Air tighness in a passive house is a key element so don´t assume that something is sealed – always check. Calculate air changes using the ATTMA volume, the PHI institute has much more detailed calculation requirements and the resulting volume can be 20% smaller, making it harder to pass the air tighness test. Don´t allow the construction program to override quality requirements – remedying air tightness defects at the end of a build can be disruptive, costly and significantly delay completion.
When carrying out an air tigthness test, a steady-state condition is established, when the air blown out of the building by the fan is balanced by the air re-entering the building through various cracks, gaps and openings. Typically this is at a pressure of 50 Pascals (Pa), about the same as 20 mile-an-hour wind acting upon the volume under test, not forgetting that wind speed rises with distance above ground if testing in high-rise blocks.
Excerpt from “How to build a Passivhaus: Rules of thumb”, Passivhaus Trust, April 2015