Cottage Crawl Space
Cottage floors have many penetrations that aren't completely contained within the floor cavity. It is difficult to install an air barrier in a crawlspace without moving these protrusions into the conditioned space. Applying Sprayfoam Insulation under the wood floors or under subflooring not only creates an air seal and insulates the floor, but it also seals all the cracks and holes where rodents and insects enter our homes. The foam also will protect your wood floors from soil moisture rising from the ground to slowly rot your wood.
Home Crawl Space
The rule of thumb for a crawl space underneath your home is 'do not install sprayfoam on the floor above it".
Depending on the season, a room above the crawl space is typically cold or hot. The exterior walls of the crawl space are insulated with rigid foam however since there is a vent circulating air in the crawl space, the air will always be cold in winter. And that is going to give you a cold floor above.
Having a crawl space with a dirt floor can cause other problems, mostly from humidity. There is always moisture in soil, and that’s going to move in under your home, because soil is porous. It also contains insects, which can crawl into your home.
Another problem that you might find is odour, especially in fall or spring, when there is more moisture in the soil.
The natural air movement in your home is always upwards, due to the stack effect. Cold air pushes in from the bottom of your house through openings and cracks, and it forces warm air upward. (And let’s not forget the crawl space has a vent direct to the outside for air supply. That’s going to provide a lot of cold air.) This will make your room cold, as the heat is pushed toward the ceiling and out through your attic. That’s going to increase heating costs.
Also, this air coming from the crawl space with a dirt floor will bring mould spores, insects and bad odours with it into your living area. If you have a crawl space with a dirt floor, you need to seal it with a sheet of vapour barrier to prevent ground moisture from getting into the space. It will also keep insects out and improve indoor air quality.