What does having a pressure reading below 25 pa indicate during a blower door test?

Having a pressure reading below 25 pascals during a blower door test indicates a connection to indoor air, which suggests that the building is experiencing air leakage or infiltration. This level of pressure indicates that the correlation between the inside and outside air is compromised, allowing air to flow into or out of the structure through various unintended gaps and cracks.

In the context of the blower door test, which is designed to assess a building's airtightness, a reading below 25 pascals suggests that the building does not have the desired level of airtightness. While some air exchange is normal, a pressure reading in this range typically implies that the leakage might be higher than accepted standards, affecting the building's energy efficiency, comfort level, and overall indoor air quality.

Other options, such as a perfectly sealed building or an efficient HVAC performance, would generally be indicated by much lower air infiltration rates and higher pressure readings, which are not reflected by a reading under 25 pascals.