Air-tightness measurement can be expressed as Q50 which is air change per unit of building envelope area at a 50 Pa pressure difference between indoors and the outside environment and uses the unit of m3/(m2xh). This is the measure usually used in building code for a requirement that new buildings must meet. The values may vary across countries. The measure required in building code may be expressed in different units. For example Swedish regulations require V50 measure below 0,6 L/(sxm2).
If a house has high air-tightness you will need a forced ventilation system to ensure fresh air indoors. The Passive House principles require the use of highly efficient heat recuperation in the ventilation system in order to meet the requirements.
Air tightness can also be expressed as N50 or ACH50 (air change per hour) which is air change per building volume. This measure is one of the requirements to meet the Passive House standard. The blower door test results for N50 must be below 0.6 to meet this requirement.