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

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Looks good. Just a couple of suggestions.

You can press the bottom of XPS boards tight to the wall with the baseplate. Shim if necessary.

As I mentioned earlier, the finished basement would look much better if you boxed in the area with the two electrical panels. You don't need anything elaborate. Just a 2"x6" box with a couple of plywood doors. And you might want to replace that top hinged single pane window with a modern set of aluminum or vinyl sliders with dual panes. I can get a custom made one here for less than $100.

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?

Here's the Fairfax Virginia information I posted on the other thread.

You can use drywall or the other materials mentioned in the information. The point is to create a discontinuity between the stud spaces in the wall and the spaces between the floor joists above, which includes the rim joist area you're insulating.