Water vapor from the air condenses on air conditioning ducts in humid climates. It’s as normal as poorly insulated bonus rooms making occupants uncomfortable or cigarettes causing lung cancer^†. Condensation on ducts is most common in crawl spaces and basements, where the air is more likely to have a higher dew point.

But it also happens in conditioned space and occasionally even in attics. That’s why you have to be careful about burying ducts in attic insulation in humid climates, especially in coastal areas. The 2018 International Residential Code (IRC) and International Energy Conservation Code (IECC), however, have passed new requirements they say will prevent condensation on such buried ducts. Will they work?

The problem with buried ducts

Before getting to the new IECC requirements, let’s understand the issue a little better first. The potential for duct condensation is a humid climate, cooling season problem. My friends out west regularly bury their ducts in air-permeable insulation (mainly fiberglass and cellulose) and don’t worry about condensation. The air in the ducts may be as low as 55°F, but their summer dew points are usually below 50°F. There’s just not enough water vapor to condense.

In humid climates, that’s not the case. The photo above shows condensation on a duct in a crawl space in Hilton Head, South Carolina. Outdoor dew points can hit 80°F in the summer there and average somewhere in the 70s. With 55°F air moving through supply ducts, you’ve got to have well installed insulation on the duct with significant R-value to keep the surface temperature above the dew point.

When that supply duct is in a vented crawl space, it’s really hard to keep that surface temperature above the dew point because the temperature down there is already close to the dew point. When you have a supply duct above the ceiling insulation in a vented attic, however, the surrounding air temperature is typically higher than ambient. Thus, the surface temperature of that duct most likely will be above dew point. That means no condensation.

But if you bury that supply duct in fiberglass or cellulose insulation, which allow attic air to reach the duct’s surface, all bets are off. As you pile more and more insulation on top of the duct, the surface temperature of the insulation jacket drops.

Since attic air permeates into the ceiling insulation, the dew point within is presumably the same as in the attic space above the insulation. Further, if the attic is vented, the dew point of that attic air should be about the same as the outdoor air. Right? The result is a duct surface temperature that’s below the dew point of the air in the ceiling insulation, and that means condensation.

As it turns out, ducts buried in ceiling insulation in humid climates do sometimes have condensation problems. If you live in a humid climate, especially locations near the Gulf of Mexico or Atlantic Ocean, you may now know the reason for those mysterious water spots on your ceiling. Maybe. They could also be from roof leaks.

What the research says about buried ducts

Well, that sure sounds reasonable, but is it confirmed by what actually happens? A Building America paper from 2016 titled, Compact Buried Ducts in a Hot-Humid Climate House has an answer for us. Home Innovation Research Labs did a study on a buried duct system in a home in Lady’s Island, South Carolina (a bit north of Hilton Head). One of the main questions they wanted to answer was:

What is the minimum level of duct insulation to prevent condensation at the outer jacket of buried ducts in hot-humid and mixed-humid climates?

So they set up this model home with a buried duct system and installed a lot of sensors. The ducts had the standard R-8 insulation you find on attic ducts but then were covered with R-30 ceiling insulation. Here’s what they found:

• No condensation on the ducts
• Colder air delivered to the house
• Simulated simple payback of 3.1 years, and simple return on investment of 32%

Another really interesting thing they found was that the assumption I mentioned in the last section doesn’t seem to be warranted. Remember when I said that the dew point within the attic insulation is presumed to be the same as the dew point in the attic above the insulation? Well, that’s not what they found. The graph below shows two things: (1) The dew goes through daily cycles (similar to what I found in a spray foam attic in Atlanta), and (2) the dew point peaks decrease as you descend into the ceiling insulation and get closer to the drywall. This is good to know.

Now, this is all good stuff, but it’s just a start. For one thing, the report is based on only one month’s data, August 2015. For another, they set this house up specifically as a test house and thus made sure to get as close as they could to their design conditions. (They did miss on duct leakage, though. Their goal was 1 cfm25/100 square feet of conditioned floor area and they hit 4 cfm25/100 square feet.)

It may well turn out that R-8 duct insulation is good enough for general use, but I’m not convinced yet. Neither, as it turns out, is the International Code Council.

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:

• Can you add the insulation above the ducts to the duct insulation for energy modeling and load calculations?
• 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.

Footnote

† Both of my parents were smokers. Both died of lung cancer.

Allison Bailes of Decatur, Georgia, is a speaker, writer, building science consultant, and the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.