How is fog testing typically performed?

Fog testing is a critical method used to identify air leakage in buildings during airtightness tests. The correct approach involves performing the test from the interior of the building under pressurized conditions. When the building is pressurized, it encourages any air that could be escaping through leaks to move outward. Therefore, introducing fog (usually produced by a fog machine) allows for a visual inspection: the fog will exit through any identified leaks, making them easy to spot.

Pressurization is effective because it establishes a pressure differential between the inside and outside of the building, which amplifies the visibility of air movement. Observers can see the fog flowing through leaks, helping them locate less visible air pathways that may not be noticeable under normal conditions.

This method is widely favored because it provides a clear visual cue indicating the points of air loss, helping improve the overall energy efficiency of the building by pointing out areas needing repair or upgrade. Other methods listed in the options do not effectively utilize pressurization or are limited in their ability to detect leaks clearly.