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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

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 I was sick and tired of having an outdated bathroom that was falling apart.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…

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