Metal vs Flex Ducts: Choosing the Right Type
Overview
Choosing between metal and flex ductwork is one of the first decisions in any HVAC installation. Each type has distinct advantages and trade-offs that affect performance, cost, and installation complexity.
Metal Ducts
Metal ductwork — typically galvanized steel or aluminum — is the industry standard for main trunk lines and commercial installations.
Advantages:
- Smooth interior surface = lower friction loss
- Durable and long-lasting (25+ years)
- Better structural integrity
- Easier to clean and maintain
- Lower noise at high velocities
Disadvantages:
- Higher material and labor costs
- Requires skilled fabrication
- More difficult to route around obstacles
Flex Ducts
Flexible ductwork uses a wire helix covered with plastic and insulation, allowing it to bend and flex around obstacles.
Advantages:
- Lower installation cost
- Easy to install in tight spaces
- Built-in insulation
- Quick installation time
Disadvantages:
- Higher friction loss (corrugated interior)
- Prone to kinking and compression
- Shorter lifespan
- Not suitable for main trunk lines
Friction Loss Comparison
The key calculation difference is that flex ducts have significantly higher friction loss due to their corrugated interior. When using our calculator:
- Metal ducts: Use standard friction rates
- Flex ducts: The calculator automatically increases equivalent length by 50% for bends
When to Use Each Type
| Scenario | Recommended |
|---|---|
| Main trunk lines | Metal |
| Branch runs to registers | Either |
| Tight retrofit spaces | Flex |
| Commercial buildings | Metal |
| Budget-conscious residential | Flex |
The best systems often combine both: metal for the main trunk and flex for the final connections to registers.
Use our HVAC Duct Calculator to size ducts for either material type. The calculator automatically adjusts friction calculations for flex duct.