The thickness at which the marginal cost of the next millimetre equals the marginal value of the energy it saves — beyond it you're buying insulation that never repays. It moved sharply upward in the 2020s: energy prices plus carbon pricing (EU ETS ~€77/t) raised the value of each saved kWh, so specifications written in the cheap-gas era under-insulate today. Modern practice: compute with current fuel + carbon price (the free calculator does both), check the personnel-protection constraint separately (it often governs above 250 °C), and remember the largest economic win is usually not thickness at all — it's covering the bare components that have NO insulation.
Compute your own case: free heat-loss calculator · materials library: 12 datasheets · related: What is CUI (corrosion under insulation)? · R-value vs k-value (λ) — what's the difference? · What insulation works at 600 °C? · Can insulation touch the hot pipe directly?
