The heat balance in a house depends on heat inputs and heat losses. By combining measured fuel consumptions for the furnace and hot water system with estimated heat gains from occupants, solar radiation, and electrical appliances, total useful space heat inputs were estimated as a function of outdoor temperature. Heat losses were also calculated as a function of outdoor temperature. From the resulting graphical relationships, estimates were made of heating system efficiencies.

Changes in the fuel consumption-outdoor temperature relationship are given for several houses to which insulation was added. In two cases, addition of RSI 2 insulation to the attic and RSI 1.5 to basement walls (previously uninsulated) reduced fuel consumption by about 30 percent.