Greetings.
I’m converting my 1920s bungalow attic for additional living space. I may be a little ahead of myself as I try to figure out a way to add more insulation to the ceiling. I have about 9′ from floor to ridge and 5′ knee walls.
The rafters are 2×6. I have baffle vents between all rafters that run from ridge vent to soffit. I have no collar ties. I’ve already installed R-19 faced batts over the baffle vents, but the more I read, I realize I need more. I would like to use rigid foam and realize it can be installed perpendicular to the rafters. My first question is how can I install strapping material to which I can add the insulation and insure strength to secure drywall? I would like to avoid having to scab 2x material as it would require removing insulation I’ve already installed.
My second question has to do with the attic dormer. There is no way to vent the valley, so do I bother installing baffle vents in between the rafters in the dormer, above it and on either side of it?
I look forward to your suggestions.
Replies
Since no one else has answered this, I will.
Personally, I'd think you would have trouble installing the rigid foam in an air tight fashion. Around here (NC) rigid foam is readily available in 1/2", 1" and 2", and the thickness your strapping would pretty much have to match. If you can find 1.5" thick rigid foam, your idea might work, but that would only be ~R-7.5 which is somewhat minimal, although it would probably be fine if you did a perfect job of installing it. Check with Mike Smith. I think maybe he did some installs with rigid foam on the inside... Strap the ceiling with 2x4s ripped in half (1.5x~1.75), 16" OC perpendicular to the rafters. Take a razor knife and cut squares of R-19' vapor barrier between the strapping and remove the paper. You don't want a vapor barrier here, and the foam is a vapor barrier. Carefully cut your foam into strips, and install it with roofing nails. You may need use some minimally expanding foam as you are installing the rigid foam to make it fit tight. There really should not be an air gap between the fiberglass and the rigid foam. I hope you face-stapled the fiberglass batts, when you first installed them.
Another idea is to rip out the fiberglass and have spray foam blown in there. Might cost around $4 a sq-ft or more though. Then you could skip the strapping.
Most people here at BT don't like fiberglass insulation a lot though, and I'll probably get flamed for saying it, but personally, I think it's OK, just so you live in an area that doesn't get real cold. The problems with fiberglass is 1) it starts loosing it's R-value when it gets around 10 degrees F (+-). and 2) it's not that great at stopping air infiltration.
Another thought - you said you installed vent baffles. This is good, but does your 1920s bungalow have a continuous ridge vent?
BTW - what area do you live in?