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michael76 01-04-2009 08:56 AM

Drywall over 1-1.5" rigid foam?
We have a typical cape with a stairwell added on the back for access to the second floor. It is not heeated, so I'm putting up rigid foam - Dow's blueboard - over new batts in 2x4 walls. When I say "over," I mean that the foam extends full across the interior and not in between the studs or rafters.

I'd like to put the drywall right over the foam...will that work?

I concerned that, since the foam is less dense than wooden studs, the drywall will crater as screws grab the wood and pull the drywall snug to the foam. I've seen and heard of strapping being put over the foam, but I really want to avoid another step if it's possible.

Thanks very much!

Termite 01-04-2009 10:05 AM

Foamboard shouldn't be installed on the interior side of the studs. Your concerns about attachment of the rock are justified. Foamboard is installed on the exterior side beneath the siding.

michael76 01-04-2009 11:52 AM

test run...
1 Attachment(s)
I'm getting completely different opinions on this...
so I took some extra 1.5" foam and 3/8 drywall and did a test run, and it seem to be fine. Assuming we take care in securing the drywall, I think it will work out. Any reason to think otherwise??

I attached a photo of the test.

Ininkus 01-04-2009 12:54 PM

Interesting project. I would have originally said do something else, but it's cool that you tried it and it appears to have worked okay.

My concern now would be the joints. I think if you make sure that the seams of the foam DO NOT align near the seams of the drywall you might just be fine.

So what do you figure you have, somewhere around R20 when combined with the batts? It also sealed all your joints... added bonus!

michael76 01-04-2009 01:43 PM

My boss, a former builder and energy auditor, suggested this method...Ininkus, what would you have suggested? Always looking for other ideas.

Seems will be offset...foam seams sealed with spray foam and tape.
Yes, on the R-20, plus air barrier.

AtlanticWBConst. 01-04-2009 03:15 PM

If you continue installing sheetrock over rigid foam board, I guarantee that the screws will pop, and the seams will crack.

The other option is to build the walls out using strapping or studs,(as you stated). You can do this horizontally or vertically, and install the rigid foam between.

Termite 01-04-2009 03:26 PM

To expand on what Atlantic said, you could add 2x2 furring strips to each stud and secure the rock to that. The insulation would of course have to be cut into strips to fit between the studs.

Sorry, but you shouldn't trust your former boss on this one.

sunthas 01-04-2009 03:30 PM

The space isn't heated?

how are you going to do your outlets?

photopeter 11-28-2010 07:47 AM

rigid foam board over interior studs
I am in the process of doing exactly what Michael76 described.
I don't understand why the screws could pop or the taped seems might separate.
The foam board will first be fastened to the 2x4 studs with nails that have a large plastic washer.
3" drywall screws will be used to fasten the 1/2" drywall boards through the 1 1/2" foam board into the studs on 16" centers (which will be marked on the floor).
Going right across the studs, rather than going in between, prevents thermal bridging by the studs and should be better than going in between.
I already have pink insulation between the studs right now.
I am removing the original vapor barrier covering the fiberglass insulation
before I will put the foam board on.

The electrical boxes are a bit of a problem, but there are a few options here, too.
An electrician told me I can just use longer #6-32 screws and mount the actual outlets (or switches) right against the drywall, leaving the box where it is. If the wire in the box has not been left long enough, pigtails can just get maretted onto them.
I don't like this too much, since I think over time the outlets and switches might wiggle loose rubbing on the drywall.

Since I work in a machine shop, I cut a small steel plate, the hight of the electrical box and about 4 1/2" long and riveted it to the side of an electrical box.
Now I can fasten this plate to the stud, bringing my box out to 1/2" proud of the foam board.
I thought another option could be by just fastening a 1 1/2" strip of wood on top of the stud where the box is mounted right now and just bringing the box forward to the proper depth.
Of course the foam board will have to be cleared for that.

Has anyone done something similar to this or see problems with it?
Michael 76, how did your job turn out?

oh'mike 11-28-2010 08:13 AM

I've done exactly what you are thinking about----never had a call back,or a crack---

Axecutioner-B 11-28-2010 08:56 AM

I wouldn't go with just putting longer 6/32's in, to me that's just a little too sloppy. I would use wood to attach the electric box to & i'd extend it out to where the box should be sitting if things were normal. Like 1/8 back from the face of where your drywall will sit. I'm thinking maybe take a 2 X 6 & nail the plastic box to it at the proper distance for the drywall & then sink 3 or 4 --- 3 inch screws on the back of the 2 X 6 to the 2 X 4. That should be nice & sturdy & solve your problem with only a little extra work.

Sometimes its hard to explain things with a keyboard, i hope what i just tried to explain makes sense :)

michael76 11-29-2010 11:42 AM

finished project feedback
It's almost 2 years from my original post, and the spaces (2nd floor and stairway) have been completed for almost that much time.

I ended putting up 1/2" drywall directly over foam (blue XPS) in 2 ways:

1. On most of the 2nd floor ceiling, I put the drywall directly over the foam.

Result:In these sections (about 500 sf), there's maybe 8 screws popping.

2. For the stairway walls and ceiling, I ripped 3/8" ply into 2" strips and went over 1" and 1.5" foam. I kept the strips right over the studs/rafters because -- strips were flimsy (they needed support and wouldn't alone hold drywall); and, to keep my head clear on where romex traveled behind the foam. To secure the drywall, I went thru the strips and foam with screws long enough to give me at least 1/2" grab in studs/rafters.

Result: Over about 650 sf, no screws popping - yet.

This project was my first time doing ANY kind of remodel work, so I think part of the screws popping is my poor job on improving existing framing, hanging drywall, and taping/mudding.

That said, from my experience and the results so far I would follow advice of other posters - and general online GC advice - and put up some decent furring. I didn't do so because I was trying to lower ceilings no more than I had to.

photopeter -- On the electrical boxes, I found boxes that can be mounted to the stud and adjusted by a machine screw to whatever depth you want. They were able to reach flush thru 1.5" foam and drywall. Just don't make my mistake and get the cheap elec. boxes made of plastic. Use the metal or fiberglass (?) ones, or whatever other recommend.

The whole idea was insulation, and that's been a big improvement. The spaces are warmer, holding the heat better, and far fewer air leaks than before.

photopeter 11-29-2010 12:26 PM

good idea
It makes perfect sense and sounds like a good and sturdy solution.

stephen1234 01-13-2011 10:53 PM

foam on rafters
i was going to do this for my 1 storey and half in canada. i bought this house and found out no insulation in the attic (well the guy put the insulation fibreglass directly on the roof, wich was pretty bad for moisture and the insulation wasnt doing what it was supposed to do).

i currently was able to push some foam trough the rafters from the storage side to the begining of the attic where its not accessible but now i find that i still need to put more than the 2 inch foam board. so im wondering
1. how i can attach another 2 inch foam board, reflectix and dry wall to the current drywall or
2. if i have to get rid of the current drywall and what type of fasteners i should be using, i dont mind the extra steps if necessary but its a bedroom right now and it needs to be done soon because we're freezing!
any other options would be appreciated thanks

masterofall 01-14-2011 01:29 PM

Sounds like you did a good job. the foam covering the studs has eliminated thermal bridging. It is also psi rated so screwing the drywall on top should not compress the foam

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