The ceiling of a crawl space is the underside of the home's main floor. Insulating it in a vented crawl space can save on heating bills, seal out cold drafts and the insulation process also helps control moisture in the space. Vented crawl spaces have screened openings near the sill, the rim where the joists meet the foundation. These vents allow air to circulate in the crawl space.
Cover the crawl space floor with 6-mil plastic film, overlapping each row by 12 inches. Run the plastic up the sides of the walls at least 6 inches and secure it with duct tape. This will help control moisture and reduce the possibility of mould or mildew growing on the insulation.
Place a stone or brick about every 8 to 10 feet on the overlapped plastic to hold it down.
Fit batting insulation between floor joists with the vapour barrier facing the subfloor. The vapour barrier is usually on the Kraft-paper side of the insulation. Reducing air space between the subfloor and the insulation is critical, but don't stuff the insulation up against the subfloor. Push it gently into place without compressing the insulation. The material's loft helps provide the rated insulation value.
Cut slits in the insulation for wires and pipes to pass through.
Hammer nails into the joists 6 inches from each end and no more than 24 inches apart down the length of the joists. Twist stiff wire around a nail, stretch the wire across to a nail on the parallel joist and twist it around that nail. This will secure the batting in place. Keep the wire tight, but don't let it push up into the batting more than an inch.
Continue placing insulation between joists and secure the batting with stiff wire until the crawl space is complete.