I have an older home (60's era) with rather cold floors. I was thinking of adding fiberglass batting to help with this, but when I went to measure the joist spacing, it seems MUCH larger than I expected.
Spacing between the joists in the photo is approximately 45.5 inches, which seems too large for fiberglass.
When the floors are insulated out this way, (west coast) the insulation is held up by wood strips. The strips are typically called "wood lath" like the type used in old homes with plaster walls. It is tacked or nailed or staple in place, starting about a foot from a wall or other starting point and spaced about 2 ft apart. It can be closer to get around obstacles, I would not suggest going any farther apart. The material is 3/8" thick, 1 1/4" long fasteners will hold very well. I used my 1/4" crown stapler powered by air to set the ones I did. Put them up first, then thread the insulation in place.
I would also suggest laying 6mil vapor barrier down first. It is tough stuff, it may get shuffled around a bit, but can be straightened out. You will need to wear a Tyek suit in any case, it is miserable work, and DUSTY..... wear a respirator, a good one, cartridge type.
Batts are typically 47" long. Trim them to 45" (friction fit with some compression).
You'll probably need to use the bag the insulation comes in to help hold them up, cut it such that the plastic will span and just staple it. I'd probably also use a 1x4 in the mid span of the 45" to help keep the insulation in contact with the floor. I like rock wool over fiberglass since it is fire proof, mold proof and insulates even when wet.
Your floor assembly looks like post and beam (not joist) and you have 2"+ subfloor boards spanning the beams? 45" seems like a lot for a subfloor to span interested to know how thick those subfloor boards are.
Forget batt insulation. It harbors rodents and this is the usual end result:
You need to:
The Earth is a natural source of heat. Your crawl space is cold because it's drafty.
