Flex Duct vs Sheet Metal: A Contractor's Comparison

The flex-vs-metal debate has been running in the HVAC trade for decades, and it usually generates more heat than light. Both materials have a place. The problem is that flex duct gets used in situations where its performance limitations cost homeowners comfort and energy, while sheet metal gets avoided in situations where it genuinely is overkill. This guide gives you the numbers so you can make the right call on every job.

Friction Rate: The Core Difference

This is where the comparison starts and, for many applications, where it ends. The corrugated inner liner of flex duct creates dramatically more friction than the smooth interior of sheet metal duct. How much more depends on how well the flex is installed.

Even in the best case (fully stretched, perfectly supported, no turns), flex duct has roughly twice the friction rate of sheet metal. In a typical field installation with some sag between supports, the friction rate is 3x or more. With kinks, sharp bends, or excessive length, it can exceed 5x.

What this means practically: a 25-foot run of 6" flex duct at typical installation quality has the same friction loss as 75-100 feet of 6" round sheet metal duct. If your duct sizing calculation was based on sheet metal friction charts, you need to either size up the flex duct (typically by 2 inches in diameter) or keep the run extremely short.

Velocity and CFM Impact

Higher friction means lower airflow at the same static pressure. On a system sized with Manual D for sheet metal, substituting flex duct without resizing can reduce delivered CFM by 15-30% per run. The rooms at the end of those runs get noticeably less heating and cooling.

ACCA Manual D provides correction factors for flex duct. The standard approach is to use flex duct friction charts (published by manufacturers and ACCA) instead of sheet metal charts when sizing runs that will use flex. If you size on sheet metal charts and then install flex, the system will underperform.

Velocity also behaves differently. Because flex duct has a corrugated interior surface, the effective diameter is slightly smaller than the nominal diameter. A 6" flex duct has an actual interior diameter closer to 5.75" when compressed at typical levels. This further increases velocity and friction beyond what the nominal size suggests.

Cost Comparison: Material vs. Total Installed

Flex duct has a major advantage in material cost and installation speed. Here is a realistic breakdown for a typical residential branch run (6" diameter, 15-foot run from trunk takeoff to register boot):

Flex duct is roughly half the installed cost of round sheet metal and one-third the cost of rectangular sheet metal per run. On a 15-run residential system, that difference adds up to $600-$1,800 in total installed cost. This is why flex duct dominates residential new construction, especially in production homebuilding where labor is the biggest cost driver.

However, the total cost of ownership tells a different story. The increased friction of flex duct means the blower works harder, consuming more electricity over the life of the system. A DOE study estimated that poorly installed flex duct can increase HVAC energy consumption by 10-25%. On a system running $150/month in energy costs, that is $180-$450/year in extra operating cost. The sheet metal premium pays for itself in 2-5 years.

Code Limitations on Flex Duct

Building codes and mechanical codes place restrictions on flex duct that do not apply to sheet metal. Most jurisdictions adopt some version of these rules:

• Maximum length. Many codes limit individual flex duct runs to 14 feet (IRC M1601.4.1). Some jurisdictions allow longer runs but require engineering justification.
• Support spacing. Flex duct must be supported at intervals not exceeding 5 feet, and at each connection point. Maximum sag between supports is typically 0.5 inches per foot of spacing.
• Bend radius. The minimum bend radius is typically one duct diameter. A 6" flex duct cannot make a turn tighter than 6" radius without kinking and severely restricting airflow. In practice, larger radii are strongly recommended.
• No kinks or compression. Flex duct must not be compressed, kinked, or pinched by framing, wiring, plumbing, or other obstructions. This is the most commonly violated code provision and the primary cause of poor flex duct performance.
• Connector requirements. Connections must use approved clamps or ties and be sealed with mastic or UL 181B tape. Cable ties alone do not meet code in most jurisdictions.
• Return plenums. Many codes prohibit flex duct for return air plenums. The return side is often limited to sheet metal construction.

Commercial buildings typically have stricter requirements. Many commercial mechanical codes prohibit flex duct entirely, or limit it to final connections under 5 feet between a sheet metal trunk and a diffuser.

When Flex Duct Makes Sense

Despite the performance limitations, flex duct is the right choice in specific situations:

• Final connections (last 5-6 feet). The short run from a sheet metal trunk takeoff collar to a ceiling or wall register is the ideal flex application. The run is short enough that friction penalties are minimal, and the flexibility makes alignment with the register boot easy.
• Retrofit work in tight spaces. When adding a duct run through an existing attic or crawlspace with obstacles that would require multiple elbows and offsets in sheet metal, flex can navigate around obstacles with fewer joints.
• Vibration isolation. A short section of flex between the air handler and the first sheet metal fitting dampens blower vibration and noise. This is a legitimate engineering use that most codes explicitly allow.
• Temporary installations. Temporary heating/cooling during construction does not justify the cost of sheet metal duct that will be removed.

When Sheet Metal Is the Clear Winner

• Main trunk lines. The trunk carries the full system CFM. Friction losses here affect every room in the house. Sheet metal trunks are non-negotiable on any properly designed system.
• Runs over 14 feet. Beyond code limits, the performance degradation of flex duct on long runs makes it impractical. The friction penalty stacks linearly with length.
• Return air systems. Return ducts are already undersized in most residential systems. Adding flex duct friction to the return side starves the blower. Use sheet metal straight duct and sheet metal return boots.
• Commercial applications. Code restrictions aside, commercial systems have higher CFM, longer runs, and tighter static pressure budgets that demand the lower friction of sheet metal.
• Static-pressure-limited equipment. High-efficiency furnaces and mini split air handlers often have 0.20" to 0.40" w.c. of available static pressure. Every tenth of an inch matters. Flex duct can consume the entire static budget on a single run.
• Any exposed or accessible duct. Sheet metal looks professional, holds its shape permanently, and does not sag, tear, or deteriorate. Exposed flex duct in a basement or garage looks like a problem.

The Hybrid Approach

The most cost-effective residential duct system uses both materials in their strengths. Sheet metal for the trunk system (plenum, main trunk, trunk reductions via transitions and reducers) and sheet metal for all return ductwork. Flex duct for the final 5-6 feet of each supply branch run from the sheet metal takeoff collar to the register boot.

This hybrid approach captures 80% of the labor savings of an all-flex system while maintaining the airflow performance of an all-metal system. The trunk handles the high-CFM, high-friction work in sheet metal. The short flex connections handle the final alignment and vibration isolation where their flexibility is genuinely useful.

Get Sheet Metal Duct Fabricated to Your Specs

At PMX Ductwork, we fabricate every type of sheet metal fitting in galvanized, aluminum, or stainless steel: straight duct, elbows, tees, wyes, transitions, reducers, and more. Any size from 2" to 48" per side, with your choice of connection types. Custom dimensions, instant pricing, no minimum order.