Hi Leen,

I typically use the formula: ach (normal pressure) = ACH at 50Pa/20, based on Kronvall and Persily - this is known as the KP model.

A more detailed model for converting ACH at 50 can be found in Sherman 1987:

Estimation of Infiltration from Leakage and Climate Indicators
Energy and Buildings, 10 (1987) 81 - 86

If your modelling a passive house it may be worth using a more sophisticated model than the KP model. You can also look in the EnergyPlus Engineering Reference manual, it discusses some methods developed by DOE etc for IBlast and DOE-2 to modify the actual infiltration rate for wind and other factors on an hourly basis according to the weather file.

Regards,

Hayes Zirnhelt