Infiltration is customarily assumed to increase the heating and cooling load of a building by an amount equal to the mass flow rate of the infiltration times the enthalpy difference between the inside and outside air—with the latent portion of the enthalpy difference sometimes neglected. Calorimetric measurements conducted on a small test cell with measured amounts of infiltration introduced under a variety of conditions show convincingly that infiltration can lead to a much smaller change in the energy load than is customarily calculated; changes as small as 20 percent of the calculated value have been measured in the cell. The data also suggest that the phenomenon occurs in full-sized houses as well. Infiltration Heat Exchange Effectiveness (IHEE), ε, is introduced as a measure of the effectiveness of a building in “recovering” heat otherwise lost (or gained) due to infiltration. Measurements show that ε increases as: (a) flow rate decreases; (b) flow path length increases; (c) hole/crack size decreases. There is a clear correlation between large values of ε and large values of the exponent, n, so fan pressurization results may be useful in predicting ε for buildings.