Cracks in building fabric lead to air infiltration due to wind and buoyancy driven forces. In the heating season, the cold air entering the building needs to be heated up to room temperature, thus leading to an energy demand. Conventionally, the measured infiltration rate at 50 Pa (expressed as n50 or q50) is recalculated into an average infiltration flow rate (yearly based) using leak infiltration ratio's (LIR) ranging from 0.033 to 0.1, the origin of these values being sometimes unclear. Apart from the yearly based energy demand, heating load calculations (according to the standard EN 12831-1) are used to size heat generators and heat emission equipment and require leak infiltration ratios on a short-term base (1 hour - 1 day). It can be expected that this short-term evaluation leads to much bigger LIR's. The BBRI investigated the infiltration rates, using CONTAM simulations on some typical dwellings. This paper presents the results of these simulations, and reports on various dwelling properties, influencing the actual infiltration rate, among others: the infiltration rate at 50 Pa, the ventilation system, the distribution of leaks over the building envelope, the variable temperature and wind conditions, even in a small country as Belgium, the shielding of the building, height and orientation. 2 important conclusions will be reported on: first the leak infiltration ratio at room scale can be much higher than at building scale, and third; because the actual leak infiltration is mainly wind-driven and the strongest winds in Belgium occur at higher outdoor temperatures (and not the lowest design temperature) in some building configurations the biggest heat load doesn't occur at the lowest design temperature, but at somewhat higher temperatures.