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The New Code Requirements for Buried Ducts

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Two articles ago, I wrote about what can happen when you bury ducts in attic insulation. The Home Innovations Research Lab has done a study in which they found ducts buried in attic insulation didn’t have condensation problems. The ducts had R-8 insulation and were well-sealed. It was only one house, but it’s part of the justification for the buried duct requirements in the 2018 building codes. Here’s the scoop on what’s coming.

The new I-code requirements

Through the 2015 versions of the IRC and IECC, the codes haven’t said anything about buried ducts. They’re not prohibited, but there’s also no guidance on how you would design and install them if you wanted to try it as an alternative to bringing the ducts into the conditioned space or moving the building enclosure to the roofline with spray foam. But that’s now changing as the 2018 I-codes have addressed the issue for the first time.

The new buried duct requirements are pretty simple. It’s basically just two things. All supply ducts completely or partially buried in ceiling insulation must:

  • Be insulated to at least R-13 in IECC climate zones 1A, 2A, and 3A and to at least R-8 in all other climate zones;
  • Have a total of R-19 of insulation above and below the ducts, excluding the R-value of the duct insulation.

But of course there’s more. Once we put the ducts down into the insulation, other questions arise. Here are the two relevant to this discussion:

  1. Can you add the insulation above the ducts to the duct insulation for energy modeling and load calculations?
  2. Can you consider buried ducts to be in conditioned space?

The answer to both of those questions is yes, but only with more rigorous requirements. Let’s look at them separately.

Deeply buried ducts. The first question leads to what is called “deeply buried ducts.” They get this designation if they (1) are no more than 5.5″ above the drywall, (2) have at least R-30 on either side, and (3) covered with at least 3.5″ of insulation (on top of the R-8 or R-13 duct insulation). If they meet all three requirements, then you can enter R-25 as the effective duct insulation in load calculations and other energy modeling.

Buried ducts inside conditioned space. The second question gets a yes if the buried ducts meet even more stringent requirements: (1) The air handler must be inside conditioned space; (2) duct leakage must be no higher than 1.5 square feet of conditioned floor area; and (3) the sum of the ceiling insulation above the duct and the duct insulation must equal the prescriptive R-value for ceiling insulation. If you achieve all of that, you can model the ducts as being located in conditioned space.

Will the new buried duct requirements work?

Since so many homes are built with ducts in the attic and blown fiberglass or cellulose insulation on the ceiling, this new option may appeal to some builders. We know that putting ducts in unconditioned attics is stupid from an energy perspective. But we also know that a builder will absolutely get a callback because of water spots on the ceiling whereas energy bills that are 20% higher than they might be with ducts in conditioned space just won’t register.

There are pitfalls here, though. Here are a few:

  • Buried ducts that are poorly insulated
  • Too much duct leakage
  • Improperly sealed vapor barrier on the duct insulation

Any of those things could lead to condensation on the ducts. A strap around a duct compressing the insulation, for example, reduces the R-value at the strap. That lowers the temperature at the vapor barrier and may put it below the dew point.

As with just about every other part of construction, the buried ducts method will work fine if implemented properly. It could also lead to some major headaches if it’s not. We’ll see what happens if builders choose to try it.

Images from HVAC Ducts Buried within Ceiling Insulation in a Vented Attic (Buried Ducts), a TechSpec published by the Home Innovation Research Labs, used with permission

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

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