FGA Presents at AIA National Conference

Wildfire Smoke: Systems for Air Quality in Single-Family Homes

FGA's practice is exclusively focused on the single family home, primarily in the California Bay Area. In 35 years of practice, none of the homes FGA has designed have been threatened by a wildfire, however, in the last 3 years nearly all of FGA's homes have been impacted by the smoke that these wildfires produce. Thus, it has become increasingly important for the design of new and remodeled homes to protect the quality of the air that is inside them. 

Meanwhile, current California code doesn’t address the impact of wildfire smoke on indoor air quality. The code requires houses to be constructed much more tightly than they were when construction was unregulated and actually demands that outside air be mechanically brought into the house - regardless of the quality of the air outside.

FGA's seminar is a synopsis of the work the firm has done to think through these issues to better meet client’s expectations of protecting the quality of the air inside their houses.

Objectives include: Identify passive and active indoor air quality management strategies and systems for existing and new homes; Recommend air-monitoring systems for occupants; Educate homeowner clients on the uses and activities that support or exacerbate indoor air quality; Knowledge of the efficiency and resiliency of employing indoor air quality management systems and strategies.

Wildfire Smoke and Indoor Air Quality - FULL TEXT


Our practice is focused on the single family home exclusively. Our work is located mostly in the Bay Area of California. In 35 years, none of the homes we have designed have been threatened by a wildfire - but in the last 3 years nearly all of our homes have been impacted by the smoke that these wildfires produce. 

During this time our current and new clients have begun to demand and expect that the houses we design for them also protect the quality of the air that is inside them. Current California code doesn’t address the impact of wildfire smoke on indoor air quality. The code does require houses be constructed much more tightly than they were when construction was unregulated. The Code actually demands that outside air be mechanically brought into the house to compensate for the air that previously permeated the house’s skin passively - regardless of the quality of the air outside.

This seminar is a synopsis of the work we have done to think through these issues to better meet our client’s expectations of protecting the quality of the air inside their houses and we’ll present a small project that we are using to test the efficacy of one solution. Learnings from that project will then be applied to our other larger projects. By way of introduction, we thought it would be helpful to do a quick review of the history of air quality to put into context the issues that we are dealing with, then walk through the wildfire threat, the physics of air and its particulates, code issues, some research that has been collected on indoor air quality - before sharing our test project with you. 

Air and Health

Air quality is fundamental to our health. This sounds so obvious to us. It was knowledge that has been known at least since 400 BC - Hippocrates made the Greeks and Romans aware that the air in crowded cities and mines could be contaminated. In the 1830’s Chicago adopted the motto the 'City in a Garden.' And Dr. John Rauch convinced the City’ to set aside 60 acres of the cemetery that then fronted the lake to begin what is now Lincoln Park. “By the middle of the nineteenth century, promenading along the lakefront to breathe in the healthful lake air became a popular activity.” Fredrick Law Olmsted, well known as the father of American landscape architecture, also became convinced that fresh air was key to our health. It was his growing understanding of how to connect people to clean air that in part convinced New York City to hire him in 1857 to clean out the huge trash dump that eventually became Central Park. Four years later, when Civil War broke out, President Lincoln signed the bill that created the United States Sanitary Commission, and it was Olmsted who was assigned to be the executive secretary of that Commission. Synergistically Louis Pasteur’s groundbreaking work that resulted in germ theory was published the same year, 1861. While this would eventually give the science to why the quality of the air we breathe is critical to our health, it took decades for the medical community to recognize and generally accept it. Cities were recognized as producers of bad air. Central Park became, as it was described, the “Lungs of the City”. Carving into dirty cities to create clean and green parks was and still is considered a powerful way to counteract the effects of bad air. And not just cities. Olmsted’s convictions helped him convince others to rethink the siting and design of hospitals, sanatoriums, other structures and landscapes as critical to sustaining the health of our country. When awarded the assignment to design the campus of Stanford University, Olmsted’s original plan was to spread the buildings out in the hills that are behind today’s current campus. This way the students and professors would benefit by copious amounts of light and air rather than organizing them into the traditional academic quadrangles that Mr. Stanford ultimately demanded Olmsted revise his plans to recall. Alas, I have been unable to find a sketch of Olmsted’s original proposal, the episode was politely referred to as “the gentleman’s disagreement” and recorded only in letters. 


Despite the growing understanding, the quality of our air was not defined and regulated until the events that occurred in Donora Pennsylvania. In 1948, nineteen people died from air polluted by the particulates emitted from two US Steel factories in that small town; the Donora Zinc Works and the American Steel & Wire company. Following the deadly smog, President Truman convened the first national air pollution conference in 1950 and Congress passed the first Air Pollution Control Act in 1955. My grandfather worked for US Steel in Pittsburgh at this time, and he brought a second white shirt with him to work in the morning to change into at lunch when his shirt became so soiled it was no longer white. While Donora anchored the understanding that industry was the cause of polluted air, it wasn’t recognized that there were other major contributors of pollution until Congress passed the Clean Air Act of 1970 and President Nixon established the EPA (Environmental Protection Agency), giving the agency broad responsibility for regulating the pollution from cars and other sources. The impact of these regulations on the country’s health since then can be demonstrated in several EPA studies including this one that correlated the amount of lead in gasoline and the amount of lead in our blood. Over a four year period, from 1976 to 1980, the levels of both were reduced by more than 50%. A remarkable outcome. 

Outdoor versus Indoor 

When outdoor air quality did become dangerous the public was advised to stay indoors and keep their windows shut. The implicit assumption was that there is a clear separation between the indoor and outdoor environments. Quoting from Mr Sundell’s 2004 paper on the History of Indoor Air Quality and Health: “…More [than] half of the body's intake during a lifetime is air inhaled in the home. Thus, [he felt that] most illnesses related to environmental exposures stem from indoor air exposure. Indoor air was believed to be a major environmental factor for more than a hundred years, from the start of the hygienic revolution, around 1850, until outdoor environmental issues entered the scene, and became dominant around 1960”. For the most part the legislation of clean air was focused nearly entirely on the outside environment - not inside our buildings and homes. It was axiomatic that the solution for bad air inside was to flush it out with good outside air. This is the assumption underlying California’s current inside air regulations. However, the wildfires remind us that the complete separation of the outside and inside can not be assumed. Bad air can get inside even the most tight of homes, and while most of us are willing to make some changes to reduce the contaminants we bring into our homes, few of us are willing to make larger changes without regulations requiring them. Our clients, it appears, are among the few motivated to do so. 

So. How do we design the house to best manage the quality of air within it? 

Next, we will break down the problem we are dealing with and share some of the research that has been collected on indoor air quality.

Breaking Down the Problem

What are the threats to human health that come with wildfire smoke? What are the physics of the air and particulates that we need to understand? How do current building codes interact with these threats?

Threat: Particulates 

Wildfires produce smoke that is made up of particulates including soot and other pollutants. Particulates as a type of air pollution are categorized by size because the smaller a particle is the easier it is to enter your bloodstream. Wildfires produce smoke particulates of a broad range of sizes, but this generally includes a substantial amount of particulates that are less than 10 and 2.5 microns in diameter. As a reference, 2.5 microns is roughly 100 times smaller than the human hair. Particles that are 2.5 microns or smaller are considered especially dangerous to human health because they bypass many of our body's defenses. They can travel deep into the respiratory tract, reaching the lungs and entering the bloodstream. This is especially true the longer one is exposed to these particles. Studies have linked increases in daily exposure to 2.5 micron particulates to increased respiratory and cardiovascular hospital admissions, emergency department visits and death. Long-term exposure to fine particulate matter may be associated with increased rates of chronic bronchitis, reduced lung function, asthma and increased mortality from lung cancer and heart disease. People with breathing and heart problems, children and the elderly can be particularly sensitive. 

Threat: More Unhealthy Air 

This chart shows the Air Quality Index for Sonoma County in California last year (2021), which had a number of wildfire events. You can see the numbers vary from Good to Moderate for most of the year, and then a few weeks of Unhealthy air due to wildfires in September and October.

Threat: Bigger Fires 

Historically the number of acres burned by wildfires is increasing. This EPA chart shows the general ‘upward to the right’ trend of the growing number of acres burned over the last 40 years. 

Threat: More Smoke 

Professor Marshall Burke and his team at Stanford have been compiling data on Wildfire smoke exposures. This map shows two 2-year snapshots of the mean ambient wildfire smoke that are 10 years apart. The yellow and orange areas show the intensity and extent of smoke exposure, which in this particular comparison is moving from pockets of the West Coast to most of the Western and Mountain West of the US. “Smoke is now responsible for half the annual pollution in most of Northern California”. And it is becoming more prevalent across the country. 

Threat: More Smoke Inside 

These maps were produced by Professor Burke's team and document the amount of particulates that accumulate inside a home from wildfire smoke. What is of particular interest in this study is that Professor Burke has not found any statistically significant correlation between the Air Quality inside houses and household income or the airtightness of the home. 

Threat: Complicating Issues - Inside Combustion and the Occupant 

The quality of indoor air is further complicated and degraded by the particulates that are produced by combustion inside the house; burning wood in a fireplace, or burning gas or propane at a cooktop that is improperly vented to the outside. Additionally the homeowner themselves will bring particulates into the house through their normal, typical and daily use of their house. Dust and pollen on their shoes and clothing, dust mites and mold on their pets, for instance. We managed to get some data during an actual wildfire event from a friend who has an air monitoring system as well as a good air filtration system. We’re looking at indoor air quality over the course of a couple of days during the September 2021 wildfire events that polluted most of the Bay Area. This happens to be a Passive House home– so it’s built and tested to be very airtight – less than 0.6 Air changes an hour at pressure difference of 50 Pascals, whereas a typical home measures about 10x that at 5-6 Air changes per hour at 50 Pascals. It’s also equipped with a balanced heat recovery ventilator system with a MERV 13 filter, which in this case is turned off. Note the spikes in high AQI corresponding with activation of clothes dryer and Bathroom fan. When we have exhaust fans that induce infiltration in order to function, that infiltration is bringing in smokey air into the home. Any management of indoor air quality must include the complicating factors of the ventilation needs of the Occupant’s use of the house as well. 

Physics: Passive Issues - Infiltration vs. Exfiltration 

Lets review the physics that will structure any solution that we come up with to manage wildfire smoke. Smoke comes into our buildings through infiltration. Infiltration requires exfiltration. For air to come into a house, it has to go out. Infiltration and Exfiltration occur when two conditions are met: First– There is an open air flow path between the inside and outside, and Second– there is an air pressure difference between the two sides In this diagram, we have this generically indicated the opening but in reality is an open window or a crack around a non-weatherstripped door. 

Physics: Passive Issues - Wind driven infiltration 

There are two main sources of natural pressure. The first, and perhaps most intuitive, is wind. The second is stack effect. Wind blows on the windward side of a house and induces a positive pressure difference (relative to the inside) on that side resulting in infiltration given an airflow path On the leeward side, there’s a relative negative pressure difference resulting in exfiltration. 

Physics: Passive Issues - Stack effect infiltration 

Stack effect can be described as thermal buoyancy. Warmer air is pushing out of the top during the winter season with cooler outside air coming in lower in the house. The reverse happens in the summertime cooling season, where cooler more dense air is inducing a positive pressure at the bottom of the house with hot air infiltration towards the top.

Physics: Active Issues - Mechanical Ventilation 

Here are the three typical examples of mechanical ventilation systems that actively move the air from one place to another – bathroom ventilation exhaust fans, Kitchen hoods, and clothes dryers. Anytime these exhaust fans are running, makeup air will come through any opening available– including both unintentional leakage and intentional pathways though the exterior walls of the house.

The Building Codes

 In recent years, building codes including Title 24 in California, and more recent forms of I-codes have incorporated mandatory continuous ventilation systems into new homes. This follows a decade or more of these requirements being prerequisites for green building certification systems such as LEED. The most common standard that is referenced is ASHRAE 62.2 The typical approach for a house is to use one or more bathroom exhaust fans and run it continuously >all day/everyday/throughout< the year to induce fresh air infiltration in the home and dilute indoor air pollutants. In the next few slides, we’ll see why. I want to emphasize that these ASHRAE standards do not mandate that homes have exhaust ventilation. They simply require continuous ventilation and are currently agnostic on the approach. The most common approach we’ve seen to comply with those requirements, is an exhaust fan. This is a chart that compares the amount of 2.5 micron particulate matter that is in homes with and without continuous mechanical ventilation. The further data is to the right, the more particulate matter. Note: this isn’t outside wildfire smoke – this is interior generated and/or imported particulate matter. The black line of triangles is further to the right and shows that homes without continuous ventilation have higher indoor levels of particulate matter than the blue line of circles that shows homes with a continuous ventilation system. There is similar data for other kinds of indoor air pollutants such as VOCs and formaldehyde. Continuous ventilation systems are generally positive and a healthy thing to put into a home. So long as the outdoor air quality is good!

Summary of Learnings 

Let's summarize our learnings. Outside particulates enter the house because of pressure differentials which drive infiltration and exfiltration through three types of pathways: Intentional openings such as vents and hoods, Controlled openings such as windows and doors, and Uncontrolled, which are unintentional air pathways in the building assemblies. There’s a mantra in the building science world which I’m sure most of you have heard– ‘build tight, ventilate right’. Building tight seems obvious enough, both from the perspective of energy efficiency and comfort, but also in this additional framework of smoke infiltration. 

If we build tight, we can substantially reduce the amount of infiltration from wind and stack effect pressure differences. And, to be clear, we don’t believe that we can build perfectly airsealed houses, but we can sure do a lot better. The least we can do to manage smoke infiltration is reduce these unintentional openings to any practical extent. 

If we ‘ventilate right’, we can substantially reduce the various sources of air pollution inside the home that we need to exhaust to the outside. Consequently it is important to think about how to provide makeup air from intentional openings as opposed to unintentional ones. 

We need to find ways to compensate for the limitations of current building code to manage these episodic smoke events. The typical exhaust fan approach to managing indoor air quality will likely have deleterious results on smoke infiltration if not deactivated. 

Test house - Strategies and Solutions 

The first project that began our research and development of how to deal with wildfire smoke was a request about two years ago to design a small house that a recently retired couple asked us to design in Novato California, so they could be close to their grandchildren. Novato is about 40 miles north of San Francisco, at the top of the Bay. The clients for this project were particularly concerned about their health given its location at the bottom of the Napa / Sonoma wildfire smoke channel. Additionally, the site is up against the hills and is itself under direct threat of wildfires in the Wildland Urban Interface area.

The house is a new 1,200sf ADU on a 13 acre rural site and is designed to complement the existing 100 year old farm house that is on the property that is occupied by one of the client’s children and their grandchildren.

Lets walk through the strategies and solutions that we developed for this house; it’s Passive and Active Systems, and owner/Occupant operation of the house. 

Passive Strategies - Tight House 

Key to our passive strategy is that the building needs to be tight. To make the house air-tight in a cost-effective way, we’ll employ many of the guidelines and construction techniques that are found in the Passive House certification system, including performing a blower-door test to benchmark air-tightness of the home during construction to achieve a maximum 1 Air change per hour at 50 pascals. 

Passive Strategies - Eliminate or Reduce Indoor Combustion 

The house will be all-electric to eliminate the standard types of internal combustion. No gas water heaters, no gas stoves or ovens, no gas dryers, no fireplaces. 

Passive Strategies - The Homeowner 

The wide porches around the house add to the utility of the small house, allowing for the overflow of interior activities onto protected outdoor areas making the small house feel larger than it actually is. The covered porches also protect the walking surfaces from airborne detritus; like pollen and leaves, to some degree. The paving provides walk off space before the homeowners enter the house. And it creates a relatively protected space to locate controlled air pathways into the house. Further, the house's entry is smaller and compartmentalized from the living spaces in another effort to keep outside airborne particulates from entering the parts of the house that the occupants spend more time in, the gathering and resting areas. 

Active Strategies - Scaling the Solution 

Filtering the smoke out of the air is the solution to manage indoor air quality. What are the pros and cons of the various ways to accomplish this? Can we combine different systems to develop solutions that address the circumstance being solved for? Here are the systems we evaluated: Room fans with HEPA filters. Deploying room fans w HEPA filters during a wildfire smoke event and closing up the house, turning off any and all exhaust fans, and sheltering in the house in a more dormant state is an effective and practical short term way to manage a smoke event. Smoke will infiltrate, but the room-by-room HEPA fans will keep concentrations down. There are several downsides to this approach: It is important for the occupants to resist compounding the particulate problem during smoke events, by inducing infiltration through their household activities– i.e. cooking and laundry. By not allowing the air to cycle, other indoor air contaminants may steadily creep up over time– i.e. off-gassing of finishes and furniture, resulting in compromised indoor air. For good filtration, room HEPA fans need to cycle through a lot of air, and this means they get noisy. The room fans are not long lasting; they do not have the life of dedicated systems. And, They do not impact the infiltration of particles through the uncontrolled openings of the house. They simply collect those particulates as they inevitably enter. HVAC + HEPA filtration. Adding HEPA filtration to standard HVAC systems is an obvious and good next step. This should address some of the noise issues, and it is certainly a more permanent solution, but will not address the other issues. HRV + HEPA. Integrating HEPA filtration into the HRV system solves the cycling problem by bringing in filtered outside air but it does not solve the uncontrolled infiltration of smoke into the house. Supplemental Supply Air + HEPA. Adding a Supplemental Supply Air fan with HEPA filtration solves the problem of uncontrolled infiltration of smoke into the house by slightly pressurizing the house with filtered outside air. 

The solution we are testing in the Novato house combines the last three systems in addition to the standard HVAC system. Here is what we did: 

Active Strategies - Typical HVAC systems 

The house will be equipped with the typical HVAC systems standard kitchen hoods, bathroom fans, and a clothes dryer but they will be electric not gas. The house’s heating and cooling will be provided through a multi-split heat pump system, where one outdoor unit is connected to 2 indoor ducted air handlers. Air filters will be integrated into these air handlers so that they can filter and clean the recirculated air. For this project the exterior electric heat pump is a Mitsubishi PUMY and indoor units are two ducted air handlers with MERV 13 filters, though the client may upgrade to HEPA here. The kitchen hoods, bathroom fans, and a clothes dryer will all exhaust through the vent chimneys on the roof.

Active Strategies - Heat Recovery Ventilation 

The code required ASHRAE ventilation will come from a Heat Recovery Ventilators (HRV) that provide continuous efficient ventilation for much of the year when outdoor air is ‘good’. The intake air is taken from high up on the wall under the protecting patio roof. The conditioned outside air is then delivered into the supply air ducts. HEPA filters have been specified on the supply side of HRV. 

Active Strategies - HEPA Supplemental Supply Air

HRV’s with HEPA filters will be able to provide filtered fresh air to the home. However, they will not keep smoke from entering the house through natural and induced infiltration during a wildfire event. To deal with wildfire events we are adding a Supplemental HEPA Supply Air system that will slightly pressurize the house to keep smoke from entering into the house through the uncontrolled air passageways and to better support the occupants use of the house during these events as they enter and exit the house. It should effectively eliminate any smoke infiltration from unintentional openings by overwhelming any infiltration that would be induced by wind or stack effects. During its operation, we’ll be pressurizing the house with filtered outside air, and the building will therefore be continuously leaking air out of any cracks or holes.

Active Strategies - The Homeowner

 HRV systems are not designed to add pressure to a house, they are designed to trickle fresh air into the house continuously and quietly along with a equal amount of stale air exhaust. When there is a wildfire event the HRV system in the house will be shut down and HEPA Supplemental Supply Air system will be activated. These are complementary systems—they would not run simultaneously. The HEPA fan system supply air is necessarily unbalanced, so it will not have heat recovery, which is why we wouldn’t use it for most of the year. We haven’t found a ‘smart’ unit on the market yet to automate this switch over that’s appropriate for the residential market, so the homeowner is motivated to take on this responsibility and will be educated on how to use and modulate these systems. The Supplemental Supply Air fan + HEPA system will also be used to modulate supply makeup air to compensate for exhaust air from bathrooms, laundry, and kitchen activities. For this project we are using a Pureair Systems 600HS Plus variable speed HEPA supply fan..

Active Strategies - Controls

 And, because ‘you can’t manage what you don’t measure’, thank you Peter Drucker, the house will be equipped with indoor and outdoor monitoring systems. These monitoring systems are not just for air quality management during wildfire episodes, but also to track the other ambient air quality episodes and personal activities that generate or import pollutants inside the house. These air monitors have associated apps where the occupants can readily track data and received alerts. For this project we are using Purple Air monitoring devices. 


Passive and Active strategies combined with an owner who is motivated to use and operate their house to manage the indoor air quality is the strategy we are pursuing. The efficacy of this approach will go into testing mode when this project is completed later this year and the homeowner moves in. The data this house produces will be shared with Professor Marshall Burke and his team at Stanford. Additionally, we are doing another research project with Professor Burke’s team where we will be placing interior and exterior air monitoring devices into a portfolio of our past projects to better understand and model the active performance of the house and their occupants over time. While the research hasn’t yet been designed, we would love to come back here with some results as to these other ventilation and filtration options to provide some insight into what’s necessary and sufficient. 


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