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Building Science

Air Flow Pathways in a Leaky Exterior Wall

I found some interesting air leakage evidence while remodeling my bathroom

Bathroom demolition in my 1970 condo. When I opened up the exterior wall, I found some interesting air leakage patterns.
Image Credit: Energy Vanguard
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Bathroom demolition in my 1970 condo. When I opened up the exterior wall, I found some interesting air leakage patterns.
Image Credit: Energy Vanguard
The air leakage pattern shows clearly on this fiberglass batt.
Image Credit: Energy Vanguard
Here's a closeup of the air leakage pattern. Note the big black area across the middle.
Image Credit: Energy Vanguard
The air leakage pattern on this fiberboard sheathing indicates lateral air movement.
Image Credit: Energy Vanguard
A closeup of the air leakage pattern on the fiberboard sheathing. Note the big gap between sheathing and stud and how it aligns with the dirt.
Image Credit: Energy Vanguard
A peek through the fiberboard shows the brick veneer. This hole, and many others like it, connects air outside the house to air inside the house.
Image Credit: Energy Vanguard
This is what my wife found when she walked into the bathroom after returning from her trip. She had no idea I had gutted the bathroom while she was away.
Image Credit: Energy Vanguard

This spring I spent a lot of hours in my bathroom. I was sick. Really. I was sick and tired of having an outdated bathroom that was falling apart. So when my wife hit the road one Monday in late April to drive across the country, I got out my wrecking bar. The lead photo shows what it looked like at the end of my first full day of demolition.

I opened up the plumbing wall first. Lots of fun stuff, there. But the real fun came when I opened up the exterior wall. The four termite-damaged studs were part of that fun, but something else was even better.

Yogi Berra once said, “You can observe a lot by just watching.” So when I got into the exterior wall, I watched. I live in the Atlanta area in a condo built in 1970. Air leakage hadn’t been discovered yet back then, but that doesn’t mean it wasn’t happening. Check out that fiberglass batt (first two images below) from the exterior wall.

Do you see what I see? The black parts are where the fiberglass captured dirt. The dirt was traveling in air that was moving in the wall. Fiberglass is a great indicator of air leakage, and most of the fiberglass manufacturers make it easy for us to see the dirt. They make their product in light colors: pink, yellow, white. (There’s a new trend toward brown fiberglass, though, which isn’t helpful for spotting air leakage. But hey, we’re making airtight houses now, right?)

The leaking air was moving laterally through the batt

Now, let’s focus in on where the dirt appeared in this batt. Let’s observe. See that part in the red box (Image #3, below)? That indicates air was moving laterally across the stud cavity in the wall. What?!

Yes, it’s true. When I first looked at the batt, that pattern didn’t stand out. I saw it only later when I looked at the exterior wall sheathing and saw the pattern repeated there. See the band of dust near the bottom of the cavity in Image #4, below? That’s where the fiberglass batt picked up that band of dirt in the red box in the previous photo.

But how is air moving laterally across the stud cavities? Well, we know that air needs two things to move: a pressure difference and a pathway. It also likes to take pathways with lower resistance. So that band of dust on the sheathing is a pathway of lower resistance. You can see the pathway better in Image #5, below.

The fiberboard sheathing is buckling

The sheathing that I discovered in the wall is asphalt-impregnated fiberboard, commonly referred to as Celotex, one of its primary manufacturers. It’s not as stiff as plywood or OSB, and you can see below that it’s not lying flat against the studs. Those gaps create pathways.

OK, that explains air moving inside the cavity, but is it connected with air outside the building enclosure? Some people think just the presence of fiberglass is the problem. They’re wrong. The answer is shown in the Image #6, below. The seam between two pieces of fiberboard is open. You can even see what’s on the other side: the brick veneer.

It looks like that nail missed the stud. It was probably OK at first, but over the past 46 years, the fiberboard has distorted through a whole lot of wetting and drying cycles. The result is a hole in our building enclosure. And there are more holes everywhere two pieces of the fiberboard meet and at the top and bottom of the wall. That adds up to a lot of leakage area.

Where’s the WRB?

Also, the fact that I can see the brick veneer on the outside of the building means there’s no drainage plane. No felt. No house wrap. No nothing between the fiberboard and brick. Fortunately, I haven’t found widespread moisture damage resulting from this. (The termite damage is an indication of moisture but it was isolated to the two sides of the window.)

How do you fix this? You’ve got several options. You could ignore the problem and put it back together the way you found it. You could spray-foam the whole thing. You could seal the gaps and install fiberglass batts again. I’ll show you my solution next time.

By the way, demolishing a bathroom by yourself is a heck of a lot of work. I hauled out 47 bags of debris and a few larger items. The good news is that by the time my wife returned home the following week, I was done with 98% of the demolition. The last photo shows what she found when she walked in there expecting to find the same bathroom she’d left the week before.

Lead-safe work practices

Now let me end with a caveat. If your home was built before 1978, it probably has lead paint in it. If you’re doing the work as a homeowner, you’re not subject to the rules of the Renovation, Repair, and Painting (RRP) Program, which applies to contractors, but you you should still work safely. Here are their lead guidelines for do-it-yourselfers. I followed them pretty closely when I was doing the demolition in my bathroom.

Allison Bailes of Decatur, Georgia, is a speaker, writer, building science consultant, 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.

One Comment

  1. slowlylearning | | #1

    Allison I have gutted a 1965 brick veneer ranch house in Charlotte, NC. I have found similar evidence of air leakage in insulation and on Celotex sheathing. Fortunately all framing appears to be free of moisture damage with exception of some joist damage from a long term leaking toilet in master bath. Over the past month I have been searching for methods to air seal the sheathing from interior side as I am not removing brick veneer. No luck as of yet. I have spoken to fluid applied sealing manufactures and have been told their products won't work. Really looking forward to hearing what solution you have come up with.

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