Duct Insulation

What Duct Insulation Does and Why It Matters

Duct insulation is applied to heating, ventilation and air-conditioning (HVAC) ductwork to control heat gain and loss on supply and return runs, prevent condensation on chilled surfaces, reduce HVAC noise, and meet energy-code minimums. On any duct routed through an unconditioned or indirectly conditioned space — attics, crawlspaces, parking garages, mechanical shafts, above-ceiling plenums — bare sheet metal can waste 10 to 30% of delivered heating or cooling before it reaches the zone. The right specification depends on duct location, air temperature, ambient humidity and the applicable code. For broader mechanical-insulation context, see our pipe insulation and building insulation guides.

Duct Insulation Materials and Forms

Two inorganic fibre families dominate commercial and industrial ductwork: glass wool (fiberglass) and mineral wool (stone wool). Both are non-combustible and available as flexible wraps, semi-rigid slabs and rigid boards.

*Continuous service temperature is limited by the aluminium foil facing bond; select thickness so the outer foil stays below its rated surface temperature. Explore our glass wool blanket, glass wool board, mineral wool blanket and mineral wool board product lines.

Standards and Compliance

Duct insulation is specified against published material and fire standards rather than brand names:

• ASTM C1290 — mineral-fiber duct wrap (flexible blanket).
• ASTM C1071 — mineral-fiber duct liner (internal).
• ASTM C665 — mineral-fiber blanket and felt thermal/acoustical insulation.
• ASTM C612 — mineral-fiber block and board (rigid forms).
• UL 181 — factory-made air ducts and connectors (Class 1 / 181B-FX for flexible duct).
• EN 14303 — European specification for factory-made mineral-wool (MW) products for building equipment and industrial installations, including duct wrap and slab.
• ASTM E84 — surface burning (flame spread / smoke developed), commonly specified at 25/50 for plenum use.
• EN 13501-1 — reaction-to-fire classification; mineral wool products are A1 (non-combustible).
• SMACNA / ASHRAE 90.1 — installation practice and minimum R-values by climate zone.

R-Value and Thickness Selection

ASHRAE 90.1 sets minimum duct insulation R-values that rise with climate severity. Typical planning values for ducts in unconditioned spaces:

Installed R-value scales with thickness: 1 in (25 mm) duct wrap is about R-4 to R-4.5, 1.5 in (40 mm) about R-6, 2 in (50 mm) about R-8, and 3 in (75 mm) about R-12. Always verify against the local energy code and the project's green-building target (see green building insulation).

Service Temperature and Fire Performance

Glass wool duct wrap is commonly rated for continuous service around 230 °C, set by the aluminium foil facing bond rather than the fibre core; the glass wool core itself supports the broader glass wool range up to 400 °C. Mineral wool duct products carry the same ~230 °C foil limit, with the stone wool core rated higher. Both materials are non-combustible: mineral wool is classified A1 under EN 13501-1, and glass wool duct products typically meet ASTM E84 25/50 (Class A). For smoke-extraction and fire-resistance-rated shafts, specify a documented fire-rated duct system rather than a generic wrap. See our fire protection insulation guide for rated assemblies.

Vapour Barrier and Condensation Control

On below-ambient or chilled ductwork, a continuous vapour barrier is essential. Most duct wrap is factory-laminated with a foil-scrim-kraft (FSK) or all-service jacket (ASJ) facing; seams, joints and fixing penetrations must be sealed with UL-listed foil tape (UL 181A-P / 181B-FX) to maintain the barrier. Select thickness from the dew-point calculation for the design condition — a thin wrap that allows the outer surface to fall below the ambient dew point will sweat regardless of the fibre's R-value. Keep the facing intact and avoid compressing the blanket, which reduces effective thickness.

How to Choose the Right Duct Insulation

• Circular vs rectangular: flexible DuctWrap rolls suit round and oval ducts; semi-rigid DuctSlab suits square and rectangular duct and plenums.
• Internal vs external: duct liner or board is applied inside the duct for combined acoustic and thermal effect (where airstream velocity and cleanability allow); external wrap is the standard for distribution duct.
• Thermal vs acoustic priority: external wrap is thermal-only; internal liner adds sound absorption in fan rooms and plenums.
• Hot vs chilled: chilled runs demand a continuous vapour barrier and dew-point-based thickness; hot supply runs focus on R-value and surface-temperature limits.

Installation Best Practice

• Wrap the insulation around the duct and secure with appropriate stick pins, bands or adhesive per the manufacturer's guidance; avoid excessive compression.
• Seal all longitudinal seams and transverse joints with UL-listed foil tape; maintain facing continuity at branches and supports.
• Label the R-value on exposed duct segments where the code requires it; support ducts at SMACNA spacing (commonly every 4 ft / 1.2 m).
• For external installations, provide weatherproof cladding — the foil facing alone is not a weather barrier.
• Use PPE (gloves, eye protection, fitted respirator) and ventilate the work area when cutting fibrous insulation.

Where Duct Insulation Is Used

Duct insulation serves commercial HVAC, above-ceiling plenums, mechanical shafts, parking garages, cleanrooms (external only) and process exhaust. It is also specified for chilled-water and refrigerant lines, data-center cooling distribution, and industrial plants such as petrochemical, power generation and marine and offshore facilities. For related fibre-glass cost and R-value detail, our fiberglass pipe insulation guide and 2026 fiberglass insulation cost guide cover adjacent topics.