strap rafters before foam insulation?
I’m having closed cell foam sprayed between the rafters of my roof. I was going to add strapping under the rafters for drywall before the insulation goes in but I have a nagging feeling I’m making a mistake.
There are reasons to add the strapping first. The rafters are only 2×6 so the additional 3/4 in of foam would help. They could spray over the rafters to reduce thermal bridging a little. Finally there is already some strapping in place that can’t be removed so that’s happening in a few spots whether I like it or not.
There are some advantages to having the edge of the rafters be drywall ready instead. Mostly that the strapping isn’t locked into the foam so I can move it or add more if I need to, like if I miss some backing for drywall. I could add rigid foam between the straps to reduce thermal bridging instead. Finally I don’t have to scramble to get everything flat before the foam install.
Any advice on which way is better?
Replies
I'm guessing your installer might have an opinion. I'm wondering why you plan to use strapping anyway. If the spray foam comes level with the rafters instead of strapping it then I would put up a layer of 1 inch foam (3/4 is hard to find but would be a better fit against the strapping already in place) over the whole ceiling. Fitting it between strapping would be tedious and pointless. You can put the drywall right over the foam and screw into the rafters. That way your thermal break will be uninterrupted and the strapping step is eliminated. You can rip some quarter inch material to bring the existing strapping out if you use 1 inch foam, and add foam between those pieces of strapping.
https://www.finehomebuilding.com/2017/07/10/closed-cell-foam-studs-waste
Above written for walls, not ceilings, but some of the details seem to apply (like closed cell density making it difficult to trim exactly)
I don't mean to say strapping is unnecessary in all situations. I'm assuming your rafters are 24" on center, so you could want strapping just to be sure the drywall doesn't sag, say 16" on center. There are ways to avoid it. Using lightweight drywall helps, and if you are going over foam you can add drywall glue. The stuff I've used has an immediate grab, and will be set before any sagging develops. XPS foam is fairly rigid and would give enough additional support.
The rafters are close to 16 o.c. but pretty unevenly spaced. The strapping is mostly to let me flatten out the ceiling. I'd rather shim the strapping than the rafters. This is how the whole house was done when we moved in. Plus I can use adhesive between the drywall and the strapping.
I don't want to add XPS because there's already not much clearance and it would only be over 2 sections of the roof about 4-5 feet wide between the knee walls and collar ties.