3 Myths About Indoor Air Quality (IAQ)

Misconceptions About Air Quality and Home Performance

Recently I wrote a blog about the October 2017 firestorms in Northern California. I live and work in the city of Sonoma and was directly affected by the fires. My family and I evacuated our home and waited out the fires on safer grounds until the danger passed. We are OK, life has resumed, but the impact on the region will last for several years.

In my last post, I wrote about how air quality was impacted in the area, and how people responded by using N95 face masks to protect themselves--a strategy that will be very familiar to home performance professionals. While living through this experience, I noticed a number of other things about how people responded, which brought to mind some misconceptions many people have about air quality and home performance that I wanted to talk about.

I learned many things through this experience. Watching the hills surrounding our city burn for several days with absolutely no effort to put them out was surreal. We later learned that in major natural disaster situations, responders worry first about people, then property, which makes sense. Due to the size and number of fires, emergency resources were stretched beyond their capacity. For the first couple of days, the first response crews were focused on evacuations, not fighting the fires. After the evacuations were in motion, the crews were able to shift their attention to fighting the fire.

When a fire of this size burns, essentially unchecked, it produces a tremendous amount of smoke, which in many cases has a greater impact on people than the fire. Smoke is why the majority of folks I know left town, and smoke is still the biggest issue people are dealing with after the fire. There has been all kinds of information circulated in the community about how to deal with the smoke, the smell, and the residue it has left behind. Having some background on residential indoor air quality, it was interesting to see what people were being told and how they decided to take care of the problem.


I am fascinated by the ideas that people have about their homes. For example, during the fires, people were told to stay inside, close the windows and doors, and put their air conditioning system on recirculate in order to keep the smoke out. This was good advice for the most part, but it also created some serious misconceptions.

First off, people think of their homes as hermetically sealed boxes, and believe that closing them up will keep all the nasty stuff out. The reality is that most houses are pretty leaky and have much more communication with outdoor air than you might imagine. Air leaks from doors and windows are a factor; a bigger issue is the outside air that enters from holes in the floor and attic. Perhaps the largest source of infiltration is leaky duct systems.

In California, ducts are typically located in the attic or crawlspace. As the system runs, even in recirculation mode, air is pulled into the home from outside. How much? believe it or not, the average duct system in California leaks between 30 and 50 percent. The bottom line: closing up your house and recirculating the air with your forced air system is better than being outside, but it’s really not a great solution if the contaminant level is extreme. If you have a tight duct system and good filtration, closing up the house can be very effective, but very few houses are that well sealed.

I did have a former customer in the area who built a very tight home. We air sealed the entire house as it was being built, packed the walls with dense packed, blown-in cellulose and shaved it before the drywall was installed. The floor was a slab and the attic was air sealed and insulated with blown-in cellulose. The last piece of the puzzle was a Heat Recovery Ventilator (HRV) which included a MERV 12 pre-filter and its own dedicated duct system. Shortly after the fires, this client contacted us to report how the entire neighborhood ended up in his house because it was the only one in the area that didn’t smell like smoke. So in this case, staying inside and recirculating the air was effective, but if your home is any less tight than that and doesn’t have a dedicated, filtered source of fresh air, I doubt simply staying inside the home with the fan in recirculation mode would be very effective. It’s better than being outside, but if the risk is extreme I would not rely on this approach.


During and after the fires, people were spreading all sorts of interesting information on the Internet. One thing I saw over and over was people buying a 16” box fan and then taping a paper furnace filter onto the supply side of the fan. There were a lot of variations on this theme. Some people got pretty fancy, taping several filters together and making a box to draw air through.

Several people posted comments and pictures about how effective this solution was. A few stated that running the fans for a couple hours completely removed the smell of the smoke. I would like to believe that this approach was effective, but the tech geek in me questions its ability to have a serious impact. First off, the cheap furnace filters people taped to the fans are not able to catch the small PM 2.5 particulates, which are the most dangerous, as they can directly enter your bloodstream through your lungs. I am sure that the filters did catch some of the larger particles, but I question whether it was enough to really be successful. The most likely reason people found this approach to be effective was due to the principle of dilution.

Many people were suggesting that homeowners install new furnace filters before they run their systems. I totally support this approach, but I have a different perspective than you might expect. Many people are convinced that the allergen or microbiological filters are the best option, while others like to promote the washable filter approach.

My advice: buy basic paper furnace filters and change them often. In normal operating conditions, I would suggest that you change your filters monthly. For the first few weeks after a wildfire I would advise changing them weekly. I believe it’s better to change your filters more often than to buy a more expensive or washable filter. This is because basic quality filters tend to be thin and less restrictive. Airflow is key for performance and comfort in forced air systems. Anything you that reduces airflow, such as a restrictive filter, decreases performance. If you look at the allergen filters and other options, you will notice that they are often very thick or dense. While this may capture more particles in theory, the minimal benefit you get from improved filtration isn’t worth the cost of reduced airflow.


This leads me to the next topic, which is “dilution is the solution”. This has long been a mantra for indoor air quality. The best thing you can do to improve indoor air quality is to flush the air in the home. Now that the air is clear outside, I am suggesting that people put their systems on “fan only” mode, open the windows, and run the system for several hours to flush out the duct systems and the rest of the house.

Many people have been asking me about duct cleaning services. In general, I am not a fan of duct cleaning. I have seen cleaning jobs destroy duct systems on more than one occasion. Many duct systems are 40+ years old and are held together by tape and good will. If a cleaning service runs a brush down the ducts, they often break apart or fall off all together. Some services use forced air to clean the ducts, which I suspect creates less physical damage, but is still a concern.


I think the best solution after a fire or smoke situation is to replace your duct system entirely. You don’t have to be in this business very long before you recognize that duct systems are typically not done very well. In California, the furnaces are too big and the ducts are too small. Replacing your duct system creates an opportunity to install new ducts that are properly sized and better insulated. Making sure your ducts are sealed is important too. If your new ducts are installed with a permit, they must be sealed to leak 5 percent or less leakage (CA Title 24 code). If you don’t get a permit, you should still require the contractor to meet the 5 percent sealing target.

Ducts are not cheap or easy to replace, but this could be a perfect opportunity to get a great bang for your buck. If you consider that the average duct system leaks 30-50 percent and is undersized, replacing it with a new duct system that is sealed to 5 percent leakage represents a 25-45 percent improvement in efficiency. Improved insulation plus better sizing will absolutely make your home more comfortable. Even at an average cost of $5,000, it could be the best value per dollar for energy savings in the long term. If you are filing an insurance claim, I encourage you to use this opportunity to replace your ducts.

In summary, fire and smoke are serious, and the contaminants they leave behind are difficult to remove and can impact your health. Your home is most likely not as tight as you think, which means that those contaminants are making their way inside. Replace your furnace filters often, and if you have the chance, replace your ducts. These principles apply across the board and are a few simple best practices that will go a long way to improving the indoor air quality in your home.

Charles Cormany
Executive Director
Efficiency First California

Image by Charles Cormany


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