Do I need a vapour barrier with Celotex

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Celotex is one of the largest manufacturers of rigid foam insulation in the UK. Their products can be used in a variety of installations. Including:

  • Roofs (flat and pitched)
  • External cavity walls
  • Internal stud walls
  • Floors
  • Etc.

In some cases, Celotex may be used in combination with an additional vapour barrier. This is most common when used in roofs, or insulation on the inside of external walls.

do i need a vapour barrier with celotex

Celotex does have a built-in vapour barrier layer, so it is already quite effective in this regard. If used for roof insulation, a common way of sealing the boards is to use a foil tape to seal any of the joints between the insulation sheets.

The main goal is to reduce air movement through the insulation, which will reduce condensation on the cold side between the insulation and the roof. This avoids the risk of excess moisture and condensation affecting the roof timbers.

It will also reduce the risk of moisture passing through to the internal surfaces, which could result in damage to the plaster and decorations.

In a roof, rigid foam insulation like Celotex, is generally fitted between the rafters. In order to do this correctly, the insulation board should be fitted tightly, with little or no gaps. If there are small gaps between the timber and the insulation, a common fix is a squirt of expanding foam, and also taping joints between boards.

Gaps do cause an issue with regards to a consistent vapour barrier. They can also reduce the performance of the insulation and result in cold spots. These cold spots can result in condensation, which can lead to damp patches and even mould.

Adding additional vapour barrier with Celotex

As we have already mentioned, the insulation already has a vapour barrier layer. However, this can be improved further in several ways. Some of the most common are:

  • Gapotape
  • Over boarding with additional insulation
  • Separate vapour barrier
  • Foil backed plasterboard


This is a relatively new product, and it solves the issue of gaps around the insulation. Spaces between the timber are often uneven, there could also be bows and twists in the wood, that cause gaps. This means it can be very difficult to get every board fitting tightly.

If you do fail to fit them tightly, the insulations vapour barrier and its overall thermal performance is affected.

Gapotape is a product that you add to the sides of your insulation. It is made from a softer foam material, which allows you to friction fit your boards between the timber rafters.

To do this, you cut the width of your insulation 8-10mm thinner than the gap between the timber, then the Gapotape fills the gap.

This works because the Gapotape is softer and more flexible than the rigid board. Therefore, it evens out any inaccuracies between the board cut and the timbers.

You can see a video of how this product works below:

Over boarding with additional insulation

Another way to add an additional vapour barrier is to add additional insulation over the top.

Once you have added the insulation between the rafters and sealed all the gaps, you can overlay another layer of insulation (20mm for example). This would be fixed to the face edge of the rafters.

When doing this, you should aim to stagger the joints, this will reduce the risk of any vapour or cold passing through the boards. You should also tape all the joints completely sealing the wall.

This additional layer of insulation, will provide extra thermal performance, and even out any cold spots. As well as completely sealing the surface and providing a fantastic vapour barrier.

Adding a separate vapour barrier

I have personally never used this method. However, it is a viable option. This simply involves using a sheet of waterproof membrane between your insulation and the finished plasterboard.

The sheet could be stapled in place and overlapped with all the joints taped. This would result in a completely sealed vapour barrier. Following this, you can simply add your plaster board over the top.

Foil backed (vapour barrier) plasterboard

This is a method I have used many times in the past. I personally prefer it to the previous option of a separate vapour barrier. This is mainly due to less hassle and ease of installation.

As we mentioned previously, Celotex already has a vapour barrier layer. The foil backing on this type of plasterboard is also a vapour barrier.

If you do a good job of fitting and sealing your insulation, the additional barrier provided with this plasterboard, will create a complete, unbroken vapour barrier.

If it is used in combination with Celotex and something like Gapotape, the finish would be far superior to just adding insulation and over boarding with normal plasterboards.

Times you don’t need a vapour barrier with Celotex

Using a vapour barrier with Celotex is not required in many other instances. If there is no contact with a roof or external wall, its use is unnecessary. This means you don’t need a vapour barrier for floor insulation, stud walls or any other type of internal insulation.

With that said, you should still aim to keep your cuts nice and tight, and you can still use tape and other materials to completely seal gaps. This will ensure you get high thermal performance from your insulation.


You will benefit from a vapour barrier with Celotex, in certain situations. One of the primary examples, is when you are insulating a roof.

The main consideration, is whether there is a risk of vapour penetration. As well as the risk of gaps and a cold void behind the insulation. This can result in condensation, which can lead to future problems.

In my opinion, a combination of Gapotape and foil backed plasterboards, is one of the best solutions, for both thermal performance, and a high-quality vapour barrier.

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