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Pineapple 02-26-2008 06:55 PM

Rigid Insulation on Interior?
Hi everyone, first time poster looking for some simple advice. I'm in the process of thoroughly remodelling our 35 year old family bathroom. The entire room has been stripped to nothing more than a floor and studs! However, I've come across a slight issue:

An entire wall was covered in some old wood panelling, which we ultimately decided to scrap. It was thick panelling (3/4"), with 1/2" drywall underneath. There is a window on this wall whose carpentry work was made to come flush to the panelling. Now that the panelling is gone, I'm left with a window which protrudes into the room about 3/4" IF I put back on 1/2" drywall. So, basically, I'm trying to pad out my drywall 3/4" so that the finished wall comes flush with the window sill and jambs.

This is where the rigid insulation comes in. I don't know too much about this stuff, so I'd like to get some opinions on whether or not this would be a good idea. 3/4" rigid foam board between the studs and the drywall would give me the padding I need, plus some added thermal performance :thumbup: right? My concerns are this: will moisture be an issue given that this is a bathroom, will the foam board hold up to being installed under the drywall. I don't want to crush it while screwing in my drywall and then have an uneven wall :(.

Anyway, any advice you guys can lend me would be very much appreciated. I'm flying solo on this project and can use all the advice I can get! Thanks, Dan

Hancockian 02-27-2008 06:44 AM

Hi Dan...Why not use 1X2 furring strips on the studs to gain the bump out the attach the drywall to them. Put the rigid insulation between the strips if desired. I'd use green board for the drywall in a high humidity area. It costs a little more but stands up to moisture better.

AtlanticWBConst. 02-27-2008 06:46 AM

Got a picture of the window?

Maybe the window jambs can be altered?

Pineapple 02-27-2008 07:01 AM

Thanks for the feedback. I have considered using furring strips, but thought this would be a more creative and energy efficient way of solving the problem (albeit a little more pricey). I don't know how much thermal performance I'll gain, but I've read that placing the rigid insulation over the studs (instead of between them) creates a thermal break. Without a thermal break, the studs act as a thermal bridge conducting heat through the wall.

I have also considered altering the window jambs. I don't have a photo of it right now, but I can certainly take one. There wouldn't be too much involved in taking in the carpentry 3/4"... However, I'd like to avoid tearing into it if possible. I've torn out everything else in this room, but the window is the one thing I definitely want to keep since it's in good condition.

Maintenance 6 02-27-2008 12:49 PM

I would definitely not drywall directly over foam board. The problem is that the foam will not be solid and if someone leans against the wall it will compress. Then the screws or nails will pop through the surface of the finished drywall. Either nail 3/4 furring strips perpendicular to the studs and lay foam between them, or cut the window jamb back. I had a similar situation one time. I screwed 3/4 thick boards around the window and used a router to trim the jamb back almost flush with the drywall. Then finished it with a sander. The other option is to add a 3/4" strip to the back of the window casing. It may be hidden behind the curtains anyway. Hope that helps.

AtlanticWBConst. 02-27-2008 03:46 PM

I missed that point about the insulation between the sheetrock. I too, absolutely DO NOT recommend this, for the same reasons.

Pineapple 02-28-2008 05:49 AM

Yeah, I too had the same concern about the foam crushing up underneath. That would probably be bad....

Anyway, this method of putting rigid insulation directly over the studs is meant for the exterior of the wall, but the principal is the same on the interior. When you have a stud making direct contact between the interior drywall and exterior surface, it creates a thermal bridge. Heat conducts directly through the stud without being interrupted by anything. Putting rigid insulation over the studs, rather than between, creates a thermal break.

I work in the aluminium window industry where this principal is applied all the time. Most commercial aluminium windows are "thermally broken", meaning the aluminium on the interior and the aluminium on the exterior are separated by some sort of insulator. It shows up as a black strip running around the jamb perimeter. Of course, aluminium is a lot more conductive than wood.... but same principal!

Thanks for your help everybody. I think I'm just going to do some carpentry work around the window to make it look right.

integlikewhoa 02-28-2008 10:02 PM

I think you guys might be thinking about the wrong foam board. Its not the white highshcool science project foram board!
Dow makes some tuff-r and super tuff-r (which I have used the tuff-r) and the foam board doesnt look like a foam board. Its very light but is rigid and has stong reflective backing on both sides. (25psi breaking) If held up only by the 2 outter sides and you were to jump into the middle of it is going to break (same as drywall), but put up against studs its just as good as drywall. The board is required to be covered drywall per codes anyways. Its very common on both the outside of new homes and the inside of remodels. Its strong enough and ridgid enough to have stucco and any other type of siding over it. It would be perfect for you install (no I dont sell this for a living) It has a R value of 6.5 per inch. 3/4" thick would be R-5. That on top of a R-13 ins. batt would be nice. Heres a link of the stuff I used before. The 1" thick was 16.00 from homedepot. I think the 3/4 was like 12.00 for a 4x8 sheet, but not 100% sure.

Pineapple 02-29-2008 05:41 AM

Yes! That's exactly what I was thinking! I should have mentioned the reflective side. Now I'm bouncing back and forth on whether I drop the extra bucks or not. Oh well, we'll see where it goes. I finally got my shower installed so I'm just happy to be clean again :no:

Brik 02-29-2008 07:04 AM

The correct way to do this job is to use a proper sized jamb. Likely they were extended to accommodate the 3/4 paneling just remove those extensions, trim to size or replace.

Maintenance 6 02-29-2008 02:51 PM

I assumed he meant isocyanurate foam or blue styrofoam TG board. I would not hang drywall directly over either of these. The first time somebody leans against the wall the foam board will compress over the studs allowing the nails/screws to pop through. Make sure somebody didn't just nail extentions onto the existing jambs that you could remove...................... On second thought forget it. That would be too easy. Home improvement projects aren't meant to be easy.

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