Pipe Heat Loss Calculator — BS 5422:2023 Insulation

A bare 22mm copper pipe carrying 60°C water in a 20°C space loses approximately 24.5 W/m. Over 20 metres, that’s 490 W — almost half a kilowatt wasted continuously. At 2,000 running hours per year (980 kWh) and the Ofgem gas cap of 7.33 p/kWh (Jul–Sep 2026), that is about £72 a year. Adding 19–40mm of lagging typically cuts this conduction loss by roughly 55–75% on the figures this calculator uses.

BS 5422:2023 (which superseded BS 5422:2009 in June 2023) and Approved Document L set a minimum insulation thickness by pipe diameter and insulant thermal conductivity (λ), sized to a maximum permitted heat loss in W/m. The thickness is a single value per bore and λ — it does not change with heated vs unheated location (that only governs WHERE insulation is required). For 60°C domestic heating and hot water at λ = 0.040 W/m·K, the TIMSA HVAC Compliance Guide §6.1.1 table (the Part L domestic figures) gives 20mm for 15mm copper, 23mm for 22mm, 25mm for 28mm, 27mm for 35mm, 28mm for 42mm and 29mm for 54mm. A better insulant (lower λ) allows a thinner figure — e.g. at λ = 0.035 the 22mm minimum drops to 18mm. Always confirm against the current BS 5422:2023 / Part L table for your actual λ.

Building Regulations Part L requires insulation on heating pipework in unheated areas (loft, under floors, in walls, garages). Pipes within the heated envelope contribute useful heat and may not need insulation, though it's still good practice to prevent heat reaching unintended areas. Use our Radiator Pipe Sizing Calculator to determine pipe diameters for insulation specification.

Common materials include nitrile rubber (Armaflex, K-Flex) with thermal conductivity ~0.035-0.040 W/mK, and polyethylene foam (Climaflex) at ~0.038-0.045 W/mK. For higher temperatures (above 100°C), mineral wool or glass fibre sections are used.

For insulated pipes: Q = 2πkΔT / ln(r2/r1) W/m, where k is thermal conductivity, ΔT is temperature difference, r1 is pipe outer radius, and r2 is insulated outer radius. For uninsulated pipes, heat loss depends on pipe diameter and natural convection conditions.