If you have or will have an unventilated crawl space, then your best approach is to seal and insulate the foundation walls rather than the sub floor. The advantages of insulating the crawl space are as follows:
• You can avoid the problems associated with ventilating a crawl space.
• Less insulation is required (around 400 square feet for a 1,000-square-foot crawl space with 3-foot walls.)
• Piping and ductwork are within the conditioned volume of the house so they don’t require insulation for energy efficiency or protection against freezing.
• Air sealing between the house and the crawl space is less critical.
Steps for Installing Crawl Space Wall Insulation
1. Review plans for this method of foundation insulation with pest control and local building officials to ensure code compliance.
2. Eliminate or seal the foundation vents.
3. Ensure that combustion furnaces and water heaters located in the crawl space are sealed-combustion units equipped with a powered combustion system.
4. Seal all air leaks through the exterior wall during and after construction, including the band joist.
5. Locate the crawl space access inside the home or install an access through the perimeter that will remain airtight after repeated use.
6. Install rigid foam board or batt insulation—exterior foam, interior foam, or interior batt—to achieve complete insulation coverage. Insulate the band joist with batt insulation, as well as the crawl space access if it’s located in the wall.
7. Install a continuous termite shield between the band joist and masonry foundation wall that covers the wall insulation and extends completely outside (or leave a 2- to 4-inch insulation gap at the top for termite inspection).
8. Install a supply outlet in the crawl space, relying on the leakiness of the floor to provide the return air path.
Information provided by Department of Energy.