You don't seal the vents, ventilation of the crawlspace is needed to prevent moisture buildup and rot.
I had to take on the very same project. There are three objectives.
- Under house air can have mold in it, you shouldn't be breathing it, block all air exchange between the living space and the crawl space.
- Under house moisture levels must be brought down so your house under-structure doesn't rot. Reduce the amount of damp soil in contact with the air, allow no pooled water, maintain ventilation to remove the rest.
- Reduce heat exchange into the crawl space. Insulate the floor, it completes the side benefit of sealing the floor seams and from personal experience, makes quite a difference.
Our '70s stick-built house is built with 1 1/8" T&G plywood subfloor. Normal construction now is to lay a heavy bead of Liquid Nails or similar subfloor adhesive in the groove and drive the sheets together, something they didn't do back then. I stripped the shag rug carpets out, caulked every seam shut, laid down 3/8" plywood and refloored the whole house. This eliminated all the drafts from the seams.
The crawl space was built on two levels. The next step was to improve the drainage so there was never any standing water under the house. We dug in a drain to the ditch and then laid lay down two layers of 6 mil black poly on the upper level and cemented over the lower level with about 2" patted into place which converted it to a useful storage area. A tremendous evaporative surface for moisture was eliminated and the dampness of the air much reduced.
We then insulated between the floor joists with fiberglass bat. 1/4" black expanded plastic mesh stapled in place on the floor joists holds it in.
One added thing to watch for is anywhere air leaks. If each seam in the floor had a 1/8" gap over a span of 28ft, you have 42 square inches of air slot there. Caulk kills a world of sins.
Following that thinking, the place was built with these decorative beams in the ceiling. Close inspection showed there was a 1/4-1/2" slot on each side of the three beams. All the heat from the stove headed for the ceiling and through the slots, you could see the air trails in the dust under the insulation. If you figure average slot width of 3/8" for a 14ft span, this was a total of 378 square inches.
Holy stinky stuff, that's like having a large hole cut in your ceiling... (18"x18" = 324 square inches).
The next winter was rather oppressive the first time we started using our wood stove. We can actually overheat the place in the wintertime now.