Buried Ducts Allowed in 2018 Building Code

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Buried ducts in attic insulation

Are you confused? In my last article, I said buried ducts are at risk for condensation. Now I'm telling you they'll be allowed in the 2018 code. What's going on here? The answer is simple. Yes, when you bury ducts in attic insulation in a humid climate, you increase the risk of water vapor condensation on the jacket of the duct insulation. But in my last article, I also showed you some preliminary results from a research study on buried ducts in a hot humid climate. They found R-8 duct insulation was sufficient to prevent condensation. Now, let's see what the 2018 versions of the International Residential Code (IRC) and International Energy Conservation Code (IECC) will require.

The basic 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 enough ceiling insulation that the sum of what's above and below the buried ducts is at least R-19, excluding the R-value of the duct insulation itself. (If that's not clear, see the diagram below.)

R-value requirement for ducts buried in attic insulation [Image courtesy of Home Innovation Research Labs, used with permission]

How are the ducts treated in energy modeling?

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 cfm25 per 100 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.

For more detail on the new buried duct requirements, be sure to check out the TechSpec titled HVAC Ducts Buried within Ceiling Insulation in a Vented Attic (Buried Ducts) (pdf) on the topic published by the Home Innovation Research Labs.

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 with a homeowner.

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

  • Buried ducts that are poorly insulated, even in one small area
  • 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.

Another way to bury the ducts in a humid climate is to encapsulate them with closed cell spray foam. The Building America program has done research on that method as well, and although I've been meaning to write about it for years, I haven't covered it yet. I'll try to get to that soon now that the issue of buried ducts is moving to the front.

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.

Related Articles

Case Closed: Get Those Air Conditioning Ducts out of the Attic

Home Performance Flaw #212: Ducts Placed High in the Attic

What's That Ice Chest Doing in This Attic Duct System?


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


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Feb 16 2017 - 9:04am

Just to clarify you wrote "duct leakage must be no higher than 1.5 square feet of conditioned floor area" is that cfm/100sf or something else I'm not familiar with. Either way it's just plain hard to get attic duct work right. There would be a need for some kind of duct attached insulation depth marker at regular intervals to ensure the install is correct.

Feb 16 2017 - 9:13am

What are you talking about, Andy? It says "1.5 cfm25 per 100 square feet of conditioned floor area." Uh...because I just changed it. Thanks for pointing that out! It was just a typo.

Feb 16 2017 - 9:36am

Would you clarify once more please. To what does the #25 refer?
"1.5 cfm25 per 100 square feet of conditioned floor area."

Feb 16 2017 - 9:38am

Craig, sorry about that. Cfm25 means the amount of duct leakage in cubic feet per minute when the ducts are tested at a pressure of 25 Pascals. I used to have some pages on duct leakage testing but they seem to have gone AWOL when we got a new website in December.

Feb 16 2017 - 10:49am

I'm looking forward to reading about ducts encapsulated in CC spray foam. I've seen a few attempts in the field, and the reality seems to be that is a process very very difficult to execute properly.

Meanwhile, I wonder when / if our HVAC supply houses here in the humid southeast will offer R13 flex, ductboard, and duct wrap.

Feb 16 2017 - 12:30pm

As with everything, execution is critical. And as usual, it's easier to get good execution with a thorough design process that thinks through all the potential obstacles.

Supply houses, I believe, were pretty good about stocking R-8 when that became required for attics. This is different though because it's optional. I'm interested to hear about contractors' experience with this so keep us posted, Curt. Next time you're in one, ask them about it.

Feb 16 2017 - 1:36pm

I suspect you'll see it when your State updates their code (circa 2026 at the current adoption rate of my state). In my opinion the driver will be production builders (Is there any other type in the Southeast?) because builders will either #1 stop putting mechanicals in attics or #2 continue to place mechanicals in the attic space and also bring the attic within the bldg. envelope as an unvented/semi-conditioned space. In my market builders are adding interior space by going vertical via 3-4 story townhomes. There's not a lot of interior space to place the mechanicals and I'm seeing them in garage closets , in the attic, or both.

Apr 17 2017 - 10:59am

Hi Curt - We tested ducts encapsulated in spray foam then buried beneath attic insulation in Jacksonville a few years ago. These were existing homes, and we couldn't get ducts perfect (you're right - it can be tricky). But heating and cooling energy was reduced by 5-20% (in 3 houses). More info than you probably want:

I think ducts encapsulated (in foam) and buried in fiberglass have been prescriptively allowed since the 2009 IRC. Code requires an ignition barrier over the foam, and my understanding is that fiberglass insulation counts. Some guidelines for doing this:

Feb 16 2017 - 3:49pm

The only reason we can lay our hands on R8 is that our supply house in Jacksonville also services South Georgia where all y'all require R8 in some situations, so I won't hold my breath on obtaining R13...There would have to be a heckuva lot of call for it.

R8 duct board, despite having only 33% more material than R6, costs almost 3x R6. If that pricing pattern persists to R13 duct board, assuming a manufacturer were to actually offer it, it might be cheaper to foam the whole attic!

Feb 16 2017 - 1:20pm

We have a builder that tries to hold their ducts down to the warm ceiling and then encapsulates the ducts with open cell foam and then blows cellulose over top to the same depth as the rest of the attic. We have tried for years to get them to move their ducts into the conditioned space, but they won't. We have also recommended that they move to a closed cell phone verses an open cell foam, but their insulator is claiming at 3.5+ inches the soft foam becomes an air barrier. This is in Michigan so its a cold climate, but we do reach temps in the 90s in the summer with fairly high humidity. We have been working with this builder for about 5 years and haven't had any issues arise from their homeowners. Many of the building inspectors over here are considering this system as within conditioned space and not requiring a duct leakage test. What do you think of this practice?

Feb 16 2017 - 3:41pm

The most troubling thing to me about this is being able to claim that the ducts are inside the building envelope. They simply are not. Yes they will perform better but any leakage will leak to the outside. Also, this points out that RESNET and CODE are not in alignment with each other. Code may say you do not need to test for LTO but RESNET would continue to say you do. Still a lot of work to do to get this all straight and modeled correctly.

Feb 16 2017 - 4:01pm

@Allison, what's the source for the modeling guidance?

BTW, I think your 2nd bullet point would be clearer if you change to "Have a total of R-19 of insulation above and/or below the ducts, excluding the R-value of the duct insulation.

Here's what the proposed code says: "At all points along each duct, the sum of the ceiling insulation R-values against and above the top of the duct, and against and below the bottom of the duct is not less than R-19, excluding the R-value of the duct insulation." Clear as mud.

Your accompanying drawing makes this requirement crystal clear.

Feb 16 2017 - 5:02pm

You're right, David. My description of the R-19 requirement wasn't clear. I'm not a big fan of "and/or" so I wrote it a little differently than you suggested. Better?

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