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Can you flash and batt with the spray foam on the inside?


I have been experiencing analysis paralysis for a while now on a wall for a house addition, currently, it is wood siding, 1.25 inches ventilation (double furring) then 1.25 inches of rockwool (R5), then 1.5 inch of woodfiber board (R4). My plan was to then fill the 2X6 cavity with cellulose and put 1 inch rigid foam insulation on the inside (also acting as the vapour barrier, and preventing condenstion on these odd days when AC is running). Unfortunately, I have issue with finding a way of putting the rigid foam inside given the usual bulge when cellulose gets injected, and all insulation contractor in my area insist on having a poly film installed before injecting the cellulose which makes it a problem with putting the foam afterward, unless I puncture the poly film. I have not yet have the cellulose injected.

I was therefore wondering, has there been any experience done with the flash and batt where the batts are on the outside and the spray foam on the inside within the 2X6 cavity. The cavity would therefore look like this: Sheeting, then 3.5 inches of rockwool batt, then 2 inches of sprayfoam which would go over the 2X6 in the inside to provide a continous air barrier. The strapping would be done with 2X3 on edge to give space to the spray foam to go over the 2X6. It feels like a more natural place for the spray foam where it can act as a vapour retarder and air barrier in the warm side, and can also properly glue the batt in place as it is sprayed after the batt is installed.



Asked by fallwanderer
Posted Jul 11, 2018 3:19 AM ET
Edited Jul 11, 2018 5:53 AM ET



Yes, you can do that.

Answered by Martin Holladay
Posted Jul 11, 2018 5:52 AM ET


You won't be able to apply a full 2" of closed cell foam in that space, since it is impractical to trim it flush with the stud edges after the fact.

If this is in Canada, be sure that the vapor retardency of 1.5" of the foam would still be under 60 ng/(Pa•s•m2) (about 1 US perm). Nearly all closed cell foam would meet the specification at a full 2", whereas some closed cell foam products would still meet the requirement at 1.5", some would not. eg:

Icynene's MDR-C-200 is specified to be 0.9 perms at 1.5”, just squeezing under the limit...

...but MD-C-200-v2 would not, coming in at about 56 ng/(Pa•s•m2) @ 2", which becomes (56 x 2"/1.5"=) 75 ng/(Pa•s•m2) @ 1.5".

There are other closed cell products from other vendors that are significantly more vapor open than MD-C-200-v2, so read the specs carefully and have them handy for making your case when discussing it with code officials.

Answered by Dana Dorsett
Posted Jul 12, 2018 1:52 PM ET

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