About this Project

Floors over basements, porches, garages, or other unheated zones need insulation, too. How you handle these heat-eaters depends on the situation underneath.

If the joists are covered, as with a finished garage ceiling, your best bet is to have loose-fill blown in by a contractor. With open joists, install batts, blankets, or rigid planks, as shown on the following page.

Use blankets or batts if the spaces between the joists aren't chopped up with lots of pipes, wiring, ducts, or bridging. Rigid planks allow you to drop below obstacles, but check building codes before you do so. You'll probably have to face rigid urethane or polystyrene insulation planks with a noncombustible material; some building codes don't permit these types at all.

Unless you're simply adding another layer to existing insulation, get faced materials and install it with the vapor barrier side facing up (that is, toward the heated area). Foil facing works well here because it reflects heat back into living areas. Pay special attention to achieving good coverage at joists and headers around the floor's outside edges.

Cold slab floors present a special problem because you can't get material under them, where it would do the most good. It may help to wrap the outside of your foundation, as illustrated on the next page, but talk to a professional first. You might end up with a higher R-value by insulating the floor itself. To do this, glue down wood sleepers, place rigid insulation planks between the sleepers, then lay new sub- and finish flooring.