Those are Tongue & Groove panels with glue/sealant on the plywood side and in the T&G.
They were just about a perfect fit. There aren't any real cracks or openings where I could inject sealant.
I can't figure out how I could get an 'air seal' without covering the whole bottom with plastic..
When I probed around between the floor and the sub-floor, I was amazed
how roomy it is down there. My guess is there is about 4" of R-11 with foil,
topped off with 8" of air space..
One idea that occurred to me was cutting a bunch of holes in the floor,
and filling up all that dead air space with blown in insulation..
Maybe when we replace the carpet.?. That's on the list..
It was laid in June 1989.. 22 years, 4 months ago, but still looks pretty good..