Flexible duct, known as flex duct, is very convenient ducting for attaching supply air outlets to rigid trunk ductwork, and, in many cases, flex duct comprises the entire duct system. Flex duct made for HVAC applications is typically constructed of a plastic inner liner attached to a metal wire helix (or coil) to make round, flex duct. The duct comes with a layer of fiberglass blanket insulation already attached around the duct. The insulation is covered and protected by a polyethylene or foil vapor barrier. Flex duct is typically available in insulation values of R-4, R-6, and R-8. For residential HVAC systems, insulated flex duct typically comes in diameters of 4 inches through 10 inches; above 10 inches, it comes in even sizes of 12, 14, 16, etc., up to 22 inches measured at the radius of the metal helix.
For proper performance, flex ducts in unconditioned space should not be installed in cavities smaller than the outer diameter of the duct, including the insulation. Compression of the insulation around the duct should be avoided, especially in humid climates where compressed insulation could encourage condensation to form. Even when ducts are installed in conditioned space, they should not be installed in cavities smaller than the inner duct diameter.
During the design phase, duct routing should be identified. Open-web trusses between floors allow for ducts to freely pass through the floor system without compromising the structure. Manual J and D calculations should be performed to identify the size of ducting needed and thus how much space is required for the ductwork. Remember to take into account the thickness of the insulation (Figure 2). For example, if R-6 flex duct is used, then the exterior diameter of the duct will increase by four inches. Plan air handler locations, making sure there is clear access for the flex duct (or other specified ducting) to run from the mechanical space (or room) to the duct zone (Figure 3). Manual D Residential Duct Systems (ACCA 2009) sets standards for flex duct design and installation. See Appendix 17 for a detailed discussion of duct installation. The Flexible Duct Performance Standards (Fifth Edition) by the Air Diffusion Council (ADC) provides guidance to designers, architects, engineers, contractors, and installers.
For more on flex duct installation, see No Kinks or Sharp Bends in Flex Duct Installation, Support at Intervals for Flex Ducts, No Excessive Coiled or Loped Flex Ducts and Sealed and Insulated Flex Ducts.
How to Install Flex Ducts within Appropriate Cavity Space
1. Design ducts in compliance with Manual D. Once the appropriate duct diameter is determined, establish the duct layout in coordination with the framing plan. Ensure that adequate space is allowed for all trunk and branch duct runs.
Figure 2 - R-6 flex duct has 2 inches of insulation around the inner liner. If the required duct run is a 12-inch round, the exterior radius will be about 16 inches. At a minimum, a 16x16-inch duct chase or opening would be required along the full length of the duct.
Figure 3 - Ensure that the air handler location does not conflict with duct routing.
Figure 4 - The height and width of a dropped soffit used to house ductwork should be the duct diameter plus the thickness of the insulation to prevent compression of the duct.
2. If room is tight and there is fear of compressing the flex duct when making a 90-degree turn, use a metal elbow.
Figure 5 - Use a metal duct elbow instead of flex duct at boot connections to prevent compressions.
3. Coordinate with the framer, plumber, and electrician for effective duct installation with adequate space.