Hello,

I am a new member to your sight. I have a question about using XPS between the stud cavity for my home in North Dakota.

I have been working on a remodel project for about a half year now; I added two rather large dormers to my home (about a 200sq ft addition) and am just about ready to start insulating. I was thinking that the best way to insulate my second floor would be to cut two pieces of 2” thick XPS and fill the stud cavity with it, so I would have 4” of XPS in each stud cavity.

To install the foam, I was going to cut the XPS so that it is about 1”-0.75” short of the studs/plate, glue the two pieces against the OSB and then fill the gap around the stud cavity with closed cell foam. You might ask why I want to create so much work for myself. The answer is that I have more time than money and XPS is about 1/3 the cost of closed cell spray foam.

I have checked on the compatibility of using closed cell foam on XPS, I am confident that I will not end up with issues there. Building codes in the area specify that I have to use an inner wall poly vapor barrier. My question is if I am asking for a lot trouble in the future by insulating this way? I realize I will have almost a two vapor barrier system with the foam coming 1.5” short of the inner vapor barrier but I will have the 2x6 studs able to breathe between the empty space and the outer wall. Do you think I am also asking for condensation problems?

Regards,
Chris

The quality of the foam air-seal around the XPS is EVERYTHING- you can't let interior air transport moisture to the colder parts of the stud or the sheathing to condense.

If the foam is well sealed adding a poly vapor retarder on the interior would be superfluous. But if you have a 1.5" gap between it and the XPS you run the risk of a moisture trap (but not the usual wintertime condensation issue) in that 1.5" of stud since it can't dry toward the exterior through 4" of XPS, or toward the interior thorugh the poly (even if the poly has holes in it (which it will.)

If they'll let you, leave the poly out, and 'splain it real good to the inspector why, and/or offer to use vapor-retardent latex on the interior (~0.5 perms). If they still balk, the alternative is to spray at least an inch of cc foam on the sides of the studs and top/bottom plates so that no naked wood is exposed to the narrow cavity. Then the poly could only trap the moisture in a fully foam-faced cavity, protecting the wood.

The studs have about the same vapor permeance as 1.5lb density XPS, so the studs' moisture levels will pretty much be whatever the exterior sheathing average is. If there's a 3/8" rainscreen gap between back-ventilated siding, it'll always be low enough to never matter. (Which is why 10mm rainscreens are now code in Canada.) Vinyl siding is inherently back ventilated, so it would be fine without additional measures.