Faced EPS as vapor barrier in double stud wall
Here’s a proposed wall assembly from the outside in. Climate zone 6
Wood siding
1x furring rain screen
SIGA Majvest or equal WRB
1/2 CDX plywood
2×4 @ 16″ o.c. (structural) w/ Timber HP batts
2″ rigid faced EPS w/ foil taped seams as air barrier
2×4 @ 16″ o.c. (non structural) w/ Timber HP batts
1/2″ GWB standard paint
All the typical plywood bucks at openings and top plate to tie the air barrier and two walls together.
I was hoping to use 2″ rigid wood fiber in between the walls but Timber HP rigid is not available yet. Also, I wouldn’t have an easy way to secure the air barrier to the rigid before I build the interior stud wall since there is nothing to staple or tape to.
The goal is to have the air barrier protected by the interior stud wall which serves as the service cavity but will also be insulated with wood fiber batts. Remodeling in the future can be done without damaging the air barrier.
Any potential issues with using the faced EPS and taping seams in the middle of the wall assembly as the air barrier?
I also considered salvaged kraft faced polyiso with taped seams. Would that be considered an adequate air barrier?
Any other low global warming potential rigid products on the market that might be better than EPS?
Thanks,
Noah
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Replies
The dewpoint inside a double-stud wall (and all walls, but more apparent with thick walls) moves with changes in temperature. I would not want a vapor barrier in the middle of the wall. When it's 5°F outside and 70°F inside, typical design temperatures in my CZ6 location, the center of the wall will be around 37.5°, cold enough for condensation. It's ok to have a vapor retarder in the center--something that slows down moisture--but I would never introduce something that completely blocks moisture movement.
A time-tested approach is to use the exterior sheathing or WRB as the primary air control layer, with the bays filled with dense-packed cellulose. Why do you want to stray from that simple approach?
Thanks Michael. We are trying to allow for a relatively serviceable space within the interior 2x4 stud bays. Dense pack cellulose for the entire thickness of the double stud wall would require us to install yet another layer of 2x furring for the service cavity. I'm trying to future proof the wall to some degree for minor alterations further down the road while also trying to protect the air barrier behind the finish to minimize intentional or accidental leaky penetrations in the future.
I just learned that Rockwool now has their SmartRock product which looks like it is just their comfortboard with Intello pre-applied to it. Seems like this would work well for what I'm looking to do. Anyone have any experience with that product?
https://www.rockwool.com/north-america/about-us/news/2025/introducing-rockwool-smartrock-interior-insulated-mass-walls/
Got it. I have cross-strapped the interior with 2-bys on a few occasions, which is enough for electrical changes, but not much else. This one's a Passive House.
I have seen that product but I have not used it, since I only use Rockwool when cellulose or wood fiber won't work.