Pipe Insulation Calculator — How Much Pipe Lagging Do I Need?

Yes. UK Building Regulations (Part L, via the Domestic Building Services Compliance Guide) require hot water and central heating pipes to be insulated, and this is most critical in unheated spaces (loft, garage, under suspended floors). Compliance is set as a maximum heat loss in watts per metre of pipe — for example 9.12 W/m for a 22mm copper pipe at 60°C — and the standard BS 5422:2023 is used to convert that limit into a wall thickness based on the product's thermal conductivity (λ). It applies to new installations and to replacement pipework.

There is no single fixed legal thickness — BS 5422:2023 and Part L work to a maximum heat-loss limit (W/m) that depends on the product's thermal conductivity (λ). In practice, the BS 5422:2023 advised wall for a 15-22mm domestic heating pipe is around 20mm for mineral wool or phenolic, which lines up with the widely-stocked 19mm foam and rubber (Armaflex) tube. Larger pipes (42mm+) typically step up to 25mm. Our calculator shows the typical advised wall thickness for your pipe size and material, but always confirm it against the declared λ on the product datasheet.

For most domestic applications, foam tube insulation (polyethylene) is the best value — cheap, easy to fit (pre-slit), and effective. For higher performance or commercial work, rubber (Armaflex) offers better thermal performance and moisture resistance. Mineral wool pipe sections are used where fire resistance is needed.

For foam tube insulation, mitre-cut two pieces at 45° angles and tape together around the bend. For a neater finish, pre-formed elbow pieces are available for common pipe sizes. Use self-amalgamating tape or PVC tape to seal all joints and prevent heat loss at connections.

Yes, insulate cold water pipes in unheated spaces (lofts, garages, external walls). Frozen pipes can burst and cause severe water damage, and Building Regulations require cold water pipes in vulnerable locations to be protected. In heated living spaces it is optional but reduces condensation.

Not on its own. The thickness this calculator shows is sized to limit heat loss, which is a different job from frost protection. BS 5422:2023 frost-protection tables (Table 30, 12-hour) show the requirement rises sharply as the pipe gets smaller: a 15mm copper pipe needs about 50mm of mineral wool inside the building (60mm inside an unheated thermal envelope), built up in layers — far more than a 19-20mm heat-loss wall and often impractical. A 22mm copper pipe by contrast only needs about 20mm. The standard notes that for the smallest bores lagging alone may not prevent freezing; combine generous lagging with trace heating, a trickle of flow, or routing pipes within the heated envelope where possible.