Diagnostic Tools-Smoke

The blower door is my number one tool when diagnosing air leakage problems in a building.  This tool gives me an idea of how leaky the building is, but often we also need to identify the location of the air leaks.  There are several ways this can be done, my go to method is usually a thermal imaging camera, these cameras will help get a good visual of temperature differences that can be created during blower door testing, that is if there is a temperature difference between inside and outside the home. If that difference doesn’t exist, thermal imaging won’t work.  Another option is to use your senses.  Sight, look for moving curtains or cobwebs, smell, raw fiberglass has a distinct odor, hearing, sometimes there is a whistling noise as air moves through a hole, and touch or feel, using the back of your hand is an effective method, are all ways to find an air leak.  Using smoke to find holes in the building envelope is another method that can also be effective.

I own four different tools that can produce smoke, a smoke pen, a smoke puffer and two different theatrical foggers.  The smoke pen is a simple test tool used to pinpoint air leaks.  It uses a wick that is lit on fire using a lighter or match.  After the wick has burned for a few seconds, you blow the fire out and the wick will smoke.  This smoke will only last a few minutes, when the smoke pen goes out, you’ll have to relight.  Cheap and reliable, but you have to work with an open flame to get the product to work, something that can be hazardous in tight spaces such as an attic or crawlspace.

When using a smoke pen or puffer, the smoke will “hang” in the air or move straight up if there are no air currents that scatter the smoke.  This is what we want to see, no scattering.  Once the smoke finds an air leak, it becomes turbulent and will typically move away from the point of the leak during a negative pressure or into the hole during a positive pressure testing.


A better option for spot testing air leaks is with a smoke puffer.  There are many versions and price points of this type of tool.  I’m aware of a unit that costs nearly $1000 with others under $100.  There’s one that was originally made as a toy that has found its way into the construction industry.  The smoke puffer I use is new and affordable.  It’s a kit you can purchase from The Energy Conservatory and is a cross between a turkey baster and e-cigarette.  It uses a nontoxic glycol to generate the smoke.  Simply squeeze the baster and smoke comes out.  There is an integrated battery that needs periodic recharging, but the unit is simple to use.

I own two different theatrical foggers.  The foggers are used to fill an entire home with fog, then pressurize the home using the blower door to see where the fog exits the structure.  This can be especially useful when air leaks are hidden behind building assemblies or when you just can’t find a troublesome leak.  This testing is usually done when constructing new but can be used in existing structures as well.  One requirement, be sure to contact your local fire department and let them know what you are doing, the building appears to be on fire during this type of testing, there’s a good chance they will be showing up if you don’t contact them.  Another suggestion, when purchasing a theatrical fogger, be sure to get one with a higher output, my first fogger was a 400-watt unit, too small to fill an entire home with fog.  My second is a 1500-watt machine that recently filled a 52,000 cubic foot home in 15 minutes.

After the fogging is complete, it will take some time to evacuate the fog from the home.  It’s best to simply open windows while the blower door is operating in a positive pressure and push the smoke out, turning the fan around and sucking the smoke out will pull the smoke through the fan, which could result in damage to the fan motor.


When using smoke to find air leaks, having the home under either negative or positive pressure is a must, it is possible to create a pressure inside a home without using a pro-grade blower door.  One option is to use a homemade blower door.  Start by sealing a panel in a doorway.  This panel can be a piece of rigid foam or plywood cut to fit a door or window opening.  Cut a hole in the panel that fits some sort of fan, and then seal the fan to the panel.  I would recommend facing the fan so that it pushes air out of the building, creating a negative pressure.  Without a monometer, you have no way of determining how much pressure difference between inside and outside the fan is producing, but it may be enough to detect air leaks.  Another option is using exhausting equipment already present in the home.  By turning on the dryer and any bath fans and/or kitchen exhaust equipment that exhausts to the outside, it may be possible to create enough negative pressure to detect air leaks using smoke.  Using both methods together may also be beneficial. 

This photo shows a homemade blower door I tested on my own home.  The small fan was capable of moving 400 CFM of air, when combined with the home’s other exhausting equipment, the dryer and a bath fan, I was able to create almost a 10 Pascal difference between inside and out, enough to detect some air leakage.  When using the method of a homemade blower door, there are a couple safety concerns to be aware of.  First, when I conduct blower door tests producing either positive or negative tests, I turn off any combustion appliances, including space and water heating equipment.  If enough negative pressure is produced inside a home, even sealed combustion appliances can backdraft bringing carbon monoxide into the home.  After testing, make sure you check any standing pilot lights on heating equipment, they may have been blown out.  Second, don’t induce an intentional negative pressure with a fire burning in a fireplace or woodstove.  I could not perform a blower door test with a fire burning.  Make sure all ash is cool and cover or better yet, remove any that is present in a fireplace or woodstove.

Tools to find or see air leaks are an essential part of an energy auditor’s arsenal.  Thermal imaging is my #1 air leak finding tool, but smoke can be effective and is usually a less costly tool.

Leave a Reply

Your email address will not be published. Required fields are marked *