Economic Thickness of Insulation (ASTM C1057 / C680)

Q: What is the economic thickness of insulation?

The thickness at which the total cost is lowest — where the cost of adding one more layer of insulation just equals the value of the heat it would save. Thinner than that wastes energy; thicker spends more on material than it saves. It is formalised in ASTM C1057 (economic thickness) using heat loss from ASTM C680.

For design engineers, “how thick?” is an economics question, not a guess. Economic thickness is the point where the total cost — energy + insulation — is lowest.

TL;DR. Heat loss falls with the log of thickness, so the first 25–50 mm removes most of it and each extra layer removes less. The economic thickness (ASTM C1057) is where the next layer's cost equals the energy it saves — typically 50–100 mm for hot service, and it moves thicker as fuel and carbon (EU-ETS/CBAM) prices rise.

Diminishing returns: heat loss vs thickness

Insulation does not save heat linearly. Conductive resistance grows with the log of the radius ratio, so the biggest cut comes from the first layers; beyond that, each millimetre buys less. Computed for a DN100 line at 250 °C (stone wool, ASTM C680):

Thickness	Heat loss	Loss cut vs bare	Saved (100 m)
0 mm	826 W/m	0%	0 €/yr
25 mm	177 W/m	79%	31 644 €/yr
50 mm	112 W/m	86%	34 845 €/yr
75 mm	86 W/m	90%	36 088 €/yr
100 mm	72 W/m	91%	36 759 €/yr
125 mm	64 W/m	92%	37 184 €/yr
150 mm	58 W/m	93%	37 480 €/yr

Per metre / per 100 m, gas at 0.05 €/kWh ÷ 82% efficiency, 8000 h/yr (ASTM C680). The economic optimum also depends on installed insulation cost and carbon price — run your case.

Too thin / economic / too thick

The total-cost curve is U-shaped: energy cost dominates on the thin side, material cost on the thick side. The minimum is the economic thickness.

	Too thin	Economic thickness	Too thick
Heat loss	High — money out the door	Near-minimum	Marginally lower
Insulation cost	Lowest	Balanced	Highest
Total lifetime cost	High (energy dominates)	MINIMUM	Higher (material dominates)
When it is right	Never for hot service	Most industrial service	Space/CO₂-critical, high carbon price

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FAQ

Questions on this topic

What is the economic thickness of insulation?

The thickness at which the total cost is lowest — where the cost of adding one more layer of insulation just equals the value of the heat it would save. Thinner than that wastes energy; thicker spends more on material than it saves. It is formalised in ASTM C1057 (economic thickness) using heat loss from ASTM C680.

How do you calculate economic thickness?

Compute heat loss vs thickness (ASTM C680), value the saved energy over the system life (fuel price × hours, plus any carbon price), and add the installed insulation cost for each thickness. The total-cost curve is U-shaped; its minimum is the economic thickness. Our calculator runs the heat-loss side for any case.

Why does more insulation give diminishing returns?

Heat loss falls roughly with the log of the radius ratio, so the first 25–50 mm removes most of the loss and each further layer removes less (see chart). That is why the economic optimum for typical hot service lands around 50–100 mm, not 'as thick as possible'.

Does a higher carbon or fuel price change the answer?

Yes — and this is the key 2026 point. The more each saved kWh is worth (high gas price, or EU-ETS/CBAM carbon cost on the fuel), the more a thicker layer pays for itself, so the economic thickness moves thicker. Insulation specs set when energy was cheap are now usually under-thick.

Is thicker always safer?

No. Beyond the economic thickness you spend more on material, cladding and support than you get back, and you add weight and bulk. The goal is the minimum-total-cost point, not maximum thickness — except where space-free CO₂ cuts or a very high carbon price justify going beyond it.

Bare vs economic thickness — how big is the gap?

Large. Going from bare to ~50 mm on a DN100 line at 250 °C cuts heat loss by the order of 90%+ (see table); the first layers do the heavy lifting, which is exactly why leaving components bare is so costly.

Economic Thickness of Insulation (ASTM C1057 / C680)

Diminishing returns: heat loss vs thickness

Too thin / economic / too thick

Components that get opened need covers that come off.

Questions on this topic

Find the optimum for your line