In HVAC construction, ductwork is more than just a pathway for air it’s a critical component that affects system performance, energy efficiency, installation labor, and long-term durability. Whether you’re bidding on a school in Texas, a hospital in Houston, or a commercial tower in Dallas, choosing the right duct material is essential for accurate estimating and successful project delivery.
The three most common duct materials galvanized steel, spiral duct, and flexible duct each have unique characteristics that influence cost, labor, and design. This guide breaks down the differences between these materials, explores their pros and cons, and shows how they impact your estimating strategy.
Table: Duct Material Comparison and Estimating Impacts
| Duct Type | Description | Pros | Cons | Estimating Impact |
| Galvanized Steel | Flat sheet metal formed into rectangular ducts | Durable, fire-resistant, widely used | Heavier, labor-intensive to fabricate | Higher labor cost, moderate material cost |
| Spiral Duct | Round duct formed from coiled sheet metal | Aesthetic, efficient airflow, fewer joints | Requires specialized fabrication equipment | Lower labor cost, slightly higher material cost |
| Flexible Duct | Wire coil with plastic or foil insulation | Easy to install, low cost, ideal for tight spaces | Prone to damage, lower airflow efficiency | Low labor and material cost, limited durability |
Sources: Vairtech, Heater Guides, Air Doctor Tulsa
Galvanized Steel Ducts
Galvanized steel is the most widely used duct material in commercial and institutional projects. It’s made by coating steel with zinc to prevent corrosion, then forming it into rectangular duct sections.
Advantages
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- Extremely durable and long-lasting
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- Fire-resistant and compliant with building codes
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- Ideal for high-pressure systems and long runs
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- Commonly specified in public-sector projects
Limitations
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- Heavy and labor-intensive to fabricate
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- Requires skilled sheet metal workers
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- More joints and sealants needed compared to spiral duct
Estimating Notes
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- Include labor for cutting, sealing, and assembly
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- Account for hangers, supports, and fire-rated components
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- Often used in schools, hospitals, and government buildings
Spiral Ducts
Spiral ducts are round and formed from coiled sheet metal. They’re popular in exposed ceiling designs and offer better airflow due to fewer joints and smoother interiors.
Advantages
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- Faster to install than rectangular duct
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- Fewer fittings and sealants required
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- Aesthetic appeal in open-ceiling designs
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- Efficient airflow with minimal resistance
Limitations
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- Requires specialized fabrication equipment
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- May be more expensive per linear foot than galvanized
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- Not ideal for tight or irregular spaces
Estimating Notes
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- Lower labor cost due to faster installation
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- Fewer accessories needed (e.g., turning vanes)
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- Coordinate with architectural layout for exposed areas
Flexible Ducts
Flex duct is made of a wire coil covered with plastic or foil insulation. It’s lightweight, easy to install, and ideal for short runs in residential or light commercial projects.
Advantages
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- Quick and easy to install
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- Low material and labor cost
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- Ideal for tight spaces and irregular layouts
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- Pre-insulated options available
Limitations
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- Prone to tearing, compression, and airflow loss
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- Not suitable for long runs or high-pressure systems
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- Shorter lifespan compared to metal duct
Estimating Notes
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- Include waste factor (typically 8–10%) for damage and scrap
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- Limited use in public projects due to durability concerns
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- Ideal for branch runs in classrooms or offices
Estimating Tips by Duct Type
| Tip | Applies To | Why It Helps |
| Include fabrication labor | Galvanized | Accounts for cutting, sealing, and assembly |
| Factor in fewer fittings | Spiral | Reduces material and labor costs |
| Add waste factor for damage | Flex | Prevents shortages due to tearing or compression |
| Review specs for fire rating | Galvanized & Spiral | Required for schools and public buildings |
| Coordinate with ceiling layout | Spiral | Ensures aesthetic alignment in exposed designs |
| Use pre-insulated options | Flex | Saves time and improves energy efficiency |
| Break out duct types in estimate | All | Improves clarity and helps with procurement and scheduling |
Real-World Example: Mixed Duct System in a Texas School
An HVAC contractor bidding on a new elementary school in Austin used:
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- Galvanized steel for main trunk lines
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- Spiral duct for exposed corridors and gymnasium
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- Flex duct for short branch runs to classrooms
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- Applied 8% waste factor for flex duct
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- Estimated labor separately for each duct type
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- Delivered a CSI-formatted estimate with clear material breakdown
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- Coordinated with architectural team for exposed spiral layout
The result? A competitive bid that won the project and passed inspection with zero duct-related change orders.
Financial Impact of Duct Material Selection
| Benefit | Impact on Your Business |
| More Accurate Bids | Avoids underestimating labor or material costs |
| Fewer Change Orders | Clear scope and material breakdown reduces disputes |
| Improved Profit Margins | Right material choice balances cost and performance |
| Better Client Trust | Professional estimates build credibility with GCs and owners |
| Efficient Scheduling | Knowing labor requirements helps plan crews and timelines |
| Smarter Procurement | Clear material list improves ordering and delivery coordination |
Duct Material Estimating Checklist
Before submitting your next HVAC bid, make sure you:
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- Identify duct types by zone or system
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- Include labor rates for each material
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- Apply waste factors based on duct type
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- Review specs for fire rating and insulation
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- Coordinate with architectural and structural drawings
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- Break out duct types in CSI Division 23 format
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- Document assumptions and exclusions
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- Include mockup and testing allowances (if required)
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- Reference historical productivity data for labor forecasting
Final Thoughts
Understanding duct materials isn’t just a technical detail it’s a strategic advantage. Galvanized, spiral, and flex ducts each affect labor, cost, and performance in different ways. Estimators who understand these impacts can produce more accurate bids, reduce change orders, and deliver better HVAC systems.
At MSB Estimating, we help HVAC contractors across Texas produce precise duct takeoffs and material breakdowns tailored to each project. Whether you’re bidding on a school, hospital, or commercial tower, we’ll help you estimate smarter and build better.
Need Help With Duct Estimating?
Contact MSB Estimating today to streamline your HVAC bids with expert takeoff services.
