Pipe insulation

Frequently asked questions

Why does insulating a cold pipe need a vapour seal when a hot one does not?

Because the moisture is moving the other way, and insulation that gets wet stops insulating. On a HOT pipe, vapour pressure inside the insulation is higher than outside, so any moisture is driven out and the insulation stays dry on its own. On a COLD pipe the gradient reverses: warm room air carries vapour toward a surface below its dew point, and the condensation forms INSIDE the insulation against the pipe, where it cannot evaporate. Wet insulation loses most of its performance, the pipe then runs colder, more condenses, and the material rots or corrodes the pipe under it. So cold-service insulation is specified with a continuous vapour barrier on its OUTER face and with every joint, fitting and support sealed — a single unsealed butt joint provides a path for the whole run. This is the reason closed-cell elastomeric is the default on chilled and cold services, and the reason fibrous insulation is a poor choice there however thick it is.

Is more thickness always better?

No, and on a small pipe it can genuinely be worse. Adding insulation increases the thermal resistance of the path out, but it also increases the OUTER surface area losing heat to the air. On a large pipe the first effect dominates from the start. On a small one — a thin copper pipe, a cable — the second can dominate at small thicknesses, so heat loss rises with the first few millimetres and only starts falling once the insulation passes what is called the critical radius. The practical reach of this is limited, because the critical radius for the low-conductivity materials and the pipe sizes used in buildings is usually small enough that any commercial thickness is past it — but it explains why specification tables give thickness against pipe DIAMETER rather than one figure, why the thickness rises with the pipe size, and why the answer is a lookup rather than an intuition. For freeze protection, thickness buys TIME rather than prevention: an unheated pipe in a cold void eventually reaches air temperature whatever it is wrapped in, so trace heating rather than more insulation is the answer where the pipe must not freeze.

Why is old lagging treated so seriously?

Because it is the friable end of the asbestos range, and friability is what decides risk. Pipe and boiler insulation was made by mixing asbestos into a soft, porous matrix — a moulded section, a bandage, or a plaster applied wet — precisely because that structure traps air and insulates well. That same structure means the fibres are barely bound: brushing past damaged lagging, sweeping up debris under it, or pulling a pipe out of it releases fibres readily, where an asbestos cement sheet in the same building would release almost none. That is why removal of pipe lagging sits in the licensed category in British law, why the material appears prominently in survey guidance, and why the advice for anyone finding damaged insulation on old pipework is to stop, not to tidy. It is also why the word matters: an instruction to strip the old lagging off a heating system is either a morning's work or a licensed contractor with an enclosure, and the age of the installation is what separates them.