Replies

1. 
   Jon_R | Jul 17, 2019 09:40pm | #1

   > MINIMUM R value outside the sheathing

   More like that such a rule (without additional qualifiers) never existed, neither in code nor from a "needed for a good wall" building science standpoint. If you understood that from somewhere, you should get the author to improve the writing.

   On the other hand, don't think that you can use any amount of any kind of foam and have it work - perms become critically important if you are going to use less than Class III enabling amounts of external foam. See here.

2. 
   calum_wilde | Jul 17, 2019 10:03pm | #2

   I think this is what you're looking for.

   https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing

3. GBA Editor
   Martin Holladay | Jul 18, 2019 04:51am | #3

   Actually, the best article on this question is a different one. Here is a link to the relevant article: "Rethinking the Rules on Minimum Foam Thickness."

4. 
   kenorakq | Jul 18, 2019 03:21pm | #4

   I "thought" the the rule was just that, "a rule" ...the written in stone kind... not the ..of thumb kind. I will stop worrying about the condo based on this info.

   As an associated point and an even more personal point I have been standing on my head for a couple of years (too long) planning on adding 4" of foam (EPS) to the exterior of my home expansion and renovation in Kenora NW Ontario Canada (zone 7).

   I have been torn between knowing that I need to improve my exterior wall design to include some exterior foam and learning that *any* really meant "a lot".

   In my case 4 inches.

   I'm already committed to the foam but... now have a clear(er) understanding of what's nice to have vs need to have.

   I'm going to inventory my foam stash (yup,,I've got a couple of portable garages full of a variety of reclaimed foam, both EPS and XPS in an assortment of thicknesses) and see if I can cover the house with 2 1/2" or 3" foam instead of 4".

   While I know the thinner foam compromises the thermal performance, a reduction to 2.5 or 3 makes installing and flashing details around doors and windows easier and as I'm learning sometimes good enough is good enuf!

   I would like to see an article that speaks to this...opps...there is one ... reference made by Martin (thanks)

5. 
   kenorakq | Jul 21, 2019 09:31am | #5

   I've got 270 1 1/2 " by 2x8 XPS SM roofmate and 400ish EPS 4x4 slabs in a variety of thicknesses... most (250) are 1 1/2" with the rest being a near even split of 2" and 2 1/2".

   I am considering changing my exterior insulation to 3" from 4".

   I have enough 1 1/2" XPS to do two layers... this would simplify my framing and trim and (I now believe) won't significantly compromise the durability of the wall.The use of the XPS will made a firmer nailing surface as well since its more dense... the EPS will go under the concrete floor. Looks like I'll have some to spare.

   Thanks for clearing up the misconception about exterior foam.

6. 
   charlie_sullivan | Jul 21, 2019 10:03am | #6

   My biggest concern about this is actually the use of XPS, which, in North America, is made with an extremely high global warming potential blowing agent. The best alternative would be graphite infused EPS (e.g. "neopor" brand), but regular EPS could also be used. As a bonus, either of those can dry to the exterior a little more than XPS.

1. 
   Jon_R | Jul 21, 2019 10:21am | #7

   > either of those can dry to the exterior a little more than XPS

   About 4x as much, which can make a big difference.

   Excellent interior side air sealing always helps, more so as sheathing spends more hours below condensation temperatures.

7. Expert Member
   Peter Engle | Jul 21, 2019 10:59am | #8

   Charlie,

   Tim mentions above in #4 that the foam is reclaimed and already on hand. It's GWP was spent by the original user.

8. 
   charlie_sullivan | Jul 21, 2019 02:00pm | #9

   Thanks Peter--I didn't notice that the name of that commenter was the same as the original questioner.