How to cut Celotex, Kingspan and other PIR board

Insulation is a requirement in new build properties. However, this was not always the case in older properties. Therefore, if you do find your home cold, you may decide to install insulation yourself.

A common area people install rigid insulation boards, such as Celotex, is in their roof space. It can also be installed in new stud walls and even in floors. However, for this article we will assume you are insulating your roof space.

The most common method for cutting insulation such as Celotex, is using a handsaw. The insulation should be cut tight between the rafters, and ideally the boards will have a friction fit. This should hold them securely in place without any gaps.

Ideally, you don’t want any gaps around your insulation, as this could affect its thermal performance. In the case where small gaps are present, you should aim to fill these with additional insulation. You can also use an expanding foam to completely fill gaps and hold the boards in place.

how to cut Celotex

If you are installing a thinner insulation (under 40mm), you can also cut boards with a knife. This can actually give a really clean cut, with very little dust and mess created.

The technique is similar to cutting plasterboard. Essentially you score one side of the board with your knife, using a straight edge. Following this, you can turn the board over and snap it along the cut.

This technique is not effective with thicker boards. Your Stanley knife will only cut in 10-15mm, so you are unlikely to get a clean cut. Thicker boards can easily snap wider on the uncut side. If this happens, you will still need to cut the excess away, which defeats the object of using a knife in the first place.

How to cut Celotex with a hand saw

When you cut Celotex with a handsaw, your goal should be a neat tightly fitting cut. To achieve this, there are a few things you should do:

  1. Use a fine-tooth hands saw – This will give you a cleaner cut in the foil face, and reduce the dust created when cutting. You can see a good quality, low-cost handsaw that we recommend by clicking here

  2. Measure twice cut once – What this really means, is to measure your cuts correctly. Ideally your insulation should fit snugly between the rafters, with a friction fit that holds them in place.

  3. Cut at a slight angle – Over the years I have fitted a lot of rigid insulation, such as Celotex and Kingspan. One technique that works very well, is cutting the sheets at a very slight angle. By doing this, the back of the board will fit in easily, with the front being tight.

    When doing this, you can even cut a little wide on the front edge (1-2mm), then run a handsaw down the front edge between the rafter, to remove the excess and ease it in tightly.

  4. Cut twice for uneven openings – In some cases, rafters will be slightly uneven, and the gap may be different at each end. In this case cut a straight piece to the widest measurement.

    By doing this, you leave the insulation sheet straight for your next cut. But the piece you just cut can be trimmed down to fit.

    Always remember, you can take it off, but you can’t put it back on. It’s better to cut too big and adjust, rather than to cut it too small and have a loose fit.

Ways to make fitting insulation easier

Following the above recommendations, should enable you to cut and fit your insulation correctly.

However, there are a few tools and products that can make the process easier. These can be seen below:

  • Dust mask – The dust created from rigid foam insulation is quite unpleasant. Using a mask will make the job much more bearable. A pair of goggles is also recommended.

  • Stanley knife – Even if you aren’t cutting thin boards, a knife can come in handy for little jobs. Mostly, it will be used to remove small bits of insulation from the edges. This will enable you to fit tight boards a little easier.

  • Surform – This is another tool that can help you adjust the size of your insulation. A surform is a bit like a plane. It has a perforated metal sheet on the bottom that resembles a food grater. This can be very useful for shaving thickness, to help your insulation fit.

    One thing you need to be aware of, is this causes a lot of mess and dust, making the need for your mask even greater.

  • Six-foot level or straight edge – A long straight edge will make cutting in a straight line much easier. Ideally a full set of levels would be even better. This way you can use smaller levels when a big six-footer isn’t required. You can see some good levels by clicking here

  • Insulation tape – This will enable you to tape all the joints between your insulation. This serves two purposes. Firstly, it seals any gaps, and secondly it can help to hold any slightly loose pieces.

  • Expanding foam – Where gaps are present, or the insulation is slightly loose, a squirt of foam will easily fill and hold everything in place.


Cutting Celotex and similar insulation boards is not very complicated. You don’t need any fancy tools. In fact, the best tool for the job is usually a hand saw.

Ideally, this should be a fine-toothed saw, which will help to achieve the neatest cuts. Insulation is very easy to cut manually, so using any kind of power tool is unnecessary.

In order to achieve a good result, you need to make sure you measure and cut your insulation correctly. This will ensure they fit nice and snug between the roof timbers. Resulting in the best possible thermal performance.