What does the pressure reading from unconditioned to conditioned space indicate with the blower door running?

The pressure reading from unconditioned to conditioned space during a blower door test is a critical measure of the air tightness of a building. A pressure reading of 25 pascals (Pa) is considered a standard point of reference in these tests. When the pressure is balanced at 25 Pa, it suggests that the house is neither significantly losing conditioned air to the outside nor gaining excessive outside air. This balance indicates a well-sealed building envelope.

Anything below this pressure indicates that there is some level of connection or leakage between the unconditioned space and the conditioned space, suggesting that unwanted air from the outside is infiltrating into the conditioned area. Thus, the interpretation that pressure readings below 25 Pa indicate a connection to the inside aligns with the principles of pressure differentials and air movement in building science.

Understanding this reading allows technicians to assess the overall performance of a building's envelope concerning energy efficiency and indoor air quality.