If you have fiberglass insulation in your subfloor, you may want to know if applying spray foam would make a difference in keeping your floors warm. Let’s take a look.
First of all, you’ve got to realize that fiberglass insulation and spray foam are a little different. If you go with a closed cell spray foam, it does act as an insulator, and it also keeps air from transferring into and out of the living space, while fiberglass or rock wool don’t typically do that sort of thing. You’ll also want to look at how thick the fiberglass insulation is. A six-inch thick fiberglass or rock wool insulation in the subfloor is an R19, where a closed cell spray foam is R6 per inch.
If you can imagine three inches of spray foam, you still don’t get to an R19, but the spray foam air seals where the R19 does not do that. Some of the other things you will want to think about in order to answer this question: your feet are warm, and the flooring is cold, so the only way to actually get the floor warm to your body, which is around 98 degrees, is for your floors to be higher than 98 degrees. Even if you insulate the floor correctly, but you keep your thermostat on 75 degrees in the winter, the floors are only going to be 75 degrees. So they’re still going to feel cold, because the floors are colder than your body temperature. So the only way you can insulate flooring to make it feel warm is to apply heat to the flooring.
If you insulate the subfloor, and let’s say it’s 60 degrees in the crawlspace, and you’ve got your thermostat on 75, there’s no heat source in the crawlspace. So there’s no way for the subfloor to get warm, because you’re not adding heat to the subfloor. All the insulation’s doing is keeping your living space temperature from being absorbed by the crawl space. That’s really all it’s doing–slowing that down. So if it’s 70 degrees here and 60 degrees here, insulation helps to slow the loss of heat to the crawlspace, but it actually doesn’t heat the floor.
This may not be the answer you’re looking for. What it comes down to is that f you want to heat your floors, install a heated floor system, and if you want to keep from losing that heat from your house, then install proper insulation between the subfloors.