DOUBLE-WALL AND INSULATED DUCT
Not every duct system can be bare single-wall sheet metal. When a run has to control heat loss, prevent condensation, or knock down noise, the spec calls for insulation, and that is where double-wall duct comes in.
This is a quick guide for mechanical contractors and project teams deciding between single-wall duct with an external wrap and shop-built double-wall duct.
Published July 2, 2026 · JMC Fabrication
Double-wall duct is exactly what it sounds like: an outer pressure wall, a layer of insulation, and an inner liner. The insulation does the thermal and acoustic work, and the inner liner protects it from the airstream so it does not shed or erode over time.
The alternative is single-wall duct with insulation wrapped on the outside in the field. Both approaches insulate, but they behave differently and suit different jobs.
Insulated or double-wall duct usually shows up for one of three reasons:
- Thermal control — Keeps conditioned air at temperature through unconditioned spaces, which protects efficiency on long runs, rooftop duct, and cold-supply systems.
- Condensation control — Stops sweating on cold duct in humid Gulf Coast environments, which prevents drips, corrosion, and mold.
- Acoustic control — The inner liner absorbs fan and airflow noise, which matters near equipment rooms and in noise-sensitive occupied spaces.
Construction follows the requirement:
- Solid or perforated inner liner, perforated where acoustic absorption is the goal
- Insulation thickness set by the thermal or acoustic target
- Galvanized steel for standard commercial work
- Stainless liner where the airstream is corrosive or requires washdown
- Built and sealed in the shop for consistent, clean results
External wrap is cheaper and faster where appearance and durability are not concerns. Double-wall is the better call when the duct is exposed, when the liner has to stand up to airflow and cleaning, or when acoustic performance matters. It costs more up front and pays back in durability and finish.
JMC builds both, and the right answer depends on where the duct runs and what it has to do. Send us the mechanical spec and we will fabricate to it.
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