Abstract: The inside air temperature, the outside air temperature and the temperature of the inside surface of an exterior structure whose `R` value is to be determined and these values are selectively applied to the `R` value apparatus which stores the inside and outside temperatures as voltage analogs and combines them to provide a first difference signal. The inside temperature analog is also applied to a second combination circuit. After the inside and outside temperature analogs are fed to the apparatus and stored therein, then the inside surface temperature of the structure, such as the wall or ceiling, which is being measured, is applied as an analog voltage to the apparatus. The inside structure temperature analog is combined with the inside room temperature analog to provide a second difference signal which is then divided into the first difference signal in an analog divider, a portion of the output being fed directly to a meter calibrated in `R` values.