What is BS 5422

BS 5422 is the British Standard for specifying thermal insulation on pipework, ductwork, tanks, vessels and equipment. Its main job is to say how thick the insulation should be. The current edition is BS 5422 : 2023, and that is the edition we work to.

The right insulation thickness is not one number. BS 5422 sets it out in tables that depend on a few things about the job:

• What the service is: heating, hot water, chilled water, refrigeration, process and so on.
• Why it is insulated: to save energy and limit heat loss or gain, to stop condensation forming on cold pipes and ducts, or to protect against freezing.
• The size of the pipe or duct.
• The insulation material, because some materials insulate better than others for the same thickness.

The designer or the insulation contractor reads the tables for their job and writes the thickness into the specification. We do not publish thickness tables on this site: always take the thickness from your project specification or the standard itself.

BS 5422 is about the insulation, not the metal over it. But the thickness it calls for decides the size of every piece of cladding, because cladding is measured over the lagging. A 168 mm pipe with 30 mm of insulation is a 168 × 30 section, and its casing, bends and tees are all made to fit over that.

That is why we size in insulation sections (pipe outside diameter × insulation thickness), never DN sizes. Give us the section, or the size over the lagging, and we add our 0.3 cm allowance. If the spec changes the insulation thickness, the cladding sizes change with it, so check the measurements before you send.

In Site Measure, the insulation section can be picked as you measure, so the specification travels with the measurement to our workshop rather than being retyped from a separate document. Our fitting calculators work the same way: they ask for the insulation section on the pipe, such as 114 × 50. See how to measure pipe for cladding for the tape side.

• BS 5970: the code of practice for insulating pipework and ductwork, including the finish over it.
• BESA DW/144: the specification for sheet metal ductwork. Our curved-back duct bends are sized by the throat, the way it sizes rectangular bends.

BS 5422 tells the insulator how thick the lagging should be. The lagging decides the size of the cladding. We make the cladding to fit, working to BS 5422 : 2023. If a word on a specification is new to you, our trade terms page explains it in a line.