Solid foam insulation , drywall, firring
What’s the best practice for working with drywall, firring strips, and solid foam insulation? We are going to be filling the rafter bays with cellulose insulation. But, then I wanted to put a layer of solid foam insulation over that to act as a thermal bridge – and give us just a bit more insulation.
What I didn’t figure on was the firring strips for the drywall. We could put the solid foam insulation over the firring strips, and then screw the drywall through the foam insulation and into the firring strips. However, I’m not sure about that as it seems like it wouldn’t give great support and it moves the ceiling down a bit.
We could also lay the solid foam insulation between the firring strips (use 3/4″ or whatever depth the firring strips are). We wouldn’t have foam where the strips are, but it’d probably be pretty good. Downside is that many of the foam boards would have to be cut. However, it’s probably not so much work to cut that stuff.
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I read awhile back that you could screw your drywall straight through the foam and into the existing rafters. Seems to me like you'd risk dimpling the drywall, but maybe not. I've never done it like that before.
I's prefer that the drywall not be against the solid foam insulation. Put it inbetween the rafters and glue/screw the drywall up against the wood. There is a very fine line between screw pops when the drywall is not up against something solid, and solid foam insulation is not solid.
Well, if you wanted the thermal barrier the foam provides over the ceiling joists, you could do two layers of thinner foam. One layer of 3/4" foam direct to the studs, then 1x4 strapping over that. You would have precut strips of board to fit between the strapping so you still have the full depth of foam board you would have had before. The strapping takes up the thermal variations, and the drywall is attached to the strapping.
Rebuilding my home in Cypress, CA
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That would have been the best plan, but unfortunately the strapping is already up.
So? Pull it! Not that big a job!
It's not like you are going to ruin a finished surface.
If you did the strapping with 2x4 or 2x3, put an X on the side facing the floor. Number them so you know where they will go back up again.
Set the fence of your tablesaw at 3/4", with the blade at full height so you can rip the 2x4 (or 2x3) on edge. The marked side faces the fence.
Put up the 3/4" foam board. Put up the strapping again - once you find the joist with one previous fastener hole, all the others will line up. Attach the fam board between.Rebuilding my home in Cypress, CA
Also a CRX fanatic!
Save the Whales! and Guns!
My 2 cents: seems like a lot of work to provide a thermal break ... will you cellulose cover the rafters? If so ... I think you are doing a lot of work for minimal return. If 3/4" is OK for you, I'd screw the drywall through the insulation into the rafters ... skip the 'nailers'. Still think it's probably not worth it at all. If you have cellulose over your rafters, then you create a thermal break. If they are trusses, the intermediate members are relatively small in area compared to the big picture of the entire attic.
Put your money and effort into something more worthwhile ...