If you’ve been studying the concepts of building science, you’ve probably heard of the four control layers, water, air, vapor, and thermal. Thinking about each of these building envelope control layers individually, and how they interact together helps in the planning and execution of building a better home.
This concept of separating a home’s assemblies can also be applied to a different topic, the air inside a home. Most of us probably take air for granted, we breathe it, it’s always around us, but with regards to inside our homes, airflow needs to be managed. I recently caught a presentation by Pat Huelman, a respected building science educator and researcher from the University of Minnesota. Part of his discussion focused on the need for projects to include an “air manager”, someone paying attention to how air interacts with the built environment and mechanical equipment. He discussed the importance of four different types of air in our homes and how each contributes to the quality of the indoor environment, energy conservation, and durability:
- Combustion Air
- Make-Up Air
- Ventilation Air
- Circulation Air
Air is 78.1% nitrogen (N₂), 20.9% oxygen (O₂), 0.93% argon (Ar), 0.04% carbon dioxide (CO₂) a varying ratio of water vapor, along with several other gases of lesser percentages. Even though the composition percentage of the different gases in air is usually the same for each air on the list, we need to think about how the air is used for each of these processes differently. Let’s discuss each of these individually, starting with the two I consider the most important, air that can quickly affect a person’s health if not addressed correctly, combustion air and makeup air.
Combustion air is air required to support the combustion process, which might include space and water heating, cooking and even close drying. For combustion to occur, we need oxygen, a fuel source, and heat or a spark. Where the air containing the oxygen comes from to support combustion is important. Hopefully the air supplying the combustion process is coming directly from the outside, through a dedicated vent. Sometimes the vent is tied directly to the heating equipment, sometimes it’s just in the vicinity of the equipment. Most new heating equipment is sealed combustion, there is a dedicated pipe directly connecting the heating equipment to the outside, the equipment does not use ambient indoor air for combustion. Some older heating equipment and many temporary heat sources use ambient indoor air for combustion.

If a combustion appliance does not receive enough combustion air, incomplete combustion is the result, causing an increase of carbon monoxide. CO is an odorless and colorless gas that can lead to health issues and, if in high enough quantities, death (and the reason combustion air leads my list in importance). The heating equipment also begins operating less efficiently, resulting in increased energy costs. Inadequate combustion air can also result in increased maintenance costs and reduced equipment lifespan.
This proper quantity of combustion air has been addressed for decades in building codes. We use a simple formula to make sure the area around the combustion appliances has enough air to support complete combustion. This formula is 50 cubic feet of air per 1,000 Btu/hr of appliance input. For instance, an atmospherically vented water heater might have a fuel input of 40,000 Btu’s/hr. We need 2,000 cubic feet of space in the combustion appliance zone (CAZ), the open area where the appliance is located. If the water heater is located in a 4-foot x 4-foot x 8-foot-tall utility closet without any other appliances and a door that closes to the space, we would only be providing 128 cubic feet, not nearly enough to meet codes. We would have to supply the closet with more combustion air, either by a dedicated vent of the correct size to the outdoors, or by way of connecting more of the living space to the utility closet. A louvered door or some sort of jump duct connecting other spaces to the mechanical closet might suffice.

Of course, if we do not have any combustion appliances within our home, we have nothing to worry about, the discussion about combustion air is moot.
A recent article I wrote that further discusses combustion air (and the next section, makeup air) can be found at: Considerations When Adding Exhausting Equipment to an Existing Home – Northern Built
Makeup air is our second of the four airs. This is air needed to maintain a neutral (or close to neutral) pressure within the home with respect to outside air pressure, it is accomplished by actively or passively transferring outside air into the home, to replace any exhausted interior air. This is especially important when a home has an atmospherically vented appliance, a fireplace or when a home has a very tight building enclosure. We do not want exhaust from a combustion appliance to back draft or spill back into the home.
Every cubic foot or air that leaves a structure (exfiltration) is replaced with a cubic foot of air coming into the structure (infiltration). Exhausting equipment such as dryers, bath fans, and kitchen range hoods all can produce negative pressure inside the home. Building codes realize this is a potential problem and require makeup air systems when an exhaust fan removes more than 400 cubic feet per minute of air from the home, and there is “one or more gas, liquid, or solid fuel-burning appliance that is neither direct-vent nor uses a mechanical draft venting system is located within the dwelling unit’s air barrier…” That code section is directly from the 2021 IRC, section M1503.6 Makeup air required. If a makeup air system is required, the makeup air must be approximately equal to the exhaust air rate, have at least one damper operating either by gravity or electrically when the exhaust fan operates, and the makeup air must be discharged in the same room as the exhaust fan or used ducts systems that communicate with the room.
Another area where makeup air is addressed in the building code is in section G2407.4 (304.4) Makup air provisions. Where exhaust fans, clothes dryers and kitchen ventilation systems interfere with the operation of appliances, makeup air shall be provided.
What happens if we have a very tight encloser and no fossil fuel or solid fuel burning appliances? Can a home become negatively pressurized without any issues? This is a question I asked of several building scientists and physicists. Gary Nelson from The Energy Conservatory, (you can read a full interview I had with him here) one of the people who began manufacturing Minneapolis Blower Doors back in the early 1980’s told me he’s not worried if a home goes negative for short periods of time. The occasional use of exhaust fans and dryers that can produce negative pressures of -25 Pascals, -50 Pascals or more for an hour or two are usually of little concern, as long as the home does not have an atmospherically vented or solid fuel appliances (fireplace or woodstove). The one issue that could arise is when a home is very tight, the expected exhaust rate of fans and dryers might not be realized. In the case of a dryer, it might take a little extra time for the clothes to dry.

To remedy this situation, you may want to consider moving to heat pump condensing dryers and using balanced mechanical ventilation (ERV or HRV) for exhausting bathrooms. That leaves kitchen exhaust systems as the only fan capable of producing the larger negative pressures inside the home. Knowing the CFM50 results of a blower door test (and the building leakage curve) can provide important information showing the risks of depressurization. (There will be a future post discussing the building leakage curve soon.)
Ventilation air is arguably the most important air in a building. I agree with that when there are not atmospherically vented appliances to worry about. Ventilation air is needed to dilute indoor air pollutants. This air is solely for the health of the occupants (or as Pat likes to call it, to meet the metabolic needs of the occupants.) Ventilation air, in the right condition, can also help control indoor humidity levels.
Before we start with ventilation air, I think it’s important to discuss indoor air pollutants. The first strategy is to eliminate any polluting items in the home when possible. I like Dr. Joe Lstiburek’s saying; “the solution to pollution isn’t dilution!” Get the bad indoor air quality stuff out of the home. Eliminate the bad smelling stuff instead of trying to ventilate the smell away (or even worse, cover the odor with another, supposedly better smelling odor).
What are the indoor air pollutants we are concerned with? Volatile organic compounds (VOC), particulate matter (PM2.5 and PM10), radon, and some might say carbon dioxide (CO₂). Let’s start with CO₂, which is the gas we breath out, it is also a biproduct of combustion. Outdoor air naturally has a concentration of around 425 PPM (and rising), we typically want to see indoor air CO₂ levels at 1000 ppm or less, but humans can tolerate higher levels without major issues. Submarines have levels between 5,000 and 7,000 ppm. The thing with CO₂ is it’s a good marker for where other indoor air contaminant levels might be. If we use ventilation air to maintain CO₂ levels at or below 1,000, there’s a good chance the other potential contaminate in the home are also below worrisome levels.
VOC’s are tough to qualify, they tend to be a catchall for several chemicals that might be bad for us. Paints, refrigerants, solvents, formaldehyde, and other chemicals are all considered VOC’s. Body odors, dirty dishes, and many more things typically found in a home can all contribute to elevated VOC levels, all can affect human health. Eliminating the VOC’s within the home is the best choice, but ventilation at a proper rate is also helpful. VOC’s are recommended to be below 400 ppb.
PM2.5 and PM10 are small, airborne particles that we can breathe in. The smaller particles can become stuck deep in our lungs, some can even bypass our lungs and end up directly in our bloodstream. Ventilation air can help, but poor, unfiltered outside air can affect indoor air quality. A combination of filtration and ventilation is often the cure. In May of 2024, the EPA lowered the annual average PM2.5 level from 12.0 to 9.0 µg/m³, the PM10 limits are 150µg/m³ in a 24-hour period and not to be exceeded more than once per year on average over a 3-year period. I’ve conducted short-term testing in a home (a young family with an infant) with levels in excess of 60 µg/m³. The EPA considers PM2.5 levels of 55.5 to 125.4 µg/m³ as “unhealthy”.
Radon is the last indoor air quality concern I’m going to discuss in this post. Radon is a colorless, naturally occurring gas that comes from decaying radium atoms, usually found in granite. The gas often accumulates in basements and crawlspaces but can be found in any home with a connection to the ground. Modern building codes have requirements for venting radon out, either passively or actively. This process can also be retrofitted in existing homes. Radon levels are recommended to be below a yearly average of 4 pCi/L. An effective ventilation strategy can also help reduce radon levels.
How do we get ventilation air into our homes? There are three main strategies, supply only, exhaust only, and balanced. My state of Minnesota has required balanced mechanical ventilation by code since the early 2000’s, and in my opinion, is the best of the three options. The 2021 and 2024 IRC model codes have started requiring balanced mechanical ventilation, the 2021 requirements included climate zones 7 and 8, the 2024 added climate zones 6.

Next question, what is the ideal ventilation rate? ASHRAE 62.2 covers that topic in depth. Personally, I’m looking forward to having the ability to measure the quality of indoor air and adjust ventilation rates based on those readings. Some ventilation equipment currently includes that technology, more should be on the market in the coming years.
How do we know the quality of our indoor air? We measure, and preferably over a long period of time. There are several indoor air quality monitors available to homeowners, the cost for a decent model is between $150 and $300. Most will measure CO₂, PM2.5 (and sometimes PM10), VOC’s, temperature and humidity. Some have the capability to measure Radon as well. Are they accurate? That’s a good question, they are not laboratory instruments, but they give us some idea of the quality and trends of the air we are breathing.

The last air is circulation air. This is air inside the home that is heated or cooled, humidified or dehumidified, filtered and moved around the home. It’s primarily used for comfort, though filtering does help our health. It’s the most expensive air in the home, we pay to condition it, we want it to stay inside the home as long as possible. When we are required to bring in the other three airs, we pay an energy penalty, the “new” air entering the home now needs to be conditioned. Finding the right balance of only bringing in what is needed is key, and we don’t want unneeded air exchange by way of unintended holes in our building envelope. “Build it tight, ventilate it right!”
My view of the air inside our buildings changed after Pat Huelman’s presentation, much like my view of the four building control layers changed many years ago when I started down this rabbit hole called building science. By separating the different airs inside a home, we can begin to see they all have different purposes, and just like our understanding of the four control layers, we also learn how they interact and impact each other. At least one has the potential to be eliminated, switching heating fuels and/or fuel venting systems (atmospherically vented to sealed combustion) can eliminate the need for combustion air, one less “air” to worry about.
This post originally appeared on the Green Building Advisor website.