Technical Foundation
What Is a Duct Insulation?
Duct insulation controls heat transfer and condensation on HVAC ductwork, applied either externally as wrap or board around sheet metal ducts, or internally as liner board that also provides some acoustic dampening but reduces the duct's effective airflow cross-section. R-value — the material's resistance to heat flow — is the primary spec, driven by local energy code (commonly ASHRAE 90.1 or IECC) requirements that vary based on whether the duct run is in conditioned or unconditioned space and the local climate zone.
Facing material matters as much as R-value: foil-scrim-kraft (FSK) facing provides a vapor barrier critical on cooling supply ducts where condensation on the duct exterior would otherwise cause moisture damage, while unfaced or differently faced products are used where vapor barrier isn't the primary concern. Duct liner, applied to the inside of ductwork, must meet fire and mold-resistance standards (commonly tested to UL 181 or NFPA 90A) since it's in direct contact with the conditioned airstream.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| R-Value | Typically R-4.2 to R-8 for duct wrap/board | Set by local energy code based on climate zone and whether duct is in conditioned space |
| Facing/Vapor Barrier | FSK (foil-scrim-kraft), foil, unfaced | FSK vapor barrier is critical on cooling ducts to prevent exterior condensation damage |
| Application | External wrap/board vs. internal liner | Liner reduces effective duct cross-section but adds acoustic dampening |
| Fire/Mold Rating | UL 181, NFPA 90A compliance for liner materials | Required for materials in direct contact with the conditioned airstream |
| Thickness | Commonly 1in to 2in for wrap/board | Higher R-value generally requires greater thickness at a given material density |
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Applications
Where Duct Insulation Get Used
Duct insulation is specified anywhere HVAC ductwork runs through unconditioned space or needs condensation, thermal loss or noise control.
Commercial HVAC Installation
Duct wrap and board applied to exterior of sheet metal ductwork in unconditioned attics, crawlspaces and mechanical rooms.
Residential HVAC Retrofit
R-6 to R-8 duct wrap added to existing ductwork runs through unconditioned attic and basement space to reduce energy loss.
Data Center & Cleanroom HVAC
Rigid duct board and liner specified where consistent thermal performance and airflow noise control both matter.
Industrial Process Exhaust
Insulated ductwork specified to control condensation and heat loss on process exhaust and makeup air systems.
New Commercial Construction
Duct liner board specified to meet combined thermal and acoustic code requirements in office and institutional buildings.
FAQ
Duct Insulation Questions, Answered
What R-value do I need for duct insulation?
Required R-value is generally set by local energy code (ASHRAE 90.1 or the IECC, as adopted locally), based on the climate zone and whether the duct run is in conditioned or unconditioned space — ducts in unconditioned attics or crawlspaces typically require higher R-values than ducts already inside conditioned space, so check the locally adopted code rather than assuming a single standard value.
Why does duct wrap need a vapor barrier facing like FSK?
On cooling supply ducts, the duct surface runs colder than the surrounding air, and without a vapor barrier facing, moisture from the ambient air can condense on or within the insulation, leading to mold growth and water damage. FSK (foil-scrim-kraft) facing provides that vapor barrier, which is why it's specified on nearly all exterior duct wrap used on cooling systems.
What's the difference between duct wrap and duct liner board?
Duct wrap is applied to the outside of finished sheet metal ductwork purely for thermal control. Duct liner board is installed inside the ductwork itself, forming the airstream-contact surface, which adds acoustic dampening on top of thermal insulation but slightly reduces the duct's effective cross-sectional area for airflow and must meet stricter fire and mold-resistance testing since it's in direct contact with the conditioned air.
Can duct liner be used in any HVAC system?
Duct liner must meet fire and mold-resistance standards such as UL 181 and NFPA 90A compliance since it directly contacts the airstream, and some facilities (hospitals, cleanrooms, certain IAQ-sensitive spaces) restrict or prohibit fibrous duct liner due to indoor air quality concerns — check the project's mechanical specification before defaulting to liner over external wrap.
How thick does duct board need to be for rigid duct fabrication?
Rigid fiberglass duct board used to fabricate ductwork itself (rather than wrap or liner added to sheet metal) is typically 1in thick in standard commercial applications, providing both the structural rigidity to form duct shapes and a built-in R-value, but always confirm against the specific product's structural and code rating for the duct system's pressure class.
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