skip to Main Content

How Can You Air Seal These Joists Over the Garage?

A Really Bad Job Of Blocking The Joist Cavities In A Wall Separating The Garage From The Living Space

The I-joists in the photo above run across the top of the wall between the dining room and the garage. In the old days, before anyone worried at all about air moving through those joist cavities, the builder didn’t do anything with them. You can see here, though, that the builder knows a thing or two about air sealing because they’ve put blocking between the joists. But what do they do next here?

Why do these joists need to be sealed?

Three little letters are all you need to know: IAQ. Homes with attached garages often have worse indoor air quality than homes without attached garages. When is the last time you walked into a garage and thought to yourself something like this: “I love the smell of this garage! I could stay out here all day just breathing in this wonderful air!”

I don’t recall ever having that thought. But I do remember the experience of trying to get through someone’s smelly garage as quickly as possible and wishing I had a respirator. That’s because of all the nasty stuff we keep in our garages. And it’s better out there than in the house, so let’s keep it out there.

That’s where air sealing comes in. We want the most robust air barrier possible separating the living space from the garage. The joists that run across the top of the wall separating the garage from the house must be sealed. That’s where the air barrier should be, not the drywall in the garage.

How to block and seal these joists properly

I-joists, as you see above, are used all the time in new home construction now. They present a challenge that dimensional lumber doesn’t because the web (the middle part) is narrower than the flanges (the wood on either side of the web). But it’s certainly possible to seal it up. Here’s a photo from the Building America Solutions Center page on insulating and air sealing a floor over a garage:

Because of the indentations at the web, the builder here covered the gap with smaller pieces of wood. Another way to do the blocking is with rigid foam board, which can be cut to the shape of the opening. Here’s an image from the Building America site showing this:

But getting the blocking installed isn’t the end of the air sealing job. You still have to use a sealant material at all of the edges where air can leak through. There a lot of ways to do that last bit:

  • Caulk, like Prosoco’s R-Guard Joint & Seam Filler
  • Spray-on sealant like Owens Corning’s Energy Complete or Knauf’s Ecoseal Plus
  • Tapes like the ones from Siga or Pro Clima

Another way to do it is with spray foam insulation, as shown below in another photo from the Building America Solutions Center.

The importance of design

In the photo I posted at the top of the page, the builder’s best option is to use spray foam. Not only do they have the I-joists to contend with in this home, but they also made a silly design decision here. Rather than running one set of joists across that wall, someone decided those joists couldn’t run continuously across the wall. Hence the joist ends you see protruding in the middle of every cavity.

I’m sure whoever made that decision had a good reason for it. The problem is they didn’t think through the air barrier consequences. As a result, they made their blocking and sealing job a lot harder. Their best option here will be to use spray foam because they left a lot of big gaps. With a little forethought in the design phase of this home, they could have made their job a lot easier.


Related Articles

I-Joists, Attached Garages, and the Air Leaks That Poison

Another Way to Prevent Your Garage from Making You Sick

Are You Making These Mistakes with Your Garage?


Top image by Energy Vanguard. Three images in body of article from Building America Solutions Center page on how to insulate and air seal a floor over a garage.


NOTE: Comments are moderated. Your comment will not appear below until approved.

This Post Has 9 Comments

  1. These air sealing articles
    These air sealing articles are so helpful. Getting ready to build a new home and had not thought of this air sealing detail. Please publish more on air sealing details that are commonly overlooked.

    1. Thanks, Tom. I’ll do that.

      Thanks, Tom. I’ll do that. Stay tuned.

  2. Yes, air sealing is needed!
    Yes, air sealing is needed! Please let us know the best sealent to use in each situation. We don’t want the sealent to dry out and leave gaps in the future.

    1. Dixie, there are a lot of

      Dixie, there are a lot of good sealants. See the list I mentioned in the article.

  3. Building America Solutions
    Building America Solutions Center. What a great resource. I actually came across it independently when I was researching jump ducts and transfer grills inspired after reading this article. A current client has two air handling systems for a 2800 sf house and complains of poor delivery from the 2nd floor. I noticed all the rooms had no undercut for the doors and their was a central return. This has to be evaluated by my HVAC contractor but may likely be a significant factor.

    Does anyone have a suggestion for sources of independent hvac testing providers in the Chicago area?

    1. Jim, yes, the BA Solutions

      Jim, yes, the BA Solutions Center has a wealth of great information available. Our tax dollars have been well spent there.

      Another option for relieving pressure in bedrooms is the Return Air Pathway from Tamarack Technologies. (Disclosure: They advertise here in our blog.) Here’s the link to the RAP page on their website.

      Regarding someone who can do testing for you, see Sean Lintow’s comment below. He’s in Chicagoland. Click on his name to go to his website.

  4. I got a feeling someone didn
    I got a feeling someone didn’t review their specs (or tried cutting costs) as I have a sneaky feeling those webs should have been filled in thus allowing for a 2x or even I joist blocking to work properly

    Hmmm Jim, Chicago area… that would be me for the western suburbs 🙂 — I also know a few guys that do work in the city

    1. Sean, it could certainly be

      Sean, it could certainly be that someone didn’t read the specs. I suspect they didn’t have a complete framing plan and made a bad decision to break those joists across the garage wall. Whatever happened, it’s now a mess. Pretty much the only way to seal it up at this point would be with spray foam.

  5. This is a framing mistake and
    This is a framing mistake and “cheaping” out on numerous levels, it has nothing to do with air sealing. What you are seeing here is a contractor trying to cut costs, if you look closely they are running 14″ and 16″ joists and “blocking” the 14″ joist up with and osb scrap and a 2x block to match the tops. A contractor worried about quality (either IAQ or overall job) would have used the 16″ joists thru out and installed proper blocking between the joists.

Comments are closed.

Back To Top