The actual heating load of a building differs often from the designed load. One reason for this is the uncontrolled ventilation through a building envelope. The heating load of air infiltration has, in practical calculations, been calculated according to the predicted leakage flow rate and to the indoor and outdoor air temperature difference. We suggest, however, that the value of transmission heat losses should be corrected by a factor, Nusselt number, because of the thermal interaction of leakage flows and conduction heat transfer in wall structures. According to computer simulations and experiments, the warming up of leakage flow may be as high as 90 % of the indoor and outdoor air temperature difference. Correspondingly, the Nusselt numbers describing the mean relative reduction of transmission heat losses can be 0,65. The seasonal heating energy savings are of the order of 10 - 15 %, The thermal interaction of leakage flows and wall structures was analyzed by computer simulation and experiments with some elementary flow cases in typical wall structures, The results are, however, so obvious that some theoretical and practical conclusions could be made.