Replies

1. Expert Member
   BILL WICHERS | Sep 23, 2021 09:54pm | #1

   You want that interior wall to be vapor open. Unless you go really overboard with paint, it should stay vapor open enough to be OK.

   Note that by adding additional insulation in your 2x4 wall, you're changing the R ratio of the interior batts to the exterior rigid foam. This will result in colder exterior sheathing in the winter, and more potential for moisture accumulation within the wall. My first thought here is that a layer of vapor retarder (MemBrain, etc.) on the interior side of the exterior 2x6 wall, facing the gap between the 2x6 and 2x4 walls, might help here. This vapor retarder would help to limit moisture getting into the colder, exterior 2x6 wall. I haven't ever tried this, and will be interested if anyone else here has some ideas.

   Bill

2. 
   smokey059 | Sep 23, 2021 10:30pm | #2

   Maybe I wasn't clear enough in my post but the foil faced polyiso is on the interior of the exterior 2x6 wall. Since the foil face is a vapor barrier i assumed this should stop any moisture from the interior moving outward into the exterior wall assembly , but with the 2x4 service cavity wall also to the interior of the foil face I was wondering where any mosture would go once it passed thru the interior drywall on the 2x4 service cavity wall.
   If it can't move outwards because of the foil face polyiso vapor barrier and it can't come back inwards because of the drywall paint and primer it seems it would be trapped in the service cavity.
   If the paint and drywall primer won't trap any mosture that gets into the service cavity from moving back inwards my thought was the service cavity can dry to the interior. What type of paint would be best for that to happen? Thanks

1. Expert Member
   BILL WICHERS | Sep 24, 2021 08:28am | #3

   Sorry, my default mode of thinking is "EXTERIOR rigid foam" and I completely missed that you mentioned your polyiso is on the INTERIOR side of the EXTERIOR wall :-)

   In your case, you would want a vapor open exterior siding or rain screen (rain screen is best), and you want vapor open on the inside too. You need drying towards both sides because you have a vapor barrier in the middle. The more "dangerous in winter" side is the interior wall, but with all that insulation, you shouldn't really have any issues with condensation on the polyiso in the middle of the sandwich. The exterior side would be an issue in the summer, but in your climate zone I'd expect that to be low risk too as long as you don't have a vapor barrier on the exterior side of that 2x6 wall.

   I wouldn't worry too much about paint and primer. Normal paint and primer doesn't create a vapor barrier. I would avoid vinyl wall paper though. I think you have a pretty safe assembly here. Do take care to air seal both the 2x6 and the 2x4 walls though.

   Bill

3. 
   smokey059 | Sep 24, 2021 11:23am | #4

   Thanks for the input. I'll just prime and paint and air seal the drywall and not worry about it. Thanks again

4. 
   smokey059 | Sep 24, 2021 11:24am | #5

   Thanks for the input. I'll just prime and paint and air seal the drywall and not worry about it.