Hello,

Happy Thanksgiving to everyone!

And, thanks to everyone for your help to date. During the week, we've installed the majority of the XPS in the basement; you can see a JPEG with two photos below.

In the top photo, you can see one corner of the room with XPS installed. For the wall on the left, which had a joist with a 1-in gap between it and the cinderblock wall (no rim joist), I sprayed Great Stuff Fireblock down to the bottom of the joist. I then inserted half-inch gypsum as another firebreak; there is a narrow gap between it and the cinderblock which will get filled with more GreatStuff Fireblock. Below that is the 2-in XPS, glued and mechanically attached with tapcons and fender washers at the tongue and groove seams. Cinderblocks hold the bottoms in place until the glue dries; I didn't want holes at the bottom.

In the bottom photo, you can see a closeup of the gap between the XPS and the gypsum board as fire block. Maybe I'm being overly cautious, but given the flammability of the XPS, I wanted to have this additional protection. As mentioned above, Great Stuff fireblock will go in the horizontal gap and will fill the narrow gap I left on the cinderblock wall side of the gypsum board; the gypsum board will therefore not be in contact with the cinderblock at any point and there should be a good seal at the top of the XPS.

So, before I move ahead and install the small XPS inserts between the joists, I wanted to see if anyone (worthy?) had any course correcting comments?

Also, question: if we're going to use gypsum board on the ceiling, do I need to put gypsum board on the interior side of the small XPS inserts up between the joists?

Do I need to seal any gaps at the bottom of the XPS?

Thanks for all your help!

Tiger