In the last few years, energy consultants have developed a quick and easy way to pinpoint air leaks in a building envelope. The technique uses a theatrical fog machine — a small, inexpensive device that creates smoke-like fog for dances, Halloween parties, or theatrical events. Fog machines have heating elements that vaporize “fog juice,” a solution of water and glycol or water and glycerin.
With the help of a blower door or a window fan, a fog machine can dramatically reveal holes in a building envelope.
For the last 30 years, energy raters have been using blower doors to determine a building’s leakiness. The results of a blower-door test — reported as infiltration airflow (in cfm) at a pressure difference of 50 Pascals — usually reveal how tight a building is, but not the location of any leaks.
Once a house is depressurized, however, air leaks can be located by walking from room to room feeling for drafts, or by waving a smoke pencil near likely problem areas. Finding these leaks requires experience, persistence, and a certain amount of detective work. (For more information on blower-door testing, see “Blower Door Basics.”)
The fog-machine technique adds a whole new dimension to leak discovery. It’s leak detection for dummies. Instead of crawling around on your hands and knees with a smoke pencil, you look for leaks by standing around in the front yard. The leaks don’t reveal themselves as subtle whiffs of moving air; they shout, “Over here! I’m leaking!”
During a conventional blower-door test, the fan is set up to blow outward, depressurizing the house. A fog test, on the other hand, requires the house to be pressurized, with the fan blowing inward. Fog tests are usually scheduled after a house has been insulated but before the drywall has been hung.