Replies

1. Expert Member
   Zephyr7|Nov 22, 2020 09:36pm|#1

   Don't waste money on DIY spray foam for walls. You will likely end up with BETTER final R value using mineral wool batts here. You can get standard size mineral wool batts made to fit 2x6 framed walls that are rated R23. 3" of even about the best spray foam is more like R21, and I doubt the DIY kits would hit above R18 at best. The DIY kits are NOT the most advanced spray foam material out there since they have to be made to work in suboptimal conditions. The application equipment the pros have allow more consistent applications with fancier products.

   I would seriously reconsider your desire to use spray foam. Get some canned (one part) foam, some urethane caulk, and air seal with those. Finish up with mineral wool batts and a smart vapor retarder on the interior. You'll get a good final wall that way if you're careful, and it will cost a LOT less than the DIY spray foam kits. To put things in perspective, the 620 board foot DIY spray foam kit (the usually "big" one) will only get about 19 stud cavities -- just over 25 linear feet of 9 foot high wall -- with a 3" layer, and that assumes you get perfect application which is nearly impossible to do. Most people get less than rated coverage from those spray foam kits.

   I'm not sure what you mean by the "cavity that will exist between spray foam and rigid insulation on the joists"? Are you planning on using exterior rigid foam here? If you are, then you really don't want to waste money on DIY spray foam kits in the walls. If you mean you want to use the spray foam on a cathedral ceiling, it might make more sense here, but you might be able to build a vented assembly that would work just as well, if not better, for less money.

   Bill

2. 
   DCContrarian|Nov 22, 2020 10:51pm|#2

   I did something similar, but in the opposite order -- cut polyiso foam board to fit between the studs, and then sealed with spray foam.

1. 
   tr24|Nov 22, 2020 11:04pm|#3

   Thank DCC, i was considering that but given I'm trying to create and air tight box (16x26 off grid cabin) ) with small slope cathedral ceiling, I was worried no air tight seal would be achieved given the rigid would not be always perfectly tight on the sheathing. Does the foam on top still achieve the seal?. I want the air tight seal so to as avoid venting the roof? A family member has severe respiratory ailments and being back in bush it does get damp when not in the mid of summer and being a weekend warrior cabin there is no air conditioning and only wood stove in winter.

1. 
   DCContrarian|Nov 22, 2020 11:19pm|#5

   I haven't done a blower door test so I can't answer authoritatively, but it seems airtight to me.

   The foam is not tight to the sheathing. The nails from the shingles keep it away. But it is tight to the framing.

3. 
   tr24|Nov 22, 2020 11:17pm|#4

   Thanks Bill.
   As I mentioned a moment ago it's air quality through air tight seal that is goal #1. R factor for the half dozen weekends in central Ontario winters is priority number 2. Don't expect home like toasty conditions but can't have any issues with condensation etc.
   What I meant by cavity that I had the question about is what would occur in both walls and ceiling if I spray foamed 2 inches for air seal and then put rigid over top of those spaces by attaching rigid to the 2x6 walls and 2x10 ceiling joists. That would create the cavity
   I guess the new questions are:
   1) can i install rigid on ends of the 2x6 and 2x10 over the 2" spray foam
   2) can i install rigid against ceiling sheathing and wall sheathing and the spray foam 2 " and create the air tight seal?
   3) I have already spray foamed 3 inches under the sub floor creating a seal and if I spray foam ceiling to create seal there can i install the wool batts with vapour barrier and achieve the air tight box im looking for???

   I do appreciate the help. Thanks

4. 
   tr24|Nov 23, 2020 12:28am|#6

   Also, I get below cost on dyi 2 component spray foam, so for consideration take the terribly high retail cost of that out of equation. Thanks