Your thoughts on a “pretty good wall”?
Hello All:
Long time homebuilder here, having built many, many D- code compliant homes for customers who preferred to spend the $$ on countertops and more square footage. I get that.
I’m planning a final home with main floor living for us empty nesters (blink of an eye!) with an eye toward durability and energy efficiency.
In a previous life, I managed 50+ high performance homes designed by Dr. Joe in conjunction with the Building America program. Met him once and he is a funny guy – he was shocked at how many of his details were removed by purchasing so the big dogs could get nice bonus $$. I learned a hell of a lot – don’t let the walls get wet on EITHER side. I think about those homeowners who had to clean up what that builder skimped on. End rant.
Anyway – I am planning an ICF walkout foundation with 2″ foam in and out.
2×4 stud walls 16″ OC, 1/2″ plywood sheathing air sealed and 2″ iso board continuous with 3/4″ treated plywood strips to drain and attach my siding.
Inside this wall would be roughly 1.5″ closed cell spray foam, air space and drywall. NO interior plastic for me here in zone 6, thank you very much.
This should give me about an R20.5 wall
My concerns are flame/ smoke spread.
Looking for any constructive feedback, particularly regarding fire inside and out. Would I be better off with a foil faced foam board inside, sealing the perimeter with a gun to reduce fire risk? Spraying the foam would probably give me a lower ACH number.
2nd: foam outside cause me no trouble unless a spark managed to get under the rainscreen. A mineral fiber board probably isn’t in the cards – how do I reduce fire risk outside? Foil faced foam again, and perhaps fiber-cement siding? Perhaps cover it with roofing felt before the 1x strapping?
Sorry for such a log 1st post.
Cool site- I subscribed this week.
Thanks in advance for your thoughts.
Kent
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Replies
Are you in a wildfire-prone area (or otherwise more concerned than normal with fire risk)?
I know the idea of sparks getting behind a rainscreen comes up with some frequency, but I don't think there's any data to show that it is any more or less likely to be a fire risk than any other standard assembly. As you note, Rockwool Comfortboard80 (or something similar) would mitigate this risk, as it's basically made of rocks (fireproof), but it comes at a premium price point.
If you're spray foaming the interior and covering with drywall, pretty sure that gets you the fire resistance you need. Go with 5/8" type X for more insurance. If you really wanted to go for it, you can spray CCSF with an intumescent coating that drastically reduces the risk of flame spread (I think - not certain - that this can even be used in place of gyp in some cases).
Thanks for the response, Paul.
I feel much better about my proposed assembly already.
I am not in a wildlfire area - I think it has been on my mind reading endless stories about LA burning to the ground. Perhaps all I need to do is find a metal mesh or screen for the exit point of the rainscreen that reduces entry of bugs and anything else floating in the wind?
Probably a dozen articles on that very item here somewhere...
I feel like I am drinking from a firehose this past week trying to read everything I can and get up to speed on the current best practices!