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The Importance of Interstitial Cavities

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Your building science word of the day is “interstitial.” As a good home performance contractor, you read about air leakage, duct sealing, insulation, and all those fun topics that are so important to making home perform better. Maybe you’ve seen this word somewhere already. Maybe not. The word itself isn’t that important. But the lesson behind it is. Let’s see.

Words matter

If you search Google for the word, here’s the definition they throw at you:

interstitial (adjective) – of, forming, or occupying interstices

Ah, that’s helpful. Not! Now we have to look up the word “interstice.”

interstice (noun) – an intervening space, especially a very small one

Now we’re getting somewhere. Their example uses sunlight coming through the spaces between leaves. The one I always think about is a net. The open spaces between the strings are the interstices, the interstitial spaces.


What does this have to do with building science?

Go back to the photo at the top of the article. That’s actually the air conditioning duct and boot in my bathroom. That joist bay where it’s located is a building interstice. Air moving in that bay leads to interstitial pressure differences and air movement.

That interstitial space is connected, in our case, to the duct trunkline. That trunk runs to the laundry room, which is connected through interstitial spaces to the fireplace and stairwell. The fireplace and stairwell are connected to the attic.

On the other end of that joist bay in our bathroom ceiling is the band joist above the exterior wall. The duct you see above is unsealed. It’s a supply duct. When the heating or air conditioning system is running, that joist bay develops a positive pressure. Before I gutted my bathroom to fix these problems, that positive pressure drove exfiltration through the unsealed band joist.

Wherever incompletely sealed ducts run through interstitial cavities, they can create pressure differences. Wind and stack effect also can create pressure differences in those spaces. When the interstitial spaces lead outside the building enclosure, air leakage can result from pressure differences. You can reduce the pressure differences with duct sealing. You can minimize the air leakage by sealing disconnecting the interstitial spaces from the conditioned and the unconditioned spaces they adjoin.

So when you hear the word “interstitial,” think air leakage. Think duct leakage. Think pressure differences, which drive air movement. John Tooley and Neil Moyer wrote a groundbreaking paper on this topic in the 1980s called MAD AIR – Mechanical Air Distribution And Interacting Relationships. Joe Lstiburek expanded on that work with his doctoral dissertation.

Now you’ve finished the article. You’ve reached an interstice in your day.

Allison Bailes, III, PhD
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Posted In: Building Performance, Residential Buildings

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