What
I have done is to simply install kraft-faced fiberglass insulation
between the floor joists.
My suggestion would be to use
only R-11 or R-13 insulation, each of which is 3-1/2" thick,
even though your floor joists are almost certainly deeper than
2x4's. I would install the insulation and leave it flush with the
bottom of the joists, thus leaving an air space above the
insulation (and below the floor boards). This air space will
provide some additional R value.
Of course, you can use 6"
thick insulation (R-19) or thicker if you want, but I'm convinced
that you won't get much additional benefit if you use anything
thicker than R-11. Floors don't lose much heat, as warm air rises,
but floors do get cold and even a small amount of insulation may
make a noticeable difference.
The problem I've had with
installing fiberglass insulation in floors above crawl spaces is
that the insulation batts tend to fall down over time. I recommend
getting some cheap strips of wood (plaster lath is about the
cheapest) and tacking them to the bottom of the joists to hold the
fiberglass in place.
Another option if money is tight:
just apply thin 1/4" fan-fold polystyrene foam insulation to
the bottom of the joists. This will create an air-barrier and the
air space above the foam will also help add to the insulation
value. You might need to tape the joints or else cold air might
get above the foam. The proper tape is pretty expensive... it's
the tape used for the seams of Tyvek and Typar and other types of
housewrap.
Bruce W. Maki, Editor.
