What is flexible duct used for in HVAC?

Flexible duct connects rigid trunk lines to supply registers, return grilles, and HVAC equipment. It handles bends and offsets that would require multiple rigid fittings, simplifying installation in attics, crawlspaces, and tight ceiling plenums. Typical residential HVAC systems use 40-60% flexible duct by total run length.

What is the difference between R-6 and R-8 flexible duct?

R-6 has 2 inches of fiberglass insulation (thermal resistance R-6.0). R-8 has 2.5-3 inches of insulation (R-8.0). R-8 reduces heat loss/gain by approximately 25% compared to R-6. Most building codes require R-8 in unconditioned spaces (attics, crawlspaces). R-6 is acceptable in conditioned spaces or mild climates.

How do I size flexible duct by CFM?

Common sizing: 4-inch duct handles 30-50 CFM, 6-inch handles 75-125 CFM, 8-inch handles 150-250 CFM, 10-inch handles 250-400 CFM, 12-inch handles 400-600 CFM, and 14-inch handles 600-900 CFM. These assume 0.08 inches water gauge friction rate per 100 feet — the ACCA Manual D standard for residential ductwork.

Is flexible duct UL 181 rated?

Quality flexible duct carries UL 181 Class 1 rating, meaning it has passed 17 fire and mechanical tests including flame spread, smoke density, mold resistance, and collapse pressure. Always verify UL 181 Class 1 on the label — some economy imports carry only Class 0 (air connector) rating, which limits them to 14-foot runs.

How long does flexible duct last?

Quality flexible duct with intact vapor barrier lasts 15-25 years. The inner core (wire helix + polyester) does not degrade, but the fiberglass insulation can settle (reducing R-value) and the outer vapor barrier can tear from UV exposure, rodents, or physical contact. Duct installed in accessible attics with proper support straps has the longest service life.

Flexible Duct: Types, Sizes & R-Value Guide for HVAC [2026]

Flexible duct is the workhorse connector in residential and light commercial HVAC. It bridges the gap between rigid trunk lines and the registers in every room — handling the bends, offsets, and tight spaces that would require five or six rigid fittings to navigate. Approximately 70% of new residential HVAC installations in the US use flexible duct for branch runs.

This guide covers the specifications that matter when specifying or sourcing flexible duct: construction layers, R-value options, sizing by CFM, UL 181 compliance, and the installation practices that separate an efficient system from one that wastes 20-30% of its airflow.

What Is Flexible Duct

Flexible duct is a round, compressible duct made of three layers: an inner core (wire helix wrapped in polyester or metalized polyester), fiberglass insulation, and an outer vapor barrier (polyethylene jacket). The wire helix maintains the duct's shape and provides crush resistance, while the insulation prevents condensation and heat transfer.

The inner core comes in two variants: metalized polyester (standard, adequate for most residential and commercial applications) and all-metal (aluminum or stainless, required for high-temperature applications above 250°F). Most HVAC applications use the metalized polyester core.

Flexible Duct Types

Insulated flexible duct (R-6 or R-8) is the standard for all HVAC supply and return runs. The fiberglass insulation prevents heat loss in heating mode and condensation in cooling mode. R-8 is required by code in most US jurisdictions for duct in unconditioned spaces (attics, crawlspaces, garages).

Uninsulated flexible duct has only the inner core and outer jacket — no fiberglass. Used for exhaust fan connections, dryer venting (with appropriate UL 2158A listing), and short connections within conditioned spaces where insulation is unnecessary.

Insulated return air duct is identical in construction to supply duct but often available in larger diameters (14-20 inches) to handle the lower-velocity, higher-volume airflow typical of return systems.

Anti-static / conductive flexible duct has a carbon-loaded inner core that dissipates static electricity. Required for dust collection systems, cleanrooms, and environments with combustible dust (NFPA 652 compliance). See our ATS conductive duct guide for specifications.

Sizing Flexible Duct by CFM

Correct sizing is the single most important factor in flexible duct performance. An undersized duct creates excessive velocity and noise; an oversized duct wastes material and space. The sizing relationship between duct diameter and CFM capacity (at 0.08" WG friction rate per 100 feet, per ACCA Manual D):

Duct Diameter	CFM Range	Typical Application
4"	30–50	Bathroom exhaust, small supply
5"	50–75	Small bedroom supply
6"	75–125	Standard bedroom/office supply
7"	125–175	Large bedroom, kitchen
8"	150–250	Living room, master suite
10"	250–400	Large open areas, return runs
12"	400–600	Main return, large commercial
14"	600–900	Trunk connections, RTU supply

Critical note: These CFM values assume the duct is fully stretched. Compressed (bunched) flexible duct increases friction dramatically — a 6-inch duct compressed to 70% of its extended length loses approximately 50% of its airflow capacity. Always pull flexible duct to its full extended length during installation.

R-6 vs R-8: Which Insulation Level

R-6 (2" insulation): Adequate for duct runs within conditioned spaces (between floors, in interior soffits). Lower material cost. Some mild-climate jurisdictions (IECC Climate Zones 1-3) still allow R-6 in attics, but this is changing as energy codes tighten.

R-8 (2.5-3" insulation): Required by IRC/IECC in unconditioned spaces for Climate Zones 4-8, which covers most of the continental US. Reduces heat loss/gain by approximately 25% versus R-6. The higher material cost ($0.30-0.50/ft more) is offset by energy savings within 1-2 heating/cooling seasons.

Our recommendation: Default to R-8 for all installations unless the duct is entirely within conditioned space. The incremental cost is minimal and the energy performance is measurably better. For wholesale buyers, R-8 is increasingly the only SKU worth stocking as R-6 code allowances shrink.

Installation Best Practices

Fully extend the duct. This is the single most common installation error. Compressed duct has dramatically higher friction and lower airflow. Pull the duct taut between support points.

Support every 4-5 feet. Use 1.5-inch or wider straps — never wire or narrow ties that compress the duct. IRC M1601.4.1 requires support intervals not exceeding 5 feet and maximum sag of ½ inch per foot between supports.

Minimize bends. Each 90° bend in flexible duct is equivalent to approximately 15-20 feet of straight run in friction loss. Two tight bends can cut airflow by 40%. Use rigid elbows at the trunk takeoff and let the flex handle the final connection to the register.

Seal connections with mastic + mechanical fastening. Use a draw band or zip tie over mastic sealant at every connection. Tape-only connections fail within 5-10 years as adhesive degrades. For UL 181-listed duct, use UL 181B-FX listed closure systems.

Wholesale Sourcing

USFlexDuct stocks insulated flexible duct in R-6 and R-8 configurations, 4-inch through 20-inch diameters, in 25-foot standard lengths. All products carry UL 181 Class 1 listing and meet IECC energy code requirements.

For contractor bulk pricing, distributor programs, or custom specifications (anti-static, high-temperature, or non-standard lengths), contact our team for a quote.