The Building Enclosure Duct Blunder


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The photo to to the left shows a common problem in new homes. It’s also one that can make it difficult to pass the blower door test required by many building codes these days. If I tell you that, the wall pictured here separates two rooms in a basement and one of them is not conditioned, can you see the problem? If so, how many mistakes do you see here?

The building enclosure is the boundary between inside and outside. It separates conditioned space from various types of unconditioned space. In the case of the wall above, it separates the conditioned part of a basement from a mechanical room with three atmospheric combustion appliances – two 80 AFUE furnaces and a natural draft water heater, all of which burn natural gas.

Since atmospheric combustion appliances draw their combustion air from the room they’re in, they need an adequate supply of air. The room behind that wall has a 12″ duct designed to bring combustion air from the outdoors directly into that room. How well that actually works is another matter, but the hole to the outdoors remains.

Ignoring the grade III fiberglass insulation in the wall, the air barrier problems are in the joist area above the wall. They are:

  • Open-web floor trusses with no blocking. Air can go right through those openings in the trusses.
  • Flex ducts running through the joists. It’s impossible to get a good, durable seal around flex duct, because it moves.
  • No blocking or sealing across the top of the wall. That bit of fiberglass below the pipes is a pathetic attempt to stop air movement that won’t work.

The way this part of the building enclosure is constructed is pretty near impossible to fix after the fact, especially with flex duct running through the joists. What would have worked is to put in a rigid air barrier before running the ducts or plumbing and telling every trade contractor who cuts a hole to make sure they seal it.

And the HVAC contractor would need to do more than just cut a hole and run the flex straight through. The movement of the flex would eventually defeat the air sealing, so they’d need to install a rigid connector through the rigid air barrier and connect the flex to either side of it.

As this home has been built, the mechanical room on the other side is not going to have a hope of being the “sealed combustion closet” it needs to be.

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

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