Air Sealing Basics
But before we dive in:
What even is air sealing? And why does it matter?
Air sealing, or airtightness refers to the process of preventing air from moving in between two spaces. It exists on a spectrum. On one end of the spectrum you have NASA rocket ships going to the moon- 100% airtight (I mean, think about what would happen to that rocket ship and the astronauts on board if it wasn’t 100% airtight and it got all the way to space…). On the other hand of the spectrum, you have something like a picnic shelter. Sure, it may have a roof and maybe even a few walls, but its open-ended design will allow air to freely come and go. And this can be really pleasant- who doesn’t want a cool breeze on a sweltering summer day as they sit in the picnic shelter at someone’s graduation party?
However, most houses and buildings like to be further away from the picnic shelter on this spectrum and closer to the rocket ship.
Why?
A lot of reasons! A tighter house will have lower utility bills, a more stable year-round temperature, less insect entry points, less moisture buildup, and a longer lifespan.
So, air sealing exists on a spectrum. And if you are building a house, remodeling or renovating, you can decide how much airtightness makes sense for you and your needs (while at least meeting the minimum requirements to pass energy code).
At Thermal Shop, we love to think of houses and buildings as systems with many parts that all work together to bring about a desired outcome. If one of those desired outcomes is low air leakage, then many different stages of the building process and many different parts of the structure contribute. Although airtightness is a major purview of insulators, it actually starts way before the insulation stage of any project.
Air Sealing at the Framing Stage
If you want a tight house, start with a square house. Use high quality lumber- lumber that doesn’t bow and isn’t curved around the edges. Using bowed or curved studs creates extra gaps where air could potentially enter or escape after construction ends.
We know that lumber comes from trees, and that trees are not “perfect” (by which I mean: straight and square. Because trees are obviously perfect in their perfect tree-way ❤️❤️❤️). A competent framer will know what kind of lumber to look for, and how to make the best use of the lumber they have. Tight and square framing can go a long way in contributing to the overall airtightness of a house or building.
Another important factor at the framing stage is the weather barrier surrounding the exterior shell. Weather barriers tend to be house wraps (like Tyvek) or wall sheathing (like Weather Logic Blueboard). Many weather barrier products go up without being sealed at the top or bottom, giving air plenty of space to move in and out of the house or building.
I’ll probably say this a few more times over the course of this article: airtightness problems don’t tend to come from one or even a few large gaps. This is death by a thousand cuts: hundreds of tiny gaps and spaces that weren’t properly sealed leading to air moving rather freely in and out of a home. It may seem like a small deal but it actually isn’t: terminate the top and bottom of your weather barrier product. Tape all of the seams. And, for bonus points, seal all nail and staple penetrations as well.
If you want to see these types of air barriers in action check out the “Structure: Exterior Shell & Sealants” section of Timber Creek’s portfolio page.
Air Sealing at the Insulation Phase
After mechanical, electrical, and plumbing are all complete, the (hopefully) squarely-framed house is now rife with holes, gaps, and penetrations that need to be sealed with your two new best friends: caulk and can foam.
To get you started, here is a non-exhaustive list of areas that need to be sealed with caulk or can foam:
Traditional Caulk
Top plates
Bottom plates
Multi-Studs
Left: Multi-Stud Caulking. Right: Bottom Plate and Multi-Stud Caulking.
Can Foam
Around Windows
Around Doors
Penetrations to the outside through framing or OSB (could be wires passing through, or a pipe or vent)
Penetrations to the attic through framing (could be wires passing through, a pipe, or a vent)
Behind outlets facing outward and inward
Behind electrical boxes
Weatherproof Sealant
Vertical transitions between concrete wall and framed wall (daylight wall to foundation wall, for example)
Gap between top of the foundation wall and sill plate
If you are using weather-resistant barrier sheeting (like Weather Logic), gap between sill plate and the bottom of the panels
Weatherproof Sealant
Notoriously leaky areas include attics and rim joists. These areas need special attention. However if you feel like running around and sealing 100 different air-entry points in your attic or on your rim joist is too much busy work, consider closed-cell spray foam. One broad sweep, and all your airtightness dreams can come true.
Air sealing at the Drywall Phase
Drywall makes another hefty contribution to the airtightness of a house or building. Just like framing and insulating, a higher-quality install makes for a tighter house. Air will definitely get past siding, can probably find its way through some gaps in the weather barrier, and (if fiberglass) can definitely find its way through insulation. At this point it could find its way out of (or into) your house through unsealed outlets or other electrical boxes. However, it may be stopped by good drywalling. By good drywalling I mean square drywalling, tight cuts around plates, outlets, and other protrusions, and drywall that doesn’t leave huge gaps at the top or bottom of a wall.
Cuts are always bigger than the fixture they go around, so as a *last resort* (aka at the finish stage) you may choose to seal the gap between the fixture and the pre-cut hole with can foam or caulk. This will get covered up by a plate of some sort- so no one will ever know!
Air Sealing Before Attic Insulation
One of the greatest air leakage points in a home- the intersection of the top floor interior and exterior wall top plates and ceiling drywall in the attic- is also an opportunity for creating airtightness. This is most easily sealed with can foam or preferably spray foam. Usually just a few hundred lineal feet, you can make a pretty big difference in a relatively short amount of time.
Air Sealing at the Painting Phase
Creating a solid air barrier isn’t really up to the painter. However, one contribution they definitely can make is sealing the trim with clear caulk or a similar product. In most situations this application would be overkill; however if something else was missed this could be used to make up the difference.
To sum up Air Sealing
Microleaks add up over the entire building. You can’t see them, but they are there. The questions to ask yourself is: how airtight do I want my home or building to be?
How Airtightness is Measured
The industry standard for objectively measuring the airtightness of a home is the blower door test. During this test, all doors and windows are closed except for the testing door. The testing door is fitted with a high-power fan that is set to put 50 pascals of pressure on the house. This pressure forces air to move through any present gaps, holes, or penetrations. The test then spits out a CFM measurement (cubic feet per minute) of air leakage.
When the air leakage reading is combined with the volume and envelope size of the house, it can then be used to determine how well the home performs compared to other buildings. A lot of rebates, energy code programs, and home certifications require a certain level of airtightness, and use the blower door test score as the determining factor of qualification.
If you are in need of a blower door test, seek out a competent HERS rater in your area. Thermal Shop’s sister company- Energy Ratings Group (watch out, shameless self-promotion to follow!) just so happens to administer blower door tests and energy code programs professionally throughout Eastern Iowa.
It takes a village of competent contractors to avoid a home’s death by a thousand airtightness cuts. Get ahead of the game and make plans for air sealing your project today. Talk with your framer, insulator (that’s us!), drywaller, and painter. What awaits you in your airtight home or building are lower utility bills, a more stable year-round temperature, less insect entry points, less moisture buildup, and a longer home lifespan.
And if you decide you need professional help on your project, contact Thermal Shop today.
