Flash and batt and Vapor barrier / retarder
![](https://secure.gravatar.com/avatar/0affbd470c6311537da8c000188e61e5?s=52&d=https%3A%2F%2Fimages.greenbuildingadvisor.com%2Fapp%2Favatars%2F200x200%2Fk.png&r=g)
Hello GBA,
I have a builder who is building in Zone 5 / 6 line (NH), 2 x 6 w/ a “flash and batt” wall insulation strategy. I haven’t been a fan of the F&B strategy in the past, and not sure I will become one w/ this job either.
2″ CC, R6.5?? sprayed 1st, w/ an R15 high density f’glass batt going in 2nd. The builders question is: What, if any, vapor barrier or retarder s/b used?
My personal preference is to not use a vapor barrier at all, but rely on a great air barrier to reduce infiltration through the wall AND inform the homeowners to keep RH low in winter (30 -40%). Probably easier said than done. A vapor retarder like Membrain or similar might be OK. No additional VB or VR is also OK w/ me, just some latex paint.
There will be A/C in this building.
What’s your opinion on vapor barriers and/or retarders in this F&B wall strategy?
THX,
KH
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
![](https://www.greenbuildingadvisor.com/app/themes/greenbuildingadvisor/dist/img/modules/detail-library/thumb1.png)
![](https://www.greenbuildingadvisor.com/app/themes/greenbuildingadvisor/dist/img/modules/detail-library/thumb2.png)
Replies
Kevin,
You've got it right. With a foam layer toward the exterior of the wall, you don't want an interior vapor barrier.
Kevin, your summation is correct. Advise against humidifiers. Membrain especially not needed with 2" of CC. All inner wall moisture will have to come and go past your drywall which is fine. Install drywall using an airtight method.
Spray in Spyder or cellulose would be a good upgrade over batts.
One exception for me is a shower wall. Pressure treated framing, filled full with CC only, and use Ditra over paperless drywall for a bullet proof system. Schluter products are worth their cost. I now use their products on bath floors and shower walls and expect to never find wet and rot and carpenter ants at these points when using Schluter goodies and a few pieces of pressure treated.
Thank you Dan @ AJ builders and Martin.
I appreciate the feedback. It helps to know from professionals that my concerns are on the right track before we close that wall system up for the next 50 yrs or so. With good advice like that for the builder, Hopefully it'll stay closed up, dry and functional for the next 100 yrs or more.
KH