Floors over unheated garages and crawlspaces are often cold. This can be due to poor insulation, gaps in the insulation, excessive air leakage or poor delivery of heat to the area. The ideal solution is to spray polyurethane or polyisocyanate foam insulation into the joist cavity. Not only do these materials offer excellent R value, but they do a very good job of sealing air leaks. Unfortunately spray foam is one of the most expensive types of insulation. A close second is to dense pack cellulose into the cavities. Although a bit messier, it has reasonable R value and air sealing properties.
Air sealing within the living area may have a significant impact if access is restricted to insulate the underside of the floor. To accomplish this remove the baseboards and caulk or foam all wall to floor junctions and seal all electrical and plumbing penetrations. Make sure any recessed lights in the unheated area are sealed as well.
Often the room over the garage is the farthest from the furnace and the ducting must take the most circuitous route to get there. The duct may also be uninsulated or loosing air to the unheated area. Duct sealing will reduce heat loss due to air leakage, and should be done in conjunction with insulation upgrades. Foam insulation will seal the ducts from the outside so it kills two birds with one stone! If there just isn't enough airflow, duct fans can be installed, although this may cause unpredictable results in other areas of the heating system. Often supplementary heating is the most economical solution.