect. You need that if you want your wall to actually dry out. Note that it doesn't have to be a simple air gap. They make mesh backers that can meet your needs.

Also, the reason you want something on the bottom is to prevent insect migration through the wall.
It can be vented however.

There is no reason to change up the foam types in your temperate region (or in any, really), which only complicates the assembly. To hit code-min performance on a U-factor basis with a polyiso-only assembly would only take 2.5" of continous sheathing. But 4" of foam isn't difficult to assemble, and two layers with the seams of each layer taped, with the seams staggered works fine, and enhances air tightness. See Table N1102.1.4

https://codes.iccsafe.org/content/IRC2018/chapter-11-re-energy-efficiency

It only needs to duck under U0.060, which is the same as (1/U=) R16.7 "whole wall".



A continuous 2.5" of cheap reclaimed 2lb roofing foam at R5.7/inch is worth R14.25

the 5/8" CDX is good for R0.77

the interior side air film is R0.68

the exterior side air film is R0.17.

With a rainscreen gap of 1/4" or more between the foam and siding (highly recommended in your climate- a 3/8" gap is code- minimum in western B.C.) adds another R0.68.

the 3/4" cedar is good for R1.06.



That adds up to R17.61 whole wall or (1/R= ) U0.057, even with completely open stud bays (no interior side wallboard, which would add additional air films)

With R6/inch foil faced polyiso it could squeak by with just 2", since the shiny foil facer adjacent to the rainscreen gap adds another ~R1 or so to the assembly.

There are multiple vendors of reclaimed roofing foam in WA, priced at 1/4-1/3 the cost of virgin-stock goods, which really takes the sting out of exterior foam projects. An outfit in Kent use to adverise on Craigslist regularly. Run this search every week or so and you'll find some (or an even better one-off deal):

https://seattle.craigslist.org/search/sss?query=rigid+insulation


See also:

https://www.greenbuildingadvisor.com/article/getting-insulation-out-of-your-walls-and-ceilings