Insulating a Floor
It can keep chilly drafts out, but it doesn't necessarily make your room warmer
I have an old cottage-style home on short piers. How can I insulate the floor to prevent winter drafts?
— Phillip, Chalmette, Louisiana
Tom Silva replies: Rigid insulation is one way. This entails cutting pieces of insulation that would fit in each bay between the floor joists under your flooring, fitting them in tightly, then skimming over the whole section with another layer of insulation. Finally, you then have to caulk the seam where the insulation meets the floor joists, because these grow and shrink every day.
Another way to do it, if there's a ceiling below your floor, is to have insulation blown in to fill the gap.
There is one especially important thing to remember about insulating a floor, however: although it will help eliminate the drafts, it is not going to make the room significantly warmer by itself. Insulation works in one of two ways: by keeping the heat out, as it does in a refrigerator, or by keeping the heat in, as it does when it prevents warm air from moving through the walls, floors, and ceilings that you insulate. So, the better the insulation, the colder your refrigerator can stay, and the warmer your house can.
But there have been a lot of people disappointed after insulating floors, having thought their house would suddenly become warmer, which it won't. Basically, what insulation does is impede temperature change. So the insulation will certainly help reduce the chilling effect you experience of drafts coming into your house through the floor, but it's not going to appreciably raise the temperature in your house on its own. Because heat rises, when it's cold outside, the heat in your house is in the upper part of a room and the cold is toward the bottom. That means the cold is going to enter the insulation and actually maintain it at a cooler temperature than the floor. Another way to think about it is that in effect, insulation helps maintain warmth in your home by "refrigerating" the outside. So just bear in mind, insulating the floor will help your house stay warm, but it won't warm it.