One of the questions LP® Building Solutions gets from builders often is about burying ducts in attic insulation and whether combining radiant barrier sheathing with blown insulation over ductwork can benefit homeowners. As with most technical situations, the outcome depends on a nuanced approach. Let’s dig in and explore this question. 

What are buried ducts? 

For optimum HVAC performance, the ductwork system should be installed in a basement or conditioned space. However, as builders are well aware, digging a basement or adding bulkheads may not be realistic or feasible. It's particularly challenging in certain house designs like slab-on-grade or two-story structures with an open floor plan. Therefore, the solution to this common problem-especially in the southern region-is to install HVAC ducts in the attic.

However, this presents a new set of problems as it exposes the ductwork and air handler to extreme temperatures and humidity, which can result in heat losses between 10% and 45%. 

Burying ducts in attic insulation can be easily done and does not require special tools or materials. Revisions in the 2018 International Residential Code (IRC) and the 2018 International Energy Conservation Code (IECC) allow new provisions for partially and deeply buried ducts as a way to construct an energy-efficient option in vented attics. 

Requirements for Burying Ducts in Attic Insulation 

Before the IECC adopted buried ducts, previous code editions didn’t forbid it; they simply didn’t specify the conditions for performance and compliance. Whether or not your state has adopted 2018 IECC standards or you wish to achieve net zero building performance, it’s best practice to follow the requirements, which include using the proper R-value of the duct insulation by climate zone and the minimum attic insulation for burying ducts. 

There are several insulation methods based on attic design, but ducts placed over the bottom of truss chords and buried under insulation in a vented attic is a popular builder option. 

Is performance affected by insulation depth?

The 2018 provision offers specifications for two different options:

  1. Buried ducts: 3 inches of insulation above the duct
  2. Deeply buried ducts: adding an additional 6 to 7 inches of insulation

Not only does this buried ductwork improve performance, but it also helps reduce condensation, which is a common disadvantage in high-humidity climate zones. Therefore, the 2018 IECC offers these guidelines:

  • Climate zones 1A, 2A and 3A require R-13 or greater for buried supply ducts
  • All other climate zones require R-8 or greater

Can a Radiant Barrier Improve Buried Duct Performance?

LP recommends using LP® TechShield® Radiant Barrier Sheathing when installing deeply buried ducts. Installing LP TechShield sheathing helps offer impressive energy efficiency by blocking radiant heat in the roof panel from emitting into the attic, reducing attic temperatures by up to 30 degrees.

LP TechShield Radiant Barrier panel

The combination of deeply buried ducts and LP TechShield Radiant Barrier further reduces attic temperature fluctuations and helps keep ducts cooler. In addition to having the opportunity to "right-size" the HVAC for reduced costs, this performance combination also allows you to build a more comfortable and energy-efficient home for your client.

Buried ductwork has been approved by California Energy Code Title 24 and the DOE Zero Energy Ready Home program. The 2022 Inflation Reduction Act extends ENERGY STAR tax credits for your clients who purchase energy-efficient HVAC systems and duct insulation. 

Not sure how to explain all the benefits of LP TechShield Radiant Barriers to your homeowners? Keep this list of top benefits on hand to guide you.

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