Flexible Duct and Duct Board: Materials and Installation Techniques

Flexible Duct and Duct Board: Materials and Installation Techniques

Flexible duct and duct board are two of the most common materials used to distribute conditioned air in residential and light-commercial systems. Used correctly, they deliver efficient, quiet airflow. Used incorrectly, they create pressure drop, condensation problems, and callbacks. This lesson covers both materials from the workbench perspective: what they are, how they connect, and how to install them right the first time.

Flexible Duct: Construction and Classification

Flexible duct (flex duct) is an inner liner — typically a spiral-wound wire helix covered with a plastic film — surrounded by a layer of fiberglass insulation and wrapped in a vapor-barrier outer jacket. Most residential flex is sold as Class 1 duct under UL 181, meaning it has passed flame-spread and smoke-development tests required by the standard. Always verify the UL 181 listing label on the product before installation; unlisted material cannot be used in return-air or supply applications governed by mechanical codes.

Flexible duct comes in nominal diameters from about 4 inches to 20 inches. The inner liner diameter — not the outer jacket — is the dimension that matches the collar on a diffuser, branch takeoff, or plenum boot.

The Two-Step Draw-Band Connection

A secure, airtight connection at each end of a flex run requires three elements: a properly sized collar, a listed draw band (also called a strap clamp or worm-drive clamp rated UL 181B), and an outer-jacket seal.

Step 1 — Inner liner: Pull back the outer jacket and insulation to expose 2–3 inches of the inner liner. Slide the liner fully over the collar so it seats at the bead or stop. Position the draw band over the liner within the distance specified by the manufacturer (commonly within 1 inch of the collar end). Tighten the band until the liner is compressed firmly and evenly around the collar — overtightening can tear the liner.

Step 2 — Outer jacket: Pull the insulation and outer jacket back over the connection point. Secure the outer jacket with a second listed draw band, positioned so the jacket cannot slide back and expose bare insulation. Some manufacturers also require mastic or listed tape at the outer jacket to complete the vapor seal; follow the product instructions.

Never use standard duct tape (cloth-backed) on either connection — it fails over time and is not UL 181B listed.

Support Spacing and Bend Radius

Two installation requirements directly affect airflow performance and are frequently cited by inspectors.

Support spacing: Flex duct must be supported at intervals that prevent sagging. The Sheet Metal and Air Conditioning Contractors' National Association (SMACNA) HVAC Duct Construction Standards and most flex-duct manufacturers specify supports no more than 4 feet apart for horizontal runs. Supports must be at least 1½ inches wide (to avoid cutting into the jacket) and must not compress the duct.

Bend radius: Every bend reduces airflow because the inner liner bunches up on the inside of the curve, reducing the effective cross-sectional area. The minimum bend radius for most flex duct is equal to one duct diameter measured to the centerline of the duct — some manufacturers specify 1.5 times the diameter. Always check the product data sheet and use a bend-radius template card on the job. Never allow a bend tighter than the manufacturer's minimum; a crushed or kinked flex run can increase static pressure dramatically.

Keep runs as short and as straight as practically possible. Every unnecessary foot of flex and every tight bend adds resistance (static pressure) and reduces airflow at the diffuser.

Duct Board: Construction and Classification

Duct board is a rigid fiberglass panel with a foil facing on the air-side surface. It is sold in R-value ratings; R-6 foil-faced duct board is common for supply plenums in conditioned spaces. Like flex duct, duct board must carry a UL 181 listing. The foil facing is the air barrier and must always face inward (toward the airstream).

Score-and-Fold Fabrication

Duct-board panels are fabricated using a utility knife and a straightedge. The score-and-fold method creates a corner joint without a separate piece of material:

1. Lay the panel foil-side down on a clean, flat surface.

2. Measure and mark the fold line. Using a straightedge as a guide, score the back (non-foil) face with a utility knife to a depth of about two-thirds of the panel thickness — do not cut through the foil facing.

3. Fold the panel along the score line. The foil facing acts as a living hinge; the fiberglass opens a V-groove on the outside of the fold.

4. Apply listed duct-board staples ("outward clinch" staples) along the corner at intervals specified by the manufacturer, typically every 4–6 inches, to hold the joint square.

5. Inspect the foil seams at each corner. Any gap greater than 1/8 inch must be closed with listed foil-backed tape or mastic before the section is placed in service — gaps allow moisture and unconditioned air into the insulation core.

Safety

• Fiberglass exposure: Duct board and flex duct insulation release fine glass fibers. Wear safety glasses and nitrile or work gloves at all times when cutting or handling these materials. A dust mask (N95 minimum) is recommended in enclosed spaces.
• Cutting tools: The utility knife blade is sharp enough to cause a deep laceration. Always cut away from your body, retract the blade when not in use, and use a metal straightedge — never a plastic ruler — as a cutting guide.
• Housekeeping: Fiberglass dust settles on all surfaces. Clean the work area after fabrication and wash hands and forearms before breaks.
• Ladder safety: When hanging flex duct in a mock-up frame or actual installation, maintain three points of contact on ladders and never overreach.