Duct board is a rigid fiberglass insulation panel with a factory-applied foil-scrim-kraft (FSK) facing that serves as both the duct wall and the insulation in a single material. Instead of fabricating a sheet metal duct and then wrapping it with insulation, duct board is scored, folded into rectangular shapes, and sealed with pressure-sensitive tape and mastic to create a finished, insulated duct in one step.
Duct board has been a standard HVAC material in the US since the 1970s. It is used extensively in residential and light commercial construction because it is faster to fabricate than sheet metal, costs less (no separate insulation step), and provides good thermal and acoustic performance. This guide explains how duct board works, compares it to alternatives, and helps you decide when it is the right choice for your project.
Duct Board Specifications
| Property | 1" Board | 1.5" Board | 2" Board |
|---|---|---|---|
| R-Value | R4.2 | R6.0 | R8.0 |
| Density | 6 lb/ft³ | 6 lb/ft³ | 6 lb/ft³ |
| Sheet Size | 48" × 120" | 48" × 120" | 48" × 120" |
| Max Velocity | 2,500 FPM | 2,500 FPM | 2,500 FPM |
| Max Pressure | 2" W.G. | 2" W.G. | 2" W.G. |
| Fire Rating | UL 181, Class 1 | UL 181, Class 1 | UL 181, Class 1 |
| NRC (Noise) | 0.45-0.65 | 0.55-0.75 | 0.65-0.85 |
Duct Board vs Sheet Metal vs Flex Duct
| Factor | Duct Board | Sheet Metal + Wrap | Insulated Flex |
|---|---|---|---|
| Material cost | $1.50-2.50/sq ft | $2.50-4.50/sq ft | $1.20-2.50/lin ft |
| Installation speed | Fast (score & fold) | Slower (fab + wrap) | Fastest |
| Insulation included | Yes (built-in) | No (separate step) | Yes (built-in) |
| R-value | R4.2-R8 | R4.2-R8 (wrap) | R4.2-R8 |
| Acoustic performance | Good (NRC 0.45-0.85) | Poor without liner | Good |
| Durability | 20-30 years | 50+ years | 15-25 years |
| Max pressure | 2" W.G. | 6"+ W.G. | 1-2" W.G. |
| Cleanability | Difficult | Easy | Not practical |
| Best for | Residential trunks | Commercial, long-life | Branch runs |
When to Use Duct Board
Best applications: Residential main trunk lines in attics and crawl spaces where the combined insulation-plus-duct in a single material saves significant labor. Small commercial tenant fit-outs where speed matters and system pressure is low (under 2" W.G.). Retrofit projects where sheet metal fabrication access is limited.
When not to use duct board: High-pressure commercial systems (above 2" W.G.), environments where duct cleaning is required (hospitals, food service), large trunk lines exceeding 48 inches on any side, exterior or exposed installations, and anywhere subject to physical damage or moisture contact.
When to Choose Alternatives
For branch runs in residential and light commercial systems, pre-insulated flex duct is faster and cheaper than duct board. For main trunks in commercial systems, sheet metal duct (either spiral or rectangular) with external duct insulation provides superior durability and pressure handling.
The most common modern residential system uses duct board for the main supply plenum and trunk, with insulated flex duct for all branch runs to individual rooms. This combination optimizes cost, installation speed, and performance.