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Guide

Pipe insulation sizes explained

The short answer

Insulation labelled 13 x 6 x 2m does not fit a 13mm pipe. Here is what each number means and which one to measure first.

Pipe insulation is sold with three numbers on it, and the first one is the one people get wrong. A tube described as 13 x 6 x 2m does not fit a 13mm pipe. It fits a 6mm pipe.

This catches people out often enough that it is worth setting down properly, because ordering on the wrong number means a tube that either will not go on or slides about once it is there.

What each number means

The first number is the wall thickness. It is how thick the insulating foam itself is, measured from the pipe outwards. On a 13 x 6 x 2m tube, that is 13mm of foam.

The second number is the pipe diameter it fits. This is the size of the pipe the tube is bored for. On the same tube, that is 6mm.

The third number is the length of the tube. Two metres, in this case.

So a 13 x 6 x 2m tube is thirteen millimetres of foam, bored for six millimetre pipe, supplied two metres long.

A cross-section of pipe insulation seen end on, with the 6mm bore measured across the hole and the 13mm wall measured across the foam, beside a side view with the 2 metre length marked.
The same tube, twice. The first number measures the foam, the second measures the hole, and only the second decides whether it fits.

Measure the pipe first

The pipe diameter is the number that decides whether the insulation fits at all, so it is the one to establish before anything else. Wall thickness affects how well the pipe is insulated. Pipe diameter affects whether you can fit it.

Then choose the wall thickness

Once you know the pipe size, the wall thickness is a separate decision. A thicker wall means more insulation. What thickness is appropriate depends on the installation, where the pipe runs and what it carries.

If a specification, a building requirement or an installer has told you a thickness, use that. Where a specific thickness has been called for, it has been called for on grounds this page cannot see.

Split and unsplit tubes

An unsplit tube is a continuous piece with no seam along its length. It gives a clean run, and it is fitted by sliding it onto the pipe, so it suits new pipework or a run with a free end to work from.

A pre-split tube is already cut along its length so it can be opened and closed around a pipe that is already installed.

Neither is better in general. What decides it is access. If you cannot reach a free end of the pipe, an unsplit tube will be difficult to fit.

A short ordering checklist

  1. Measure the pipe diameter. Not the insulation, the pipe.
  2. Establish the wall thickness you need, from the specification if there is one.
  3. Decide split or unsplit, based on whether you can slide a tube on.
  4. Work out the length, remembering that tubes are cut to suit and that a longer tube means fewer joins on a straight run.

The quick check

If you are looking at a size and are not sure which number is which: the smaller figure is almost always the pipe. Insulation walls are usually thicker than the small bore pipes they go on, which is why 13 x 6 reads oddly until you know the order.

When in doubt, measure the pipe and match the second number to it.

The products in this guide