Highly rigid insulation board

I've run into interior renovation situations were I really wish I could provide a thermal break from stud to drywall. In areas like bathrooms with exterior walls it would be really nice to keep the drywall insulated from the stud. Even on a perfectly done job I'll see the shower steam condense on the screw heads and around framing first, because those spots are always the coldest. I know the framing is best insulated with rigid foam on the outside of the house but I'm always working with older homes which do not have foam over the exterior sheathing.

Yes, I could put xps over the entire interior framed wall, then drywall, but I'd never mount something like a shower surround over top of foam. Foamular/xps is too squishy and would compress between the shower surround and the framing.

You could do xps then plywood, then finish the room, but now you've added a lot of thickness and that'll cause issues.

I'm wondering if there is a non-compressible material that can act as an interior thermal break from the framing. Maybe just a quarter inch thick. I don't need it to hold nails or screws as I'd plan my framing behind it.

Thanks!

 

I think you are underestimating the compressive strength of foam. It's not good with point loads, but under sheetrock you would not be able to tell it's there as long as you fasten everything into framing. After all, people put siding right over foam all the time. I kneel on scraps of foam all the time and unless I drop down on it hard, it doesn't even dent.

I suppose you could look at schluter kerdi-board panels to see if they are even stronger; they are used for shower enclosures and shower curbs and are plenty strong for that.

Another option would be to build a double stud wall with the studs offset by 1/2" or whatever you are willing to live with. Insulate between the outside wall and the outer edge of the offset stud and you would have a good thermal break.

 

If it were just a regular wall with drywall then I'd have no issue. But mounting a fiberglass tub and surround against foam seems pretty sketchy. Then I have a surround/drywall joint on top of foam and I feel like there's a good chance for movement. The screws around the flanges and for the clips supplied to secure the tub/surround would easily start to crunch the foam (I'm using Sterling Vikrel surrounds). Since the tub/surround fasteners would be sticking out 1/2" (or whatever foam thickness) from the real framing, their holding power would be very poor (compared to a fully tightened screw) which could result in movement & squeaks.

I know the foam works fine on exterior but we expect a lot of movement in our siding products. If my surround/drywall joint moves just a peep it'll break the caulk line.

 

Foam on the outside does work, but not at 1/4" thickness.

In your climate zone they call for R-20 walls and specify combining cavity and 1" of rigid, usually on the exterior. One step to make a slight improvement would be using the best cavity insulation possible. The studs lose heat to the exterior surface, but also to the cavity insulation. Fill those cavities with spray foam and the studs will be warmer, slightly. Roxul would be my next choice for the cavity.

Bud

 

1/4? You should read up a little on thermal bridging. I don't think you have thought this through.

Also, when you say "non-compressive" you are implying something rigid... 1/4" plywood comes to mind... which has an R value of roughly r-.30... next to nothing. Most things that are rigid will have next to no r-value, and will conduct heat away just like wood will. Wood is generally accepted to be r-1.2 per inch, so anything you apply as a "thermal break" needs to be far superior to that in order to have any effect at all. A 3 1/2" stud would be about r-4. Its not just a matter of introducing a separate material.

Also your drywall screws will still be screwed into a cold stud so they will still sweat, so your logic is flawed.

If you want something behind the drywall the best you could do is a "high compression" foam like Foamular 600 or 1000.

 

For the bathroom I'm working on now I've already introduced all the necessary framing to mount the tub/shower surround. In the exterior wall I've cut and filled with 2" plus 1.5" xps, sealing all edges.

Yes, I realize my idea doesn't solve the problem with the screws getting cold. I think the only way to fix that is to install foam, then sheath with ply, then install drywall (screwing into the plywood, not the framing). In my case I installed 2" xps between the existing studs, then installed some 2x blocking between studs all along the tub/shower flange with the blocking laying flat against the 2" foam, then filled in all remaining spaces with 1.5" foam. Everywhere possible I'll screw into the blocking as far away from the cold studs as I can.

I have thought this through. I'm not trying to save money on heating & cooling here. I'm just trying to shift the temperature gradient in the wall so that I don't have cold spots that encourage condensation & wall damage. Wood and drywall have about the same R-value per inch. By introducing even a small amount of a much better insulator between the two, I can much better hold the entire sheet of drywall at room temperature. If the drywall is R0.5 and I put a sheet of something R1 behind it, that's a huge deal in terms of shifting the temperature gradient back away from the face of the drywall.

I had already looked up the stiffer Foamular products but they're not carried locally, and they're only offered in thicker sheets. I have Cerazorb mat (3/16" with R1.5) that I've used for floors but it's too expensive to do a whole wall. I wonder if 1/4" cork underlayment would do what I want?

Another interesting idea is to cover the wall in a thin sheet of aluminum then drywall. This adds no R-value (might reduce it) at all but it does spread the heat/cold evenly. Essentially the warmer majority of the drywall would be helping warm the cold drywall against the studs (at the cost of making the warm spots cooler). Again, all this does is change the temperature distribution on the field of the wall. I think sheet aluminum thick enough to work would cost more than Cerazorb or cork which actually would add R-value.

 

A couple of comments for you.

1. If the Cerazorb would help but cost is an issue, only cover the face of the studs to give whatever thermal break it will provide. You could then fill the cavity out to flush OR make your last surface a foil facing and leave the gap. The radiant barrier will add between r-1 to r-2 for insulation.

2. As for the "Cerazorb mat (3/16" with R1.5)" they are stretching a bit as that would give R-8 per inch. Cork underlayment is around r-3 per inch so I would expect their real number to be in that range.

3. You could also consider just gluing the drywall to the studs. A few screws to hold it in place until it is solid and then remove most. If you have ever had to remove drywall that was glued you know it isn't going anywhere.

Food for thought.
Bud

 

Good thoughts, Bud.

Something bugs me about just treating the stud faces. I think part of is that the material will have to be even higher compressive strength since a 1.5" strip of foam/cork will be easier to squish. I guess a wider strip could be used. I thought you needed more than 1/4" air space for the foil to be effective?

You're probably right about the Cerazorb not being R-1.5.

I like the glue idea. Done very carefully you could glue with super fat beads of polyurethane glue, making it so the glue beads hold the drywall ~1/4" from stud. No data on this but I'd guess polyurethane glue has some r-value since it kind of expands/cures like a foam. If I do cork or cerazorb then glue won't work.

With cut and cobble rigid foam insulation, will I want a vapor barrier? What about with a cork layer added? I would think cork has low vapor permeability, but I couldn't find data on this.

 

Whether you cover the wall or just the face of the studs it is only the 1.5" face that is under compression.

They recommend something like ½" to reduce the thermal conduction directly through the air. But that is a minimal and ¼" will still give you the conductive break, stud to drywall, and benefits of the radiant barrier.

You might also need to glue the cork to the stud face. Or, just thinking out loud, you could use a ¾" strip of cork for spacing with your "super fat beads of polyurethane glue" on each side. Tedious, but the drywall would then be glued to the stud.

I don't think the cork would be a vapor barrier. A quick search and it needs a vapor barrier added when used as underlayment on a concrete floor.

Actual vapor barriers are being replaced by various vapor diffusion retarders. Once you have sealed all air paths the amount of moisture that can move through your wall is very low. In your case a vapor barrier would probably not be needed but if added it would complicate the glue approach. The solution would be the final coat of paint on the drywall can be your vapor retarder.

Bud

 

Yeah I've actually been using a lot less plastic in recent years since I read that latex paint is not very permeable. I focus on air sealing.

I have some 1/4" cork on the way. I'll play with it and use it if I like it. If I don't like it I think I'll glue & screw, then remove the screws.

I think it is not just the 1.5" under compression. Most materials are harder to compress in sheets because to compress a point, the point has to either break away from or compress the area around it. Also remember I've filled the bays with xps, so there's a continuous firm base for the drywall to sit against. This means the drywall would have to compress the cork everywhere.