I’m insulating 2nd floor rim joists with XPS. I had a bunch 1.5 in XPS sheets laying around so I decided to do a double layer for ~R15. Cut them 3/4″-1″ under size and gunning foam around the perimeter, first one layer then the other.
I noticed that the 2nd layer gets pushed away from the 1st as the foam expands leaving a void. This seems like a double vapor barrier and I want to avoid that. If they’re not perfectly sealed, and they probably aren’t, moisture is going to condense in there and run down to the top plate below. Now I’m just installing the one layer and saving the 2nd for later if at all. Is there a good way to keep them from floating apart? Construction adhesive? I don’t really want to put 4 in screws through the XPS.
Edit: What I meant to ask is this a problem with patching together insulation from multiple layers of rigid foam in general? Am I overthinking this?
Replies
Overthinking. Seal the edges and you should be fine.
You could glue sandwiches together before placing them in the cavity
another thing I did is to use insulation stays (metal wires that hold up insulation between floor joists) to hold the XPS in place as the foam expands.
I had a similar problem, except that the XPS I was using had warped in storage (not stored flat). I ended up using some spray foam between sheets, but first I had strapping ready to screw across the sheets to hold them together while the foam cured. I also had shingles handy to use as wedges for extra pressure if needed. Foamed the edges as well. A bit time consuming, but better than wasting material.
You are right to consider a potential moisture problem in the future. Not saying it will happen, but there’s a chance. The cool thing about foam is that when dew point temp occurs within the foam no condensation forms. This is why thickness of foam is so important. You have to make sure dew point temp occurs within the foam. If dew point temp occurs on the inside of home it can condense in the wall/floor. Both the building code and the foam industry agree with the proper thicknesses for foam according to climate region. There is a table in the IRC that will list proper foam thickness for your region. I can’t remember the IRC table number, but if you google IRC foam thickness or something similar it should pop up. If you have an air space between layers of foam and the first layer applied towards exterior is too thin moisture could potentially condense between the layers of foam. Moisture between layers of foam could take awhile to dry out. If your first layer of foam is the proper thickness, no worries. If too thin, I would have the layers of foam as tight to one another as possible. I would actually install them tight to one another regardless, it doesn’t take much time. To get them tight I would use a foam acceptable adhesive (PL makes a foam board glue) and screw with washer to make them tight. Also, I typically cut my foam with 1/4” gaps, just large enough for the tip of the foam gun. 3/4 to 1” will require quite a bit of spray foam. I would not spray foam until both layers are in place and not going to move.
By 3/4" to 1" undersized, I mean across the full dimension so that's a 3/8" to 1/2" gap. Still a bit large but workable. I've used the PL rigid foam adhesive and it works ok but it takes 7 days to cure and needs bracing. I've been using a few globs of PL premium, seems to hold ok. Tested it on the foam and the foam holds up fine. Any problems with it that I'm missing?
Is it critical that the foam be tight up against the outside wood? It's not an actual rim joist, just 1x sheathing board. I figured this provides no insulation at all. A lot of the 1st layer foam pieces I foamed in place I held in place with a finger while I foamed around them so they probably floated off some.
PL premium is fine. It will not burn the foam like some other adhesives.
I would install foam tight to the sheathing for same reason as I stated before. You have to treat it as an unvented assembly. You will have pocket of air and possibly some moisture that will not dry out between the foam and sheathing. This is assuming the rim of your floor is fairly air tight. If you have skip sheathing w/out any air barrier and a ton of air leakage you’ll be alright as the wall is basically performing as a vented assembly.