A glossary of energy terms.

Degree days: A convenient measure of heating requirements for a particular climate. The DD's are based on 65 [degrees] F as a baseline. If the average temperature for a day has been zero, 65 degree days were accumulated that day. For example: Montreal, near us, suffers from about 9,000 degree days per heating season. Washington DC has about half of that, or 4,500. The same house will require about twice the fuel to heat in Montreal as in Washington.

Masonry stove: A wood- (or coal) burning stove made of masonry materials, such as bricks, blocks, stone or tile. These stoves can weigh anywhere from 1,000 pounds to many tons. Fuel is burned hot and fast in a masonry stove, and almost all of the heat is absorbed into the mass and then given off slowly into the room or the house. This is the most efficient way to burn wood, and the home keeps a steady temperature.

R-value: A measure of a material's ability to slow down the transfer of heat--its insulating value. The higher the R number, the better the insulation. The insulation value of a material is often measured in therms of R/inch. Extruded polystyrene, for example, is rated at R5/inch. Wood varies widely according to species, from about .6 to 1.5 R/inch.

Thermal mass: The mass heat storage capability of a material. In general terms, a material's ability to store heat is inversely proportional to its insulation value. Styrofoam[TM] for example, is an excellent insulation, but has virtually no thermal mass. Cast iron, concrete and stone, on the other hand, have great thermal mass but are horrible insulators. Wood is somewhere in-between, having both insulating and mass heat storage characteristics.