It is generally recognized that uncontrolled infiltration should be minimized to avoid unwanted heat loss in winter and heat gains in summer. Air tightness standards in construction vary enormously throughout Europe, but legislation is moving towards tight air infiltration rates.

In many climates, Passivhaus require a mechanical supply and exhaust air ventilation system with heat recovery. In this case, excellent airtightness of the building envelope is required. If the envelope is not sufficiently sealed, the airflows will not follow the intended paths, the heat recovery will not work properly, and the energy consumption will rise. In very mild climates, it is possible to build Passivhaus without heat recovery systems. In such a case, if no ventilation system is present, air tightness is not quite as important. Conversely, very airtight buildings without ventilation systems run the risk of bad indoor air quality and excess humidity. Results from this Passive-On project showed that the Passivhaus proposal can also accommodate a less restricted condition in terms of air tightness in particular on milder climates and where traditionally people are used to open windows to refresh and ventilate the spaces.

Natural ventilation can be used very effectively to provide fresh air and improve comfortable and hygienic conditions for occupants. Despite the variability and difficulty to control natural driving forces, natural ventilation can reduce energy consumption, capital and running costs of mechanical and electrical plants as well as the save space for their installation. Also avoiding air conditioning units may reduce health conditions associated with poor maintenance and re-circulation of air.

3.6 Heat recovery systems

A heat recovery ventilator (HRV) can help make mechanical ventilation more cost effective by reclaiming energy from exhaust airflows. HRVs use heat exchangers to heat or cool incoming fresh air, recapturing 60 to 80 percent of the conditioned temperatures that would otherwise be lost. Models that exchange moisture between the two air streams are referred to as Energy Recovery Ventilators (ERVs). ERVs are especially recommended in climates where cooling loads place strong demands on HVAC systems. The heat recovery system is an essential part of the central European Passivhaus, and allows homes to be built without conventional heating systems.