From Drafty to Tight: Why and How We Air Seal Homes

By Brendan Kavanagh

Many homeowners come to us because they feel their homes have too many leaks, allowing their conditioned indoor air to escape and uncomfortable outdoor air to get pulled in. In winter, they know which rooms don’t get as warm as the others and where the floor feels cold even through their thick socks. Their heating bills are expensive, and still they need to bundle up inside just to be comfortable. 

Experiences like these are common in most homes here in New England. It’s often said that replacing outdated windows or adding insulation will take care of it, but that can make little to no improvement if pathways to the outside aren’t durably sealed up. Some leaks are easy to find, and some are easy to seal, but there are almost always many more that are harder to track down and deal with.

Air leakage is almost always the biggest driver of high heating usage – often estimated to account for 20 to 40% of heating energy use in the average home. In very leaky homes, the heating system may run constantly in very cold weather and still struggle to keep up. Air leaks also play a major role in comfort, indoor air quality, and durability. At Byggmeister, we focus on air sealing because if you don’t control how air moves through a house, it becomes much harder to control anything else.

What is Air Leakage? 

Air leakage is the unintended movement of air between the inside and outside of a home. It occurs through small gaps and cracks in the building enclosure — around framing joints, plumbing and wiring penetrations, attic accesses, chimneys, and transitions between materials. 

Air moves through these gaps primarily because of pressure differences. In winter, warm air inside the house naturally rises and escapes through the upper portions of the building. As that air leaves, negative pressure develops near the bottom of the house, pulling cold outdoor air inward. This helps explain why homes often feel colder on the first floor and warmer upstairs. 

Because air carries both heat and moisture, air movement affects more than just energy bills. It influences comfort, moisture behavior within building assemblies, and indoor air quality, often in ways that aren’t immediately obvious. 

Why Windows Get Unfairly Blamed First

We blame windows first because we like to spend so much time around them, to enjoy the view or the light or many other reasons. When people feel colder near their windows, they typically think it’s because air is leaking through or around them. While that’s often true, their discomfort isn’t caused by air leaks alone. What’s also happening is radiant heat loss – your body is giving off heat to the coldest surface around it which is typically the window glass. This is why you may continue to feel chilly around windows even after they have been sealed or replaced with tighter units. This sensation paired with the fact that we like being around windows makes them a prime suspect for winter discomfort, whether they are leaking or not.

This is particularly unfair because even leaky windows typically aren’t the biggest source of air leakage in a home. Most windows aren’t at the top or bottom of the home, but towards the middle. The middle is often called the neutral pressure plane because it’s in between the positive pressure at the top of the house and the low pressure at the bottom. The lower pressures around the middle means less air moves in and out when compared to pathways that allow air to move up through a ceiling to the attic or in through the basement or lower floor.  

Insulation and Air Sealing Are Not the Same Thing 

A lot of people assume insulation and air sealing do the same thing. They’re related, but they solve different problems. Insulation slows or resists heat moving through walls, ceilings, and floors. Air sealing controls air movement along with the heat and moisture air carries. A house can be well insulated and still perform poorly if air is moving freely through or around that insulation. Conversely, reducing air leakage can dramatically improve comfort even when insulation levels remain unchanged. 

This distinction is especially important in older homes, where insulation is often incomplete or inconsistently installed. Adding insulation without addressing air movement can deliver less improvement than you might expect — and in some cases can create moisture problems — because air continues to move freely through the assembly. When air is moving through a house, it carries a lot of heat and moisture with it. That’s why air sealing can make such a noticeable difference. The biggest gains usually come from addressing transitions — where walls meet roofs, floors meet foundations, or pipes and wires pass through the envelope.  

Why Air Sealing Tends to Hit a Limit

For the homes we work on, the baseline air leakage is often in the 10 to 20 ACH50 range, which is higher than what people might expect. At that level, air is moving pretty freely through the building, which helps explain why comfort issues are so persistent. 

Basic weatherization work provided through utility programs like Mass Save can help. In our experience, we’ve seen them reduce leakage down to around 9 to 15 ACH50 by addressing the leaks that are straightforward to access and seal. At this range, the house remains relatively leaky and many of the underlying comfort issues remain. 

It’s hard for programs like Mass Save to reduce air leakage further because they have to work within clear limits. The work has to be repeatable, relatively quick, and minimally invasive. Mass Save also relies on standardized materials and approaches that can be broadly applied.   

As a remodeling contractor, we can go further. Often, we’re opening up parts of the house as part of our renovation scope, and we can more easily create access to areas outside our renovation scope if there’s a good reason to. We can also spend more time diagnosing and addressing leakage pathways that are more challenging to find or seal.  

Using blower-door–guided air sealing, we’re able to see where air is actually moving and focus on the most consequential leakage pathways. With this approach, we’re often able to bring homes down to 5-8 ACH50. 

At that point, you start to notice the house behaving differently. Drafts are significantly reduced, and temperatures are more even. While this is much better, we know it’s possible to get homes even tighter but our projects are almost always partial renovations where a significant portion of the finishes are staying in place. Without removing all the interior finishes or taking off the siding and roof to add a continuous exterior air barrier, it has been difficult to find reasonable ways to get homes any tighter.

That practical limit is what led us to start using AeroBarrier. 

Why We Use AeroBarrier 

AeroBarrier is a whole-house air sealing system that uses an aerosolized sealant to fill small gaps and cracks in the building enclosure. During installation, the home is pressurized with a blower door while the sealant is released into the air. Wherever air is escaping to the outdoors, the sealant is carried with it and gradually builds up, sealing those openings from the inside. 

Aerobarrier can’t seal holes larger than about half an inch so we need to do a good job with manual air sealing before deploying it. Once our crew has sealed the larger holes, we can deploy this technology to reach pathways that would otherwise be inaccessible.  

Aerobarrier has some practical constraints though. After installation the airborn sealant settles out of the air onto upwards facing horizontal surfaces. Even with careful preparation, it’s nearly impossible to protect everything perfectly. For that reason, we can only use it in homes that are unoccupied and emptied – practically this means major renovations or maybe during a change of ownership. This limitation is the main reason we have only used it four times so far. Most of our projects aren’t extensive enough for our clients to move themselves and all their things out of their house.  

When we have used it, the results have been impressive and reliable. In each case, we’ve gotten air tightness to 1-2 ACH50 – which gets us below the code maximum for new construction of 3 ACH50

Older homes were built to be great at keeping the rain and snow outside but they weren’t designed with modern performance in mind.  Today we want our homes to provide far more comfort and do so with less energy and upkeep.  Air sealing is key to allowing us to control our indoor environment so it can be comfortable and affordable over time.

If you’re wondering how all this relates to your home’s mechanical systems – heating, cooling, ventilation, and dehumidification – that’s an important topic. It’s one we spend a lot of time thinking about on our projects, but not until we’ve done all we can to reduce air leakage.