Freestanding space heaters are portable and inexpensive, but they are only useful for heating small spaces. Unvented kerosene and propane space heaters can boost the temperature in a small garage, especially if it is insulated, but they also release carbon monoxide and moisture into the air.
Direct-vent space heaters can be great choices for supplying heat to the garage, especially if your house already receives propane or natural gas. These units have a pipe in the wall that brings in combustion air and expels exhaust fumes. If you plan to use the heater frequently, look for a super-efficient model. Suppliers of direct-vent heaters provide instructions for installing the unit yourself, but even if you feel up to the chore, have a professional install the fuel line.
Research various types of space heaters on ConsumerSearch.
A woodstove can be a great choice for heating the garage under certain circumstances. You will need to have room for a safe installation, with proper clearances from combustible materials, and you must also connect the stove to a proper chimney. A woodstove takes quite awhile to heat up, and will continue generating heat long after you’ve stopped feeding it fuel, so this is not a good option for short, quick heating needs. On the other hand, it would be ideal for daylong projects in the garage. And you could throw scrap pieces of lumber into the firebox (caution: don’t do this with pressure-treated lumber).
Electric baseboard and wall heaters are inexpensive to install, but they can be very expensive to operate. In a cold climate, electric baseboards are not a good choice for a room that will be used frequently unless there aren’t any good options. On the other hand, in a moderate to warm climate, this kind of heater may make sense for occasional use. Electric heat is quiet, clean and unobtrusive, and it does not require tapping into the furnace or boiler.