Properly insulating crawl space

There are some very good spray foams on the market. Most are a two part system and a starter kit runs around $800.00. This would include the hoses, tanks and spray wands and enough material to do several hundred square feet, depending on thickness. There are foams that are completely water and moisture resistant as well as mold and vermin proof. I sub out most of that type of work, so I don't have the brand names on the tip of my tongue, but you should be able to investigate various types of foam and equipment appropriate for your application at your local specialty distributor of insulation products.

Most crawl spaces are vented if they have an earth floor or are prone to moisture. If you insulate between the floor joists with a moisture, mold and vermin resistant insulation (foam as we discussed before) you would still want some ventilation. The only time I would seal the exterior walls would be if I also used a pretty darn water tight moisture barrier over the floor, tuck taped to the knee wall at the bottom of the joists. You would be creating a dry cell and blocking movement of cold air through and under the house. This is typically done with a rubber/neoprene type compound sheet commercially, but can be done effectively with a couple of layers of good 6 mil poly. This poly is avail at Lowe's, HD, etc. in 10 and 20 foot wide rolls, 25 to 100 foot in length. You can use Tyvek tape to seal the seams and around posts etc. Leave yourself a way to enter the space to check humidity levels occasionally for reassurance.