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Installing Drywall Over 2" XPS Foam


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03-18-17, 07:06 PM   #1 (permalink)  
Installing Drywall Over 2" XPS Foam

I'm planning an attic renovation and part of it involves putting full sheets of 2" xps over the 2x6 16"oc rafters to add another R-10 to my total insulation. Then I'd like to install drywall over the xps but I have not attempted this before.

I went ahead and bought Foamular 150 (15psi) thinking that 250 would be overkill. Now I'm thinking about it and wondering if the drywall screws will compress the foam. I'd rather not add strapping because that's more work and I'll lose even more headroom. I can confidently drive 4" drywall screws into the framing.

I like to use 5/8" drywall because it's heavier and stiffer and I expect it will also distribute the compressive force of the screw over a larger area. I suppose there will be a bigger danger that the foam compresses against the rafter than the drywall into the foam, since the rafter is only 1.5" wide.

What do you think? Will this work ok?

 
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03-18-17, 07:59 PM   #2 (permalink)  
Should be okay, but I would add that you want to make sure you stagger all your joints so that no drywall edge falls on top of any foam edge. 3 1/2" screws are sufficient. And I would add a few dots or lines of glue on every sheet so that the 2 layers act as one. You didn't mention if this ceiling is flat, but I assume it is cathedral. If it is, gravity will cause your drywall to shift downhill (ever so slightly)... so a little glue (PL300) will help hold all layers together as one.

 
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03-18-17, 09:09 PM   #3 (permalink)  
It is cathedral. I'm not a fan of gluing all the drywall - if you should ever have to remove or redo something you can kiss the (expensive) foam goodbye. I'd rather just use more screws. Sometimes if I don't feel super confident with things I'll glue all the edges of drywall to try and reduce chances of crack appearing.

Yes, staggered boards and seems on both foam and drywall.

3.5" screws means 7/8" penetration. Into old lumber that's not enough for me. I'll go 4"

 
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03-19-17, 04:55 AM   #4 (permalink)  
You might check out a metal framing product known as "Z" furring. It is used in commercial applications where a foam insulation is attached to a (usually) masonry substrate and does double duty of helping to hold the foam in place and provide an edge to screw attach the drywall to.

You probably won't find it in a big box store or most lumber yards. You should check with a drywall supply yard. They will show you the product and several sizes available.

I'm sure there are other products that incorporate foam and furring in one assembly. I believe one is "Gold Foam", you might check that out for specs and distribution of the product.

 
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03-19-17, 03:01 PM   #5 (permalink)  
The Z furring strip looks like a pretty good solution. The big problem is that not you've got to work with 16" or 24" strips of foam board instead of 48" just so that you can get the z channels into place for drywall. So you're breaking up your foam and making the seams harder to seal. AND, now you're forced to land foam and drywall seams on the same spot.

One idea I'm considering is to hang all the foam board, then make a hot wire cutter in the shape of a 1x4 furring strip. Run the cutter down the length of the room on 16" centers and now you can lay the furring strips into the foam rather than on top of. I suppose one advantage of laying the furring strips on top of the foam is that you would have 3/4" channels to push wires through if needed down the road.

In the end, if there won't be any finishing/cracking problems with simply using long drywall screws through the foam and into the rafters, I don't see why one would go through the trouble of doing anything else.


Last edited by chimpywrench; 03-19-17 at 03:29 PM.
 
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03-19-17, 05:05 PM   #6 (permalink)  
The key to hanging over foam, or any surface for that matter, is to make sure you have the board held tight to the substrate before driving the screw. Screws that are used to pull the board will inevitably break the paper face and weaken the holding power of the fastener and lead to finishing issues.

The foam and drywall ending on the same joint with the z furring is of no consequence since the drywall is screwed directly to the face edge of the furring member while the foam is under the furring. It gives you a decidedly solid surface to attach to and you get to use 1-1/4" screws instead of 3-1/2" or 4".

 
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03-19-17, 05:19 PM   #7 (permalink)  
Another idea that's popped into my head is to install the drywall over the foam with 4" screws, and while installing tape the back side of the drywall seams to permanently join them at the seams.

We kind of got at this earlier when I mentioned gluing just the edges of the drywall to the foam, but I bet a different type of backer (than foam) could be used. Something like a fiberglass tape with a bead of construction adhesive so that the tape is glued to the back of the seams and also glue would squeeze between the edges of the sheets. You would of course tape and mud the face of the seams as usual. I bet this would resist cracking really well.

 
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