Celotex Thermaclass Cavity Wall 21 is an insulation material designed for full fill 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).

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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.

Thermaclass 21 Cavity Wall Boards are easy to install and can be fitted with up to a 10mm residual cavity. This helps to ensure an easy installation and also accommodates mortar squeeze. In addition to this, Thermaclass 21 can be used a full fill solution. (without residual cavity)

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 Thermaclass

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.

Thermaclass Cavity Wall 21 is not suitable for use in the external walls of buildings that exceed 11 metres in height. This restriction arises from recent amendments to Building Regulations, which now state that only non-combustible insulation or insulation materials classified as having limited combustibility are permitted for buildings of such heights. These changes have been implemented to enhance fire safety standards and reduce the risk of fire spread in taller structures, ensuring that insulation materials used in such buildings meet stringent safety requirements. For this reason, we advise you only purchase Thermaclass where the application is appropriate.

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?

Thermaclass Cavity Wall 21 is a polyisocyanurate (PIR) insulation board designed specifically for full-fill cavity wall applications. It boasts good thermal efficiency. This is evident by its impressive thermal conductivity rating of 0.021 W/m.K. This product is designed for ease of installation, making it an ideal choice for construction projects seeking to enhance energy efficiency and thermal insulation. The full fill cavity board's PIR technology contributes to reducing heating and cooling costs, making it an environmentally friendly option for modern building designs. With its exceptional thermal performance, Thermaclass Cavity Wall 21 can help buildings remain comfortable and energy-efficient throughout the year.

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 two boards join prevents air flow without the need of tape.

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.

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 50mm 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 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.