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.