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Plastic Ductwork Systems

Below ground ductwork system of pipe and fittings for slab foundation applications

Plastic ductwork systems are well suited for underground applications because they are resistant to rust and other forms of corrosion. These systems are manufactured with either high-density polyethylene (HDPE) or polyvinyl chloride (PVC). When installed properly, they are watertight and airtight and can withstand temperatures up to 150oF.

Plastic ductwork is available in diameters of 4 to 24 inches, with a variety of fittings including tees, wyes, elbows, 45-degree elbows, register boxes, saddle boots, saddle connections, reducers, couplings, end caps, and starting collars. Plastic plenum boxes are also available for underground installation.

While it is preferable to install ductwork in conditioned space to maximize energy efficiency, it is not always possible. In some places in the country, placing ducts underground may be the next best strategy, particularly if the slab edge is insulated. In hot climates, for example, it is more energy efficient for the cooling ducts to be under the slab than exposed in a hot attic. In areas where ground moisture and water intrusion may be an issue, however, extra care must be taken when installing the underground ductwork to protect it from excess water.

These underground air ducts and fitting systems were first introduced into the market in 2003 for use with forced-air heating and cooling systems. The ducts consist of a high-density polyethylene (HDPE) liner covered with a layer of air-entrained polyethylene. These HDPE pipes have a K-value equal to PVC wrapped with R5. The ducts and fittings are typically blue in color.

Conventional PVC plastic pipe can also be used in below-ground duct applications. The pipe and fittings are typically white, black, or gray in color. Sunlight contains ultraviolet rays, which degrade PVC, so care must be taken during construction to protect the PVC from exposure to the elements. The ductwork is the same material as PVC plastic pipe used in below-ground plumbing systems, which have been installed successfully in the United States since the 1950s.

Energy Efficiency

In cooling climates, installing ducts under the slab foundation is more energy efficient than installing the ductwork in an unconditioned attic.

Safety and Disaster Mitigation

Plastic ducts are non-corrosive. In addition, they are installed below-grade and not directly exposed to a potential fire within the building.


It is relatively easy for builders and HVAC contractors to transition to plastic ducts and fittings. Additional training may be needed for installers to learn to design underground layouts and the new connection methods. There are no special tools required; installers will need a circular or reciprocating saw, drill, utility knife, and a caulk gun.

On a component-to-component comparison basis, plastic ductwork and fittings are more expensive than metal ductwork and fittings. However, the overall installation cost is driven by design of the system and the labor required. It is possible that the new ductwork design demands more or less time and materials.

While there is no direct operational cost, any modifications needed to the ducting system may be more expensive, as the duct work under the slab would be much harder to access.

Must be in accordance with IMC Section 603.8.3   The Evaluation Guideline for Underground Plastic Air Ducts: EG290 April 1, 2005 is available at:

None available

The plastic ductwork and fittings are designed to install with ease. HDPE cuts without chipping or cracking. The ductwork uses dimensionally matched fittings and has a convenient strapping fixture. The HDPE pipe and fittings are manufactured with male ends and are joined with a gasket and clamp assembly. 

The plastic ducts are installed directly on the bottom of a compacted trench excavated to the desired level. The trench should slope to allow drainage to a point provided with access. Two sections of pipe (or pipe and fitting) are placed end to end. The adhesive backing is peeled off from a gasket and the gasket is wrapped around the pipe. Before the gasket is overlapped (approximately one inch), a bead of caulking is run along the leading edge. A clamp is placed around the gasketed joint with the clamp and gasket lined up. To keep the clamp from falling off, the unknotted end of the clamp is slid under the guides and over the first notch. The clamp handle is closed to finish tightening the joint. 

Embedded air duct pipe shall not be considered a structural replacement for the displaced concrete in the slab design. The loading of the pipe from wet concrete and/or soil above the pipe shall be limited so as not to produce deflections greater than 15 percent of the original pipe diameter. 

Manufacturers typically offer a 10-year warranty on the ductwork and fittings.

Plastic ductwork systems are durable, non-corrosive, water- and air-tight ductwork and fittings that can be installed subsurface in slab-on-grade applications. They are easy to install, and don’t require any special tools or equipment. However, underground plastic ductwork systems should only be used in new construction in sub-slab applications, as they can not be installed above ground, or where there will be any exposure to sunlight. While plastic ductwork systems are energy efficient, energy savings are maximized only when used in cooling climates, when it is not feasible to install the ductwork in conditioned space.

Disclaimer: The information on the system, product or material presented herein is provided for informational purposes only. The technical descriptions, details, requirements, and limitations expressed do not constitute an endorsement, approval, or acceptance of the subject matter by the NAHB Research Center. There are no warranties, either expressed or implied, regarding the accuracy or completeness of this information. Full reproduction, without modification, is permissible.