Expansion Joint Insulation: Heat Loss & Savings

Fabric and metal expansion joints on ducts, HRSGs and hot piping are almost always left bare — they move with thermal expansion, so fixed cladding cannot follow them. That makes them one of the single biggest avoidable heat losses on power and process plants. Removable insulation is purpose-made to flex with the joint.

Direct answer: a bare hot expansion surface loses roughly 2,300 W/m² at 250 °C and 4,100 W/m² at 430 °C (20 °C ambient, ASTM C680). Inzonex removable insulation cuts that by about 91%; high-temperature cases use the appropriate build-up to meet the required touch-safe target.

Heat loss vs insulated — by surface temperature

Surface temp	Bare loss	Insulated	Outer surface	Reduction
190 °C	1,700 W/m²	189 W/m²	39 °C	88.9%
250 °C	2,300 W/m²	152 W/m²	35 °C	93.4%
350 °C	3,300 W/m²	255 W/m²	46 °C	92.3%
430 °C	4,100 W/m²	354 W/m²	55 °C	91.4%

Per-m² flat-surface flux, ASTM C680 / ISO 12241, 50 mm Lamella (≤220 °C) or 100 mm Wired mat (>220 °C), 20 °C ambient. Multiply by the bare area.

Worked examples (per year, 8,000 h, natural gas)

Item	Heat saved	Energy/yr	CO₂/yr	€ saved/yr
A fabric expansion joint (1.6 m², 280 °C)	3.9 kW	36 MWh	7.4 t	€2,113
A large duct expansion joint (3.0 m², 250 °C)	6.4 kW	61 MWh	12.3 t	€3,518

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FAQ

How much heat does a bare expansion joint lose?

At 20 °C ambient (ASTM C680): a fabric expansion joint (~1.6 m², 280 °C) loses about 3.9 kW; a large duct expansion joint (~3.0 m², 250 °C) about 6.4 kW. Insulating a fabric expansion joint saves ≈36 MWh and ≈7.4 t CO₂ per year (8,000 h).

Can you insulate round and rectangular fabric expansion joints and similar?

Yes. Expansion joints move and must be inspected for fatigue, so rigid insulation cannot be used. Inzonex modular removable joint insulation flexes with the bellows, unclips for inspection and refits with no damage. Inzonex covers round and rectangular fabric expansion joints, metal bellows and duct joints. The outer surface is reduced sharply; the insulation build-up is selected to meet the required touch-safe target while keeping the part serviceable.

How much does expansion joint insulation save?

Each item saves up to 96% of its bare loss. A fabric expansion joint at 280 °C saves ≈2,113 €/yr of energy (≈36 MWh, ≈7.4 t CO₂) at typical gas prices; payback is usually under two years.

What insulation thickness is used?

Up to 220 °C surface: 50 mm mineral-wool (Lamella); above 220 °C: 100 mm (Wired mat), because conductivity rises with temperature. The build-up is selected by temperature: higher-temperature assets may need a thicker or higher-performance layer to meet a ≤45 °C touch-safe target.