Crawl spaces have historically been difficult areas to insulate until spray foam insulation came along. The traditional thought has been to insulate the floor of a crawl space with fiberglass insulation and then to vent the crawl space area. The idea was to give moisture, mold, and radon a way to vent out. Using this method allows moisture to enter the house, through fiberglass insulation and creates a perfect environment for mold growth.
Building science and years of testing has taught us differently now. Best practice is to insulate the exterior walls of the crawl space and to install a 6 mil vapor barrier plastic on the floor, sealing the edges and overlaps with spray foam insulation. This is what is called a conditioned crawl space, essentially turning into a conditioned space. By not allowing the cold air under the house, the floor will stay warmer, pipes won’t freeze, and if there is any ductwork or furnace units in the crawl space it will create a conditioned space, keeping the ductwork in a warm environment. With the 6 mil plastic on the ground, moisture will not be drawn up into the house.
Insulating the floor underside with spray foam insulation does nothing to add to the thermal performance in the house. We do not recommend this, plus it adds unwanted costs to the project. Installing spray foam insulation in a crawl space is one of the best places to add spray foam, and will yield the most noticeable, instant results. You will notice warmer floors, less mold, and a more comfortable home with lower heating costs