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Building Science

Buried Ducts Allowed in 2018 Energy Code

The keys are duct leakage, R-values, and the vapor barrier

Condensation on the duct insulation in this South Carolina crawl space is not unusual. It can happen in attics, too.
Image Credit: Energy Vanguard

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…

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  1. User avater
    Reid Baldwin | | #1

    Do you have a theory to explain the graph?
    Do you expect that the air that flows down into the fluffy insulation mixes with air that is leaking up from the ceiling? Or do you expect that air flowing down mixes with air that is leaking from the supply ducts? Or do you expect that the air that is flowing down from above is giving up its moisture somehow?

  2. Charlie Sullivan | | #2

    Buried in what insulation?
    I would expect that the variations in dew point through the insulation would be different for fiberglass vs. cellulose. Do you know which was used in the test? That might help answer Reid's question.

    Reid, a possible part of the explanation would be that the air deep within the insulation is influenced by the air below the ceiling drywall, because of the vapor permeability of the drywall. It would be fun to run some WUFI simulations to compare to this behavior. You could then change various parameter to tease out the different effects.

  3. User avater GBA Editor
    Allison A. Bailes III, PhD | | #3

    Response to Reid Baldwin
    I don't know for sure why this is happening, but I suspect that there could be a few contributing factors:

    1. Duct leakage from boots or buried ducts
    2. Air leakage through the ceiling
    3. The hygroscopic nature of the ceiling drywall and vapor diffusion down into the house

  4. User avater GBA Editor
    Allison A. Bailes III, PhD | | #4

    Response to Charlie Sullivan
    Charlie, the ceiling insulation in the test house was R-38 blown fiberglass. I agree that there would be a difference if it were cellulose since it's hygroscopic. Also, they did run some WUFI simulations to compare with their measured data. Download the report and you can see all the results, measured & modeled. Here's the direct link:

  5. Kevin Camfield | | #5

    Great Article
    Martin just pointed me to this article. It is very informative and very relevant to decisions we are now making for our roof design and insulation. For us burying the ducts looks like a very effective way to go. We were able to move most ducts, not all, into the conditioned space but still had a couple in the attic. A non-vented roof assembly was more expensive and/or used a lot of CC foam. We are near Seattle in zone 4C our max yearly dew point is 60F. So going with a vented assembly and spending some time to detail correctly the few ducts we have (HVAC and HRV) in the attic space seems to be the way to go. The detail in your article confirmed that choice and gives us some other options on how to build the ducts. Thank you.

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