# Drywall over 1-1.5" rigid foam?



## michael76

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!


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## Termite

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.


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## michael76

*test run...*

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.


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## Ininkus

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!


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## michael76

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.


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## AtlanticWBConst.

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.


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## Termite

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.


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## sunthas

The space isn't heated?

how are you going to do your outlets?


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## photopeter

*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?


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## oh'mike

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


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## Axecutioner-B

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


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## michael76

*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.


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## photopeter

*good idea*

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


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## stephen1234

*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


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## masterofall

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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## masterofall

stephen 1234 can you provide a drawing the explanation is confusing


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## 95025

oh'mike said:


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


I did the exact same thing - on a vaulted ceiling of a family room I added on to our house - many years ago.

It was tougher to install the drywall, required extra long screws, etc. But we were living in Minnesota at the time, and I wanted the extra insulation.

We lived in the house for 5 years after I built the family room. No cracking or problems during that time. We then sold the house and moved out of state, so I cannot say what happened in the ensuing years.


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## concretemasonry

I built an 1850 sf lake home using all masonry construction in northern MN (cold winters, warm summers) over few year period. The scissor roof trusses spanned 30', starting at 9' and going up to to the peak (to about 12'). When I originally built it, I put 10" of the fiberglass junk between the trusses spaced at 24" o.c. and used poly to hold it in place for a few years. When I went to finish the ceiling a couple of years later, I opted for 1 1/2" of extruded polystyrene since the home was so energy efficient (turned off breaker so no heat from sometime in October until the middle of December with a few periods on below zero. The thermostat was still showing a temperature higher the low limit of 65F, with quite a few days of with below 0 F.). I went to 1 -1/2" instead of 2" and very liberally slashed the old poly "vb". I had a contractor put in the 1-1/2" rigid XPS and drywall with 5/8" sheet rock (traditional, but long screws and had the foam and drywall joints staggered in both directions). After a year, I had the ceiling taped. After 2 more years there is no sign of problems with screws or taping.

It worked with no problems despite the long span, but I have a feeling the uniform temperature may have had an effect.

Dick


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## Gary in WA

I believe you may have trouble with the interior (room) moisture leaving through the outside wall, depending on your climate. The idea of foam on the outside is to have it thick enough to keep the moisture from condensing on the sheathing by raising the temperature of the inside face of the foam. If the heat is stopped at the drywall, the moisture will condense on the insulation and the sheathing after going through the foam because they are colder, the reverse of this; http://www.greenbuildingadvisor.com...lating-minimum-thickness-rigid-foam-sheathing

The insulation needs to be outside to work with the vapor flow; http://www.buildingscience.com/documents/insights/bsi-001-the-perfect-wall

Another BSC article explaining the different thickness of foam required for different locations as to changing dew point temperature; http://www.buildingscience.com/documents/reports/rr-0412-insulations-sheathings-and-vapor-retarders

Of course the ceiling would be different than a wall, especially if vented, to get rid of the moisture from inside. I'm not an expert on this subject, just surmising, open to discussion.....


Gary


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## captainkidd

*Electrical boxes*

If you can't find the adjustable electrical boxes, or don't want to mess with the existing wiring, there are box extension rings that will fill the gap between the front of the box, and the face of the sheetrock. These are required by code. You can't just use longer screws on the switches and receptacles.


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## concretemasonry

Duplicate post


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## dberladyn

I don't know... I'm not sure I see any problems with doing this at all. He should be using 1/2" drywall, not 3/8".. but the foam shouldn't act any different than the vapour barriers we use here in BC. As far as screws popping... why? As long as everything gets sucked down tight when it's properly screwed off there shouldn't be problems. And he could always GLUE the drywall to the foam.

I don't understand some of the responses. But then maybe some here have more experience than I with this specific idea.


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## Gary in WA

Foam board and plastic vapor barriers are not the same. The foam will insulate (stop the heat) from reaching the sheathing better than cavity insulation alone, letting any moisture condense on the inside sheathing because you changed the dew point location and temperature. Read the links on post #19.

Gary


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## chrisBC

The only thing that strikes me is the amount of space you are building out your surface, especially if it's a ceiling. Unless it's a high ceiling, or something like a cathedral ceiling, you are going to decrease your ceiling height by an inch and a half or so, more if you use plywood strapping to attach the drywall. 

Something to keep in mind with this idea.


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## jenstotland

Hi, we did this. It turned out the studs really should have been planed flat before installation as the gypsum is a bit bowed. We also needed to have a vapour barrier underneath the foam as opposed to the foam being the barrier, just to hold everything in place. We needed 2 1/2" decking screws for the gyprock and 3" screws for the baseboards but it's done now and quite cozy. Before this I thought "Am I making this too complicated for myself as a first-time drywaller?" And then I thought "Nah"


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## Old Thomas

Dow blue Styrofoam shrinks as it ages and outgasses. A 2x8 sheet will shrink about 3/8” in length over time. If I want 1.5” of foam, I use 2 layers of 3/4” with the seams staggered. At least after it shrinks the wind won’t blow through it.


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