Air control: The balance of airtightness and ventilation
By Justin R. Wolf
Of the four main elements that we struggle to harness in our homes—earth, water, air and fire—none is more fickle than air.
This is because we can’t smell it or see where it gathers, lingers and escapes. Hot or cold, dry or humid, a healthy home controls not just temperature, humidity and airborne contaminants but the movement of air as well. And there are other related factors to consider.
WHAT DOES IT MEAN FOR A HOME TO BE “AIRTIGHT”?
An airtight home has been properly sealed top to bottom, creating a continuous air barrier (also called a “continuous air control layer”) from the cracks that exist between the concrete foundation and mudsill (or where wood framing starts) to the multiple penetrations made by plumbing, electrical and ductwork.
“In a typical wood-framed building, things are not tight,” says Jon Riley, owner of Casco Bay Insulation. “The drywall is cracked, corners are wonky, and door and window fenestrations are leaky in places.”
Air sealing prevents unwanted pollutants from entering your living space via the basement, attic, crawlspace or a connected garage. Without attention to air sealing, air will enter and leave your home through random gaps and cracks in an uncontrolled manner, moving much more quickly as the weather turns windier and colder.
A blower door test is the best method for finding the locations of major air leaks and quantifying the leakiness of the home’s thermal envelope. Common air leakage spots Riley advises sealing include major bypasses such as chimneys, utility chases, drop ceiling soffits and any place that’s adjacent to the attic where holes exist. And one thing you should know is that whatever is creeping in, it’s likely not the healthiest of air.
THE IMPORTANCE OF VENTILATION
It’s important to remember that airtightness and ventilation go hand in hand. Mechanical ventilation controls when and where outdoor air enters the home, and it can provide filtration and heat exchange as well. A well-sealed home that fails to include mechanical ventilation will likely suffer from moisture issues and a buildup of unwanted contaminants, resulting in an unhealthy interior environment.
Proper ventilation ensures that roughly equal amounts of supply air and exhaust air are moving through your home. Maine’s building code requires that any dwelling unit with an air infiltration rate of less than 5 air changes per hour when tested with a blower door (which would indicate relative airtightness) must include some form of mechanical ventilation, which may be accomplished with a continuously running fan (such as an efficient bathroom fan), an energy recovery ventilator (ERV), or a heat recovery ventilator (HRV).
Learn more about mechanical ventilation: Why exhaust-only whole-house ventilation isn't "enough"
AIR SEALING IS NOT THE SAME AS INSULATION
For starters, if you are renovating an existing house, you will want to get an energy audit to identify sources of air leakage. You will also want to identify insulation levels throughout the home and how well it’s performing. But it is important to know that a well-insulated house is not the same as an airtight one. These qualities are interrelated but not interchangeable. Some insulation products will add thermal resistance to walls and ceilings; some will stop air leakage in interior corners, wall cavities, behind cabinetry and elsewhere. But not all of them.
“You want to find the right balance” of insulation, ventilation and indoor air quality, says Riley. His preferred products and wall assemblies are dense pack cellulose in stud walls and a continuous outer layer of rigid foam board. “We remove fiberglass wherever we find it,” he says. “It’s full of dust and debris and sometimes rodents. It can become an indoor air quality issue.”
All told, Casco Bay Insulation is in the business of “fundamentally improving air quality and comfort.” To that end, Riley will work with all manner of insulation products and air sealing tech, but he prefers tried and true methods. “When I treat and air seal a house today, I’m using the same best practices that were around twenty years ago.”
Photos to the right showing before and after AeroBarrier® is installed. COURTESY PHOTO
NEW TECHNOLOGIES
AeroBarrier® being installed in a newly constructed home. COURTESY PHOTO
Incidentally, it was a little over 20 years ago when research funded by the U.S. Department of Energy led to the invention of an aerosol-based latex solution for air sealing homes. Today, products like AeroBarrier® and Aeroseal® use this technology to seal building envelopes and ductwork, respectively. The former works as such: Once interior openings (windows, doors, outlets, light switches) have been temporarily sealed and the home is pressurized using a blower door, the sealant compound is dispersed as a fog, and those particles build up around existing gaps up to a ½-inch thick to seal off air leaks.
“AeroBarrier can fill holes the size of a human hair,” says Alex Lord, owner of Lord Building Solutions in North Yarmouth, one of a few Maine contractors licensed to work with these products. He was drawn to working with AeroBarrier because of its potential to address a gap in the market of multi-family building owners who “want to dial in on an airtight envelope while dealing with budget constraints.”
For new homes being built to code, with adequate assemblies, insulation and air changes per hour, AeroBarrier may be a cost-effective option to ensure even better performance. And if sequenced properly during construction (Lord says he likes to get in “while the mud is drying”), it precludes the need to patch up utility openings and other holes in the home because they don’t exist yet. “It’s a very targeted approach to air sealing,” Lord says.
Michael Maines, a high-performance home designer based in Palermo, appreciates how Aero technology makes the benefits of air sealing accessible to more people. “It makes sense on new construction for crews that maybe don’t have experience building homes airtight enough to meet code,” he says. Maines also likes Aero products as a tool for tightening up existing homes. The prep work can be significant, he admits: The house must be unfurnished to avoid the compound sticking to obstructions, and the prep work can be considerable due to all the existing utility openings. Ultimately, he still thinks it’s worth pursuing: “I would strongly consider using it for my own house.”
Practically speaking, most homeowners will be better served by identifying their home’s biggest air control issues and addressing them one at a time. These culprits are typically found in attics, basements and chimneys. New windows or doors may be needed, as well as new or upgraded insulation, ventilating fans, air sealing tapes, caulking and so on.
In a healthy home, indoor air quality, ventilation, temperature and humidity levels are all maintained to optimal levels. Just how well-insulated and airtight your home is will directly impact those other factors.
This article appeared in the Spring 2025 edition of Green & Healthy Maine HOMES. Subscribe today!
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