Rising fuel prices have made insulation of residential structures an important factor. It is frequently difficult to determine the amount of insulation in the walls of existing buildings and excessive attic insulation is not cost effective if the walls are poorly insulated. Some means of determining the insulation in a structure is desirable when homes are sold; there is a need for a simple way to determine the insulation effectiveness in a residential structure.

Two measurement methods are potentially usable--steady state and transient. If the inside and outside temperatures were constant for a period of time long enough for the exterior envelope of a building to approach equilibrium, and if the cyclic efficiency of the heating system could be easily determined, the overall Building Thermal Resistance (BTR) could easily be calculated. Outside temperature is rarely constant, however, and the cyclic efficiency of the heating system is generally difficult to measure. Thus, the steady state method is not very attractive.

A transient method has been devised that appears to offer some advantages .1,2 Transient measurements do, however, measure a combination of thermal conduction and heat storage. Thus, it is necessary to make two measurements under different conditions in order to obtain two independent equations so that the conduction and storage effects can be separated. It has been found that measurements during a cooling period with the furnace off and during a heating transient with the furnace on continuously provide the two independent measurements needed.

Measurements with the furnace off or the furnace on continuously solve the problem of cyclic efficiency. The furnace output has been assumed to be zero during the off-period ; it will be shown, however, that fan power should be considered. Furnace efficiency and output can be readily measured during a long (60 to 90 minutes) on-period by measuring flue oxygen and flue temperature rise. The analysis, measurement techniques, and results of some specific measurements are presented to support the hypothesis.