In any heated structure the inside surface temperatures in winter will always be lower than the inside air temperature. Depending on the overall coefficients and the outside design temperature, the magnitude of the surface temperature reduction will vary from a fraction of a degree F to 15 or more.

The usual evaluation of the percent reduction in heat load due to insulation is in terms of the reduction in overall air-ta-air coefficients of heat transfer but is based on the same inside design air temperature as used for the uninsulated structure.

A secondary effect of insulating a structure is to increase the inside surface temperature of the insulated areas. This means that body heat loss by radiation from the occupants is necessarily reduced and, in consequence, for equal thermal comfort the air temperature in the occupied space should be reduced. Thus the lower air-to-air temperature difference provides a saving due to insulation in addition to that due to the reduction in overall coefficient. The radiation saving is a "dividend" due to insulation and, as shown in numerical examples in this paper, varies from a few percent up to approximately 15%, depending on the structure.

The analysis presented is for winter conditions, but can readily be applied to summer conditions; the "dividends" for summer are of the same order of magnitude as for winter.