Buried Ducts in Humid Climates Without Condensation

Consider moving cold air through a hot attic or hot air through a cold attic. Building codes require just a meager R-8 insulation on the ducts. It might appear natural to bury those ducts beneath the attic insulation to improve energy effectiveness. My contractor pals out west do this all the time and have no issues. But if you attempt to do buried ducts in damp climates, you risk of condensation.

That’s what the 2018 IRC attempted to deal with, but that first effort was a loser. The code requirement that was nearly difficult to satisfy was to utilize R-13 duct insulation in IECC climate zones 1A, 2A, and 3A. None of the duct insulation producers made R-13 duct wrap, and the code modification didn’t produce much demand for it either. Now we have a much better procedure for doing it without the condensation threat, and it’s based upon strong research study just recently completed in Florida.

What sort of buried ducts?

Before jumping in, let me define what I’m discussing– along with what’s not consisted of– when I use the term “buried ducts.” The research did not address ducts that are only partly buried. Nor did it take a look at encapsulated ducts, which are first covered with an inch of closed-cell spray foam before being buried in blown insulation.

The two categories of buried ducts in the study are “completely buried” and “deeply buried.” They’re both buried, but they differ only in the amount of insulation over the top of them. Totally buried ducts require to have 3.5 inches of loose-fill insulation on top, and deeply buried ducts need to have R-30 on top.

They call these out independently since you can design them differently. You deal with completely buried ducts as in an attic with R-25 duct insulation. When you do deeply buried ducts, you get to treat them as remaining in conditioned space.

More research led to a much better protocol

Last month we had Nelson Conarroe from Owens Corning as the speaker at our Atlanta BS & Beer conference. (BS represents developing science, of course.) He offered a fascinating presentation on the research that Owens Corning has been doing at the Florida Solar Energy Center (FSEC). (Download the slides here.)

I had doubts about how well simply going to R-13 duct insulation would safeguard versus condensation. In my 2017 short article on the 2018 IRC requirements, I wrote this:

There are pitfalls here, though. Here are a couple of:

• Buried ducts that are inadequately insulated, even in one little area
• Excessive duct leakage
• Improperly sealed vapor barrier on the duct insulation

One problem was that it was counting on duct installers to get the information right. If you comprehend ducts, you most likely know that a high percentage of duct systems are installed poorly. And the other glaring omission was leaving the attic vented with outside air– and all its humidity.

Adding the missing out on components

The brand-new research study that Owens Corning did at FSEC created a much more secure way to do buried ducts in humid environments. They even made it more achievable by reducing the duct insulation to R-8, which is the code-required level for vented attics. The key to making it work was also attending to the humidity in the attic. Here are the main elements they found that make buried ducts in damp climates work:

• R-8 duct insulation
• Unvented attic
• Percentage of supply air from the air conditioning system (50 cfm per 1,000 square feet of attic flooring area)
• Vapor diffusion port (See this article by Joseph Lstiburek to learn more about this function.)

They put the numerous attics in the FSEC test hut under what they believed might be the best series of conditions to worry the system. They differed the color of the roofing, considering that light-colored roofings reflect more heat and might make condensation most likely. They also checked with the thermostat set at 68 ° F(20 ° C), since that would make the ducts cooler and more likely to have condensation.

When they did those four things, they discovered this: “Attic air relative humidity and roofing system wood wetness in unvented attics with buried ducts and insulation on the ceiling was acceptable during summertime and winter season conditions.”

What about using a dehumidifier?

Considering that they’re sealing up the attic, you might question if they evaluated how well utilizing a dehumidifier might work. And yes, they did test that. Here’s what the full FSEC report says in the executive summary:

Utilizing a dehumidifier in unvented attic to control duct surface wetness is not reliable sufficient and needs a high amount of energy and long runtimes.

• The RH [relative humidity] sensing unit of a dehumidifier is under really various conditions than the buried duct surface. The lowest RH setting of control was required to get simply marginal moisture control throughout hot and damp weather.
• Based on measured data and estimates from attic test cells, dehumidifier energy usage is anticipated to be a minimum of an order of magnitude higher than using conditioned supply air into the attic.

So just stick with the items in the previous section if you wish to do buried ducts in damp climates.

Code requirements for buried ducts in damp climates

Even though they simply finished up this research study, some of what they advise made it into the 2024 International Residential Code (IRC). Here’s the appropriate section:

N1103.3.5 (R403.3.5) Ductwork buried within ceiling insulation

3. In Climate Zones 0A, 1A, 2A and 3A, the supply ductwork shall be entirely buried within ceiling insulation, insulated to an R-value of not less than R-13 and in compliance with the vapor retarder requirements of Area M1601.4.6.

4. In Environment Zones 0A, 1A, 2A and 3A where installed in an unvented attic with vapor diffusion ports, the supply ductwork shall be totally buried within the insulation in the ceiling assembly at the floor of the attic, insulated to an R-value of not less than R-8 and in compliance with the vapor retarder requirements of Section M1601.4.6.

Product 3 above is basically the arrangement from the 2018 IRC, which needs R-13 duct insulation but not the other things. Product 4 shows the new requirements. (Products 1 and 2, which I didn’t show, cover duct insulation.)

The 2024 IRC does not state anything about putting a little supply air into the attic, however they don’t say you can’t do it either.

Do not sleep on duct condensation

If you’re questioning how crucial all these safety measures are, increase into an attic in a humid climate at some point. Condensation on ducts is common there. Normally, it takes place where two ducts are touching. That triggers the space in between the ducts to get colder, however it’s still in contact with the damp attic air.