©2015 Green Building Advisor. From The Taunton Press, Inc., publisher of Fine Homebuilding Magazine.
In the last few years, energy consultants have developed a quick and easy way to pinpoint air leaks in a building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials.. The technique uses a theatrical fogTo fog a room or building is to use a fog machine during a blower door test, revealing locations of air leaks where the fog escapes. The fogging material is usually a glycol-based solution, completely non-toxic. 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 testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas. — 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." )
Duct Leakage Testing 
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.
The fog machine is set up indoors. Before beginning the test, it’s important to open two or three windows in rooms distant from the fog machine. (The windows only need to be opened a crack.) The fog machine is then turned on, and all the workers on the job site assemble outdoors for the show. Once fog can be seen escaping from the deliberately cracked windows, the windows are shut and the test begins. The observers then look for escaping fog, which can show up almost anywhere: at the mudsill, at the eaves, or around windows.
Small buildings are easier to test than large ones. “For a residential test, we can fill the whole building with fog,” explained Vermont-based building envelope consultant Henri Fennell. “But with a large commercial building we can’t do that. So we ‘bag’ an area with polyethylene – for example, after the first window is installed – and test a smaller area. If we aren’t getting any fog flow, we poke a hole in the bag with a knife. When we see the fog come out, we patch the hole with duct tape.”
[Photo credit: Jeremy Kovac]
Fog tests are so dramatic that they quickly convince skeptics of the value of air sealing. “Anyone who sees the fog coming out gets it right away,” said Fennell. “The test doesn’t require a college degree to interpret it.”
According to Marc Rosenbaum, a Massachusetts energy consultant, most contractors are humbled and cooperative when fog reveals a building’s holes. “My experience is that if you have a blower-door specification for new construction – so many cfm at 50 pascals – and the test comes in 10 percent more than the specification, the builder will usually ask, ‘Why isn’t that good enough?’ – especially if you are fairly far along in the construction process,” said Rosenbaum. “But when you use a fog machine, and you have fog blowing out of a hole in the building, I’ve never had anyone point to it and say, ‘Why isn’t that good enough?’ ”
You don’t necessarily need a blower door to perform a fog test. “All it takes is a window fan,” said Fennell. “Just be sure you have a positive pressure, and go out into the front yard and look at the house. The big holes show up very easily.”
If the building is large or particularly leaky, however, a window fan may prove ineffective. “If you need to create a pressure difference across a boundary, it’s easier to do when the envelope is tight,” explained Fennell.
Gary Nelson — the president of The Energy Conservatory, which manufactures the Minneapolis blower door and the Duct BlasterCalibrated air-flow measurement system developed to test the airtightness of forced-air duct systems. All outlets for the duct system, except for the one attached to the duct blaster, are sealed off and the system is either pressurized or depressurized; the work needed by the fan to maintain a given pressure difference provides a measure of duct leakage. — points out that fog machines can also be used to test for duct leaks.
“You tape up all the registers and you pressurize the ducts,” explained Nelson. “Then you introduce fog into the Duct Blaster – you aim the fog nozzle at the fan blades, without letting the fog get drawn into the vent holes in the motor, and you watch where the fog pours out. Sometimes you may be working with an HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. contractor who says, ‘This is a good duct system. This is the way we have always done it. This is normal.’ Well, when you show them the fog coming out of the leaks, they shut up really fast.”
One of the virtues of the fog test is that the necessary equipment is inexpensive. A recent Web search showed that a 400-watt fog machine can be purchased for as little as $30.
As with any tool, however, you get what you pay for. Both Fennell and Rosenbaum use Rosco  fog machines that retail for about $400. “The most common problem with most fog machines is that they gum up,” said Fennell. “You have to follow the manufacturer’s instructions for maintenance. As long as you flush them out between uses with distilled water, they will last a long time.”
A fog test quickly answers an important question: where are the leaks? But according to Rosenbaum, there’s another reason to conduct a fog test: “It’s a lot of fun.”
Last week's blog: ‘Insulating’ Paint Merchants Dupe Gullible Homeowners