GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted
Building Science

The Difficulty of Stopping Air Leakage Between the House and Garage

I-Joists and open-web floor trusses, especially with flex duct running across the garage wall, can be troublesome

Image 1 of 3
This garage wall is part of the building enclosure. The wall will be difficult to air seal because of the I-joists and the flex duct running through the cavity.
Image Credit: Energy Vanguard
This garage wall is part of the building enclosure. The wall will be difficult to air seal because of the I-joists and the flex duct running through the cavity.
Image Credit: Energy Vanguard
The I-joists running right across the wall between the garage and the house can be sealed, but you have to make sure to block the big hole first and then seal the gaps on the side.
Image Credit: Energy Vanguard
Here's another house with I-joists running across the garage wall. A better way to do this would be to have the joists running parallel to the wall separating the garage from the house.
Image Credit: Energy Vanguard

A home with an attached garage is usually a home in which people breathe more carbon monoxide (CO). Of course, having an open carport or detached garage is better for air quality (and a feature that usually gets points for you in green building programs like LEED for Homes and EarthCraft House), but what if you don’t want to give up that attached garage?

You can do some things to minimize the amount of CO you breathe, and it starts with air sealing. One place you really have to pay attention to is where the garage’s ceiling joists cross the walls into the conditioned space. If the joists run parallel to the building enclosure wall, your job isn’t that hard. If the joists are perpendicular to the building enclosure wall, however, you’ll have open cavities that have to be blocked and sealed all along the top of that wall.

Look at the photo above. The joists in this home run right across the building enclosure wall between the conditioned space and the attached garage. You can see a flex duct running through one cavity and some blocking in the cavity to the right of where the duct is. (You have to look up at the top because there’s also a lowered soffit with extra framing that complicates the photo.) This house is still under construction, and the builder will finish the air sealing.

That duct, however, presents a problem. It’s really difficult to seal around a flex duct. A better method to run flex ducts through the building enclosure is to put solid blocking in first, then cut a hole, install a collar, and attach two separate pieces of duct to the two sides.

Without getting a good rigid air seal there, that cavity is probably going to hurt the results of the blower-door test. Even if they spray a ton of foam in there to get it sealed up initially, things shift over time. Or maybe the drywall installer will reach in there and push the duct, causing the foam to break loose.

It’s tricky to install blocking between I-joists

The other problem here is that the joists in this house aren’t made of dimensional lumber; they’re I-joists. As you can see in the photo below, you have to make sure that you finish the process of air sealing and fill those gaps on the sides of the blocking. It’s extra work, but very important when you’re trying to keep carbon monoxide out of the house.

The thing that gets me when I see this is that it can be avoided. If, instead of having joists running right across that wall, they had run the framing the other way, they could have a solid piece of wood there. Seal the edges and penetrations and you’re done. They also could have kept the joists running the same direction but not made them continuous. Just butt them into a perpendicular joist on top of that wall. The key is not to leave any holes that need to blocked and sealed.

Unfortunately, few homes have detailed framing plans, and many of those that do have them don’t get reviewed for this type of problem. This failure of design makes extra work for the builder.

Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. Check out his in-depth course, Mastering Building Science at Heatspring Learning Institute, and follow him on Twitter at @EnergyVanguard.


  1. Expert Member
    CARL SEVILLE | | #1

    It only takes care and competence.
    I saw this very high quality air sealing in a bunch of entry level production homes in NC a few years back. They were building high quality, cost competitive homes. I see so many production builders selling mid to high end homes that don't even understand enough to realize that they should be doing this. lots of room for improvement.

  2. charlie_sullivan | | #2

    Use the ceiling?
    Would it be a valid option to let the areas discussed be leaky, and concentrate on making the drywall ceiling in the garage tight?

  3. GBA Editor
    Martin Holladay | | #3

    Response to Charlie Sullivan
    Your approach won't work.

    If you establish an air barrier at the garage ceiling, that's all fine and good. But what that means is that the air between the ceiling joists in your garage is outside of the garage air barrier. So that's outside air (equivalent to the air in the attic of the garage).

    The outside air in the garage attic is now communicating with the wall separating the garage from the house. While you may have (theoretically) solved the carbon monoxide problem, you haven't provided an air barrier for the house. Even with the carbon monoxide danger reduced, you still need to prevent air leaks through that wall.

  4. charlie_sullivan | | #4

    Living space over garage
    Thanks Martin. That of course makes sense for a regular attached garage. I was, without thinking about it, imagining a garage under living space, whether it's under the main body of the house or under a finished "bonus room" over the garage, which of course makes this whole air sealing thing more critical and difficult.

    That might be a good addendum or part II to this article--best practices for air sealing with a living space over a garage (in which case, or course, the true best practice is to convert that to living space and add a car port, but for various reasons that doesn't always happen.)

  5. pallbearer | | #5

    CO detection
    We will be building a house soon - with an attached garage and living space above. One thing I considered - and cannot find - is a CO detector attached to a fan - or similar (Motion detector that has a 20 min fan run?). This way - the car pollution can be pulled out. Any ideas, sources or thoughts? Now - we will be sealing the house from the garage as well as we can - and this article makes some very good suggestions. Further elaboration or a follow up article that is longer is requested!

  6. kevin_in_denver | | #6

    Best Solution: Tesla
    Internal combustion engines are still being produced and we have to cut that out.

  7. GBA Editor
    Allison A. Bailes III, PhD | | #7

    Response to Paul Baier
    A garage ventilation system is a great idea if you have an attached garage. I haven't heard of one that triggers on CO levels, and that would be inadequate anyway. There's a lot more bad stuff in typical garage air than just carbon monoxide. AirCycler makes a GarageVent that turns on for a set amount of time whenever a door to the garage is opened.

  8. littlewolfarch | | #8

    Mudroom Link
    A link between the garage and house can serve several purposes: it creates a mudroom where you can shed outerwear and gear; it provides a physical separation between vehicles and the house; roof lines often change direction on the link creating a break that can be easier to seal; a link generally looks better than slamming the garage up against the house. It does complicate the envelope if the link is conditioned, and it should be if serving as a mudroom in the northeast, but its often worth the effort.

Log in or create an account to post a comment.



Recent Questions and Replies

  • |
  • |
  • |
  • |