[Greenbuilding] underfloor insulation R-values & materials

[Reuben Deumling]

I have now used the Bonded Logic cotton batts a number of times and like
them fine for several reasons. What I don't like is that they only come in
R-13 and R-19 strengths. My floor joists (the basement ceiling) are 2x8s. I
would like to utilize the full 7-1/2", especially since I've found the
cotton batts to tend toward the thin side of their rated thickness. Another
complicating issue is that the bays vary in width from about 21" to 24". I
also now own (thanks to this list) an insulation blower that will need some
mechanical attention from me before I use it. Within a few years I intend
also to insulate the day light portions of the basement walls, which will
put the basement into a limbo category between conditioned and
unconditioned.

My questions are
(a) Is it worth pursuing insulation that is 2" thicker than R-19 in this
application? I hope to insulate the rest of the house a la Corwyn to R-40 or
better, down the road.
(b) of the choices I've thought of (unfaced cotton R-19 batts, fiberglass
batts?, wetspray cellulose?) are there obvious ones to prefer under these
circumstances?

Thanks.

Reuben Deumling