How to seal floor under built in closet

subfloor

We have a built in closet. When we removed the bottom drawers we saw the exposed subfloor (diagonals on top of joists).
subfloor under built-in

There are two drawers. They are side by side Each drawer is about 3' wide. 7" high. About 3' deep. The picture shows the right drawer removed.

It's hard for me to reach and work in there. What is the best way to seal it so that I won't get cold air, hot air, bugs etc. ?

I have a crawl space underneath.

Best Answer

That's a difficult situation. The best bet might be to caulk everything with silicone from the crawl space, then add some fiberglass batt insulation.

Otherwise I think I'll look for a heavy, flexible fabric or membrane and lay it in there under the drawers, lapping up the sides just a bit where you can. You could try applying silicone caulk to all joints and seams, maybe from a small squeeze tube, but it'll be messy. You could use rubber gloves and finger the caulk out to press and finish it.