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mbscallen 02-14-2008 10:36 AM

Venting foam board insulation
Hi, I'm a newbie to the forum. Want to install foam board insulation on the underside of a sloped kitchen ceiling, then drywall over it. Room has been gutted so I'd be securing the insulation to the underside of the roof.

Do I need to allow 1.5" of airspace above the foam board insulation? Or can I secure it right to the underside of the roof?

If I need that airspace for venting, I assume I could secure 1x2s to the roof every few feet, screw the foam board to these, duct tape it all. Then screw the drywall through the foam board to the 1x2s to finish off the space.

Whattya think?


Chris Johnson 02-14-2008 08:47 PM

You need a little help but we need some more info.

You will need airspace

how much rigid insulation are you adding? R-30 means you need around 6" of foam (about R-5 per inch)

Are you filling between the rafters with the foam? if so run 1x2 on edge on both sides of the rafter under the deck, this will give you 1 1/2" airspace. Cut the foam snug to friction fit. last sheet can be placed on underside of rafters and use long drywall screws to secure drywall to rafters (not the 1x2's). DO NOT INSTALL VAPOR BARRIER (Foam acts as a vapor barrier itself).


look at and search for ceiling technology

mbscallen 02-14-2008 10:04 PM

Thanks and a follow-up question
Thanks for your reply--very helpful. I appreciate the reminder about not needing a vapor barrier--it's good to have that suspicion confirmed.

Have one challenge I'm still trying to solve--my client wants the rafters exposed--in other words, wants me to drywall between rafters so you still SEE the rafters after the drywall's up. The rafters are 2x8s so it should be possible, even with 6" of foam, but it means I'd need to secure the drywall to the 1x2s, not the rafters. Think they can hold the weight? If not, I may need to get her to rethink her design... And how might I fill in the cracks between rafters and drywall--caulk?!? That seems unlikely...

Your opinion is much appreciated.

Have to say, this forum has provided a terrific think tank for me--have been reading much and not posting until now. I'm grateful for the expertise available here.


Ron6519 02-14-2008 10:37 PM

Sorry, but the math doesn't seem to add up. A 2x8 is 7 1/2"-1 1/2" for the air space(1x2 attached to the joist)- 6" of foam and you're even with the joist bottom. Then you add sheetrock and the joist disappears. You would have to add to the joist to have it, "exposed".
What R value do you need in your area for the roof? How much rafter does the owner want to see?
It might be easier to add whateaver R value you need in the rafter bay. Sheetrock on top of the joists as you would normally do and just screw cleats on top of the drywall to get the illusion of exposed beams.

Chris Johnson 02-14-2008 11:06 PM

Go Ron's route for the exposed beams, plus your client can choose whatever species of wood she wants to see and you don't have to caulk the edges every rafter bay as you were thinking

If your a builder check out (sister site) this site is more for DIY's with mostly repair questions.

mbscallen 02-15-2008 08:18 AM

Yes and yes
Dear Ron and Chris,
Thanks, all. Talked to the owner this morning and she wants 4" of beam left exposed--clearly impossible as I'm going for an R-factor of 30, so I'm going your route and drywalling over the studs, then adding "fake" beams on top of the drywall.

I appreciate your help. And I'll check out the sister site you suggested, Chris. After years of renovating my own old homes, I've begun working for friends and referrals on small, contained projects: tiling, drywalling, insulating, etc. I'm no pro--there are many things I can't do yet. I'm learning as I go, AND calling in pros when I hit something I can't handle. But I'll check out the contractor site to see what I can pick up and continue my education.


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