The photo at right 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-inch duct designed to bring combustion air from the outdoors directly into that room. (How well it actually does that is a topic for another article.)
Ignoring the Grade III fiberglass insulation in the wall, the air barrier problems are in the joist area above the wall. These problems are:
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” that it needs to be.
Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, 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.