Insulating an uninsulated floor can drastically save on energy costs, helping to keep the house warmer in the winter and cooler in the summer. An additional benefit to adding batts of insulation is that the dampening effect it will have on sound control. Many producers now provide more earth-friendly, formaldehyde-free types of insulation as opposed to the more toxic fiberglass formulations of yesteryear. You can install underfloor insulation in older homes with a few simple hand tools.
Wear a dust mask and a pair of safety glasses, then access the area beneath the floor through either the crawl space or basement. Inspect the areas for any gaps where electrical wires operate between the floor joists and the wall using a flashlight.
Insert a tube of outside silicone caulk to your caulk gun, and cut off the tip of the nozzle using a utility knife. Squeeze the caulk gun activate and fill any holes or gaps found in the review using caulk.
Unroll a roll of kraft-faced insulating material (R-19 or higher) using an attached vapor barrier and also loosely tuck the first batt of this roll into the space between two floor joists using the vapor barrier facing upward against the subfloor, and with the paper-faced side facing down. Begin at one end of the joists and work toward the center. Tuck the insulating material between any electrical wiring, plumbing or duct work joined to the floor joists. Unfold the newspaper flaps on the sides of the batts and staple them into the lower inside faces of the floor joists every 6 inches using a construction stapler.
Insert a second batt to the end of the first between the two floor joists until you reach the wall. Cut the insulation and kraft-paper cleanly using a utility knife where the insulating material meets the wall, then install the remainder of the batt between the next joists. Continue insulating between the joists until you insulate the entire underfloor.