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Celotex Thermaclass Cavity Wall 21 is an insulation material designed for full fill masonry cavity wall applications. The boards are made up of a polyisocyanurate (PIR) core featuring a low emissivity foil facing bonded to each side which provides improved thermal performance in sealed cavity air spaces, improving the achievable U-value. Thermaclass Cavity Wall 21 is manufactured with a blowing agent that is CFC/HCFC free and has zero Ozone Depletion Potential (ODP) with a low Global Warming Potential (GWP).

90mm Thermaclass Cavity Wall 21 – Celotex Thermaclass (1190mm x 450mm x 90mm) – 3.21m2 (Pack of 6)

£79.99 ex VAT

115mm Thermaclass Cavity Wall 21 – Celotex Thermaclass (1190mm x 450mm x 115mm) – 42.84m2 pallet

£1,250.00 ex VAT

140mm Thermaclass Cavity Wall 21 – Celotex Thermaclass (1190mm x 450mm x 140mm) – 34.27m2 pallet

£1,225.00 ex VAT

The Celotex Thermaclass 21 board is available 1190mm by 450mm (1205mm by 465mm including tongue). It comes in thicknesses between 90mm and 140mm with excellent thermal performance of 0.021W/mK.
The Thermawall cavity can be installed with up to a 10mm residual cavity (to support ease of installation and accommodate mortar squeeze) or as a full fill solution with no residual cavity. When installed correctly, Thermawall Cavity boards should have a lifespan matching the relevant buildings.

Key Features:

  • Tongue and groove edge profile
  • Long lifespan
  • Durable and rigid
  • Easy and simple installation process
  • Excellent thermal performance
  • Built with the protection from weather conditions in mind

Celotex Thermaclass 21 boards feature a tongue and groove edge providing a continuous layer of insulation with minimal gaps to avoid thermal bridging and increase the thermal quality of the product. This also provides a protective layer against wind and rain and eliminates the need to use foil tape on board joins.

This product is lightweight and easy to cut, handle, and install. The boards sizes are designed to fit within cavity wall tie spacings. The boards can be cut, if necessary, by using a fine-toothed saw or scoring each side over a hard edge with a sharp blade.

The board should be stored indoors in a dry space on a flat surface. If indoor storage is not an option, weatherproofing materials should be applied to protect the boards from weather conditions. If boards get slightly wet, they should be wiped over and left to dry however if they get wet, they should not be used.

Celotex Thermawall

Thermaclass Cavity Wall 21 should not be used in the external walls of buildings over 18 metres in height (England and Wales) or 11 metres in height (Scotland). Recent changes to Building Regulations mean that only non-combustible insulation or insulation of limited combustibility can be used in buildings of that height.


Thickness (mm) Thermal Conductivity (W/mK) Thermal Resistance (m2K/W) Length (mm) Width (mm) Pieces Per Pallet
90 0.021 4.25 1190 450 96
115 0.021 5.45 1190 450 80
140 0.021 6.65 1190 450 64

Thermaclass 21 cavity wall boards are one of the latest insulation products manufactured by Celotex. Celotex are a well-renowned brand; they have manufactured reliable and high-quality products throughout the years that are a must have within the building and construction industry. Celotex are easily one of the top brands in the insulation industry today.

What is Celotex Thermaclass used for?

Celotex Thermaclass Cavity wall is an insulation material made of a rigid Polyisocyanurate (PIR) core bonded to low emissivity foil facings on each side. Foil facings provide improved thermal performance in sealed cavity air spaces, improving the achievable U-value. This material is lightweight and easy to cut and handle resulting in an easy installation process. The boards have a tongue and groove edge profile. This helps to ensure a continuous layer of insulation and minimise gaps between boards, maximising thermal performance and providing protection from wind driven rain.

What is a tongue and groove edge?

A tongue groove edge is the finishing edge and shape of a material.
Square edge boards are flat and simply butt together. Where the boards join, they should be taped to prevent air flow going through the middle.
Tongue and groove edge boards slot together ensuring a more reliable and secure finish. The way to boards joins prevents air flow without the need of tape. See image.

The Celotex Thermaclass 21 Cavity Wall boards are 1190mm x 450mm (1205mm x 465mm including tongue and groove edge) with thicknesses 90mm, 115mm, and 140mm available. The boards provide an excellent thermal performance and a thermal conductivity of just 0.021W/mK – the best thermal conductivity in the Celotex range to date. This incredible thermal conductivity reduces the thickness of insulation needed to hit the target U-value.

Is thermal conductivity better higher or lower?

A material's thermal conductivity is the number of Watts conducted per metre thickness of the material, per degree of temperature difference between one side and the other (W/mK).
The simple answer would be the rate heat transfers from one side of the board to the other. The LOWER the thermal conductivity the better because the material conducts less heat energy.

The Cavity Wall 21 insulation board is for full fill cavity wall applications. This product can be installed with up to a 10mm residual cavity (to support ease of installation and accommodate mortar squeeze) or as a full fill solution with no residual cavity. The boards are also sized to fit standard cavity wall tie spacing without needing to cut the board to size.

What is full fill cavity insulation?

Full fill cavity wall insulation allows more of the cavity between the inner and outer leaf of the structure to be filled with insulation, providing a much better insulated structure without increasing the wall width. Keeping more space within the property.

What is the difference between Thermaclass 21 and Celotex Cavity Wall Boards (CW4000)?

Cavity wall boards (Celotex CW4000) have a straight edge and provide thicknesses from 40mm to 100mm. The Thermaclass has a tongue and groove edge and provides thicker boards between 90mm and 140mm. The Celotex cavity wall boards are design for partial fill applications and Thermaclass boards are designed for full fill applications. Another difference between the two is CW4000 has a thermal Conductivity of 0.022W/mK where as Thermaclass 21 has an even lower thermal conductivity of 0.021W/mK.

The top seller of Thermaclass 21 Cavity Wall boards is the 90mm board.
Many builders, architects, and loyal customers of Trade Insulations choose 90mm Thermaclass boards for long lasting thermal quality that will contribute towards a comfortable home or working environment. The 90mm board meets requirement whilst not taking up too much of the
internal space.

  • BBA approved – certificate 16/5343 (Full Fill Cavity Walls)
  • Euroclass E Reaction to Fire classification, according to BS EN 13501-1.
  • Products with this level of classification are not provided with a rating for smoke emission
  • Environmental Management System – ISO 14001:2015
  • CE Marked (20.DOPTCCW-01) to BS EN 13165
  • Quality Management Standard – ISO 9001:2015

How Do You Install Celotex Thermaclass Insulation?

Please note this is a guide only and a building professional should be consulted before any building works begin on your property.

Build up the first section of the walls inner leaf (concrete blocks) and install the first row of wall ties at a minimum of one course of blockwork below the DPC – Damp Proof Course.
The Thermaclass Cavity Wall 21 boards should be positioned with the tongue at the top end of the board. The position should be decided whether the tongue should appear on the ‘top and left’ or ‘top and right’ of the boards before installing. This should be continued throughout the installation.

Before installing the Thermaclass Cavity Wall 21 insulation boards first Identify where the wall ties will land on the tongue side of the board. Using a self-retracting knife, cut the facer then use a sharp specialist insulation saw to make the final small cut in the tongue profile, with a tilt to follow the downward facing direction from the inner leaf to the outer. The incision should not be more than 15mm deep. When there is a residual cavity, all ties should include a retaining clip or disc as appropriate to ensure boards are secured in place against the internal wall leaf.

The first row of the Thermaclass Cavity Wall 21 insulation boards should be supported by the first row of wall ties. Fit the boards flush against the internal leaf, maintaining up to a 10mm residual cavity where specified between the insulation board and the external wall leaf. The boards should be installed to an engineer’s requirement or to a minimum depth of 150mm below the damp proof course (DPC) to provide edge insulation to the floor and reduce thermal bridging at this junction. Ensure that the Thermaclass Cavity Wall 21 insulation boards are not in contact with the ground or that foil tape is applied to all joining board edges.

The next board can then be applied and fitted tightly to the previous board by positioning the tongue and groove edges together. This process is repeated for the first row of boards. When installing the next rows, maintain a staggered pattern to have no continuous vertical joints – a staggering pattern of 600mm is recommended. The wall tie position should not coincide with a vertical joint. Please see further information in section 11 on starting the staggering pattern in corners.

Continue to build the cavity wall and install insulation following on from the previous steps, ensuring excess mortar is removed from the cavity face and a cavity board is used to prevent mortar dropping on the top edge of the insulation. If installation of the insulation boards is terminated below the highest level of the wall, the top edge of the insulation must be protected by a cavity tray and alternate perpend joints raked out, to provide adequate drainage of water from the tray.

How do you cut Thermaclass boards?

Thermaclass boards are designed to fit between cavity wall ties however if a board should need to be cut to fill gaps, the board can be cut with a fine-toothed saw. They can also be cut by scoring one side of the board with a sharp knife, snapping, and then scoring the other side.
Our marketing team have reached out to our clients / customers to get their feedback on Celotex’s newest addition. See what they had to say about Thermaclass 21 Cavity Wall Boards below.

Clive White – Express builds
When first ordered I found the boards were too pricey and more than standard insulation boards but overall easy to install without having to use tape or leave a cavity space. Even with the price and depending on the job at hand, I would be inclined to order these board again.

Michelle Farber – Home Owner
My builder gave me the option of cavity wall boards required for my new build as I wanted to outsource them myself. In the end I went with a thicker slightly more expensive board, my builder was impressed and installed them with no problems.

Matt Dolton – M D Builders
Always a good product for Cavity walls application with no messing around and always straight forward ordering with Trade.