The idea that heat rises has prevented many builders in the past from insulating floors, but adding insulation for floor joists can actually improve a home's energy efficiency as well as its insulating capabilities. Part of the problem with adding insulation for floor joists is securing such insulation between joists underneath the floor. Batt insulation can be used, but it will need to be secured in place properly; blown-in insulation is also an option, though this can be messy and somewhat inefficient. Aluminum sheeting can also be used to insulate between joists, but this may not be the most efficient method for floors.
If you choose to install insulation for floor joists, it is best to first examine your floors and joists carefully. You will be at an advantage if you have a basement or crawl space, as installing batt insulation properly will be much easier. Otherwise, floorboards will need to be torn up in order to install insulation for floor joists properly. It may be possible to blow insulation into a small access panel or crawl space in order to properly insulate the floor, but this can be costly, and you will probably have to hire a professional to do it. Blown insulation can be messy and can coat pipes, wires, and other common items within floor spaces, and gaps may exist within the insulation, leading to heat loss.
In attics, aluminum sheeting is often used for insulation, and this same method can be used for insulation for floor joists, but this will not be the most efficient method for retaining heat within the home. It is inexpensive as compared to other insulation methods, and it is relatively easy to install, but the benefits may not outweigh the cost and labor. Aluminum sheeting is fairly lightweight, but it can also be damaged fairly easily.
Fiberglass insulation is usually the best option, though handling fiberglass can be dangerous and it is not the most eco-friendly option in some cases. Fiberglass rolls can be placed between joists, but the fiberglass itself will need to be cut to fit properly between joists. The insulation should press right up against the floor between joists; gaps between the insulation and the floor can lead to inefficiency and heat loss. The fiberglass will also need to be secured in place using wire mesh, twine, wood laths, or other securing methods, which can be an intensive project that adds to the overall cost.