Garage to ADU Air Barrier Location
I’ve got a project where we’re converting a garage to an ADU and I’m having trouble deciding exactly where to put the Air Barrier.
Existing Conditions:
– Climate Zone 4c Marine (Portland OR)
– Existing Exterior Wall, 2×4 framing, felt and T1-11 sheathing/siding
– Built in 2003
– Foundation stem wall on footer with uninsulated slab floor, there is a concrete apron around ~ 40% of the perimeter of the building
Proposed:
– Air Barrier and vapor control layer at inside of existing Framing and underside of roof trusses (see image)
– Frame new interior wall inside existing wall (double stud wall?) to avoid furring out entire wall for proud stem wall.
– Insulate wall with 6.5″ Dense pack Cellulose
– Insulate Ceiling with ~14″ of loose fill or more
Reasoning:
– I know this is close to code minimum level of insulation but figured minimizing penetrations in air barrier would be more important than maxing out R Value. If budget remains do a floating floor over some EPS, focusing on air barrier first since it’s buried in the wall.
– I’m always worried about condensation so figured a smart air barrier would allow adequate drying in either direction (in or out, ceiling or wall)
I’m sure there’s a better way and I’m also sure this community knows what it is.
Thank you all.
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Replies
You want the Intello against the drywall. I'd make the drywall the air barrier throughout with the Intello right behind it.
Roofing felt is about 5 perms, the wall shouldn't have trouble drying to the outside.
I'd pad the existing wall out about 3/4" past the concrete stem wall. That way you can run a furring strip over the concrete that's flush with the rest of the wall to give something to attach drywall and baseboard to. I'd put a capillary barrier over the concrete. Then I'd fill the whole cavity to the T1-11 with blow-in insulation. It looks like the stem is about 6" wide so that gives you 6-3/4" of insulation. You could also use batts sized for 2x8 (7-1/4") and squeeze them a bit. I don't think the Intello is necessary if you used faced batts.
If I was feeling particularly diligent I would put pieces of 3/4" foam between the furring strips on the concrete.
It looks like a 2x3 wall in front would get you flush with the concrete, if you then ran a 1x2 strip on each stud all the way floor to ceiling there's your furring. Just use the 2x6 that's there for the bottom plate, rip a piece of treated 2x to bring it flush and nail it on the face.
If you install the ceiling drywall before building the 2x3 front wall you can caulk between the 2x3 top plate and the ceiling drywall, and between the top plate and the wall drywall, and have a really tight junction.
Thank you for the Reply DC
I was considering putting the air barrier right behind the drywall but I'm skeptical of being able to adequately air seal at every outlet box, every light switch, every light... there are so many penetrations in the drywall. I've seen products out there that can make it easier to seal all those. I'm keen to do a blower door on this so I can start learning the practical steps to building air tight... but this may not be the right project for that.
With Regard to the furring strip on the stem wall .... unfortunately where the exterior wall sits on the stem wall is not consistent so it would be hard to keep the wall planar. The 1x2 furring would need to be scribed or shimmed and the studs on the wall above would need to be sistered on the existing stud to predictably keep everything planar.
Although I may be too pedantic... the NAHB Residential Construction Performance Guidelines says Walls should not be more than 1/2" out of line over 32" horizontally. A wall that far out would look wavy.