Crack data is used when the 'Calculated' natural ventilation option is set. In this case every surface in the model has a crack and its size (characterised by flow coefficient and exponent) is determined by Airtightness as set on the Constructions tab in the Model Data. The value of Airtightness is used as a key to look up corresponding crack sizes in the Cracks database.
The cracks database has been set up empirically to give typical air change rates for each of the five Airtightness categories in a range of building types.
There are five tabs on the Crack Template data dialog: