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

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Celotex Thermaclass Cavity Wall 21 90mm

Get the best prices on Thermaclass Cavity wall boards here! Celotex Thermaclass 90mm is a quality product made up of a polyisocyanurate (PIR) core with low emissivity foil linings bonded to each side, featuring a tongue and groove edge.


  • Celotex as a brand is renowned for its quality products and reliable services
  • Their high-quality products manufactured are virtually impossible to beat in price!
  • Celotex insulation has better thermal efficiency than many other materials
  • The Celotex brand offers materials to suit a wide range of applications including, pitched roofs, flat roofs, walls, floors, and ceilings.
  • Other Celotex product include Thermaclass, cavity wall boards, and insulation boards

90mm Thermaclass Cavity Wall 21

  • 90mm Cavity Wall 21 is a well-used product within the construction industry
  • The cavity boards have a tongue and groove edge to ensure boards have a continuous layer with minimal gaps between boards. This also means steps to tape joining boards together are no longer necessary!
  • The 1205mm/1190mm x 465mm/450mm board is designed to be easily fit between wall ties
  • Low emissivity foil facings for improved thermal insulation performance in cavity air spaces
  • This product means you can improve the thermal quality within the internal structure of the building itself
  • Thermaclass 21 boards are lightweight making them super easy to handle, cut, and install, this versatile product can be installed within domestic or commercial buildings
  • This size is commonly used due to its thinner size, the 90mm can be installed in walls that contain a smaller cavity to make the most of the building’s space
  • This board offers an incredible thermal conductivity of just 0.021W/mK – which for those who don’t know, means a very small amount of heat can pass through the board, keeping the heat inside your home or office!
  • Whilst staying warm in the winter and cool in the summer, you’re also providing an energy-efficient environment that contributes to the reduction of climate change
  • While you think it stops at helping the environment, with less heat escaping, Celotex Thermaclass can also save you money on your energy bills
  • These boards can be applied to full fill masonry walls applications, installed between wall ties
  • Thermaclass 21 boards are also available in sizes 115mm and 140mm to suit every application need
  • All Cavity boards are CFC, HCFC and Formaldehyde Free with zero ozone depletion potential (ODP) and low global warming potential (GWP)


  • Reaction to Fire: Euroclass E according to BS EN 13501-1
  • CE Marked (20.DOPCCW-01) to BS EN 13165
  • BBA approved – certificate 16/5343
  • Achieves a BRE Green Guide A+ rating
  • Quality Management Standard – ISO 9001:2015

More Information:

  • Thickness: 90mm
  • Size: 1205mm/1190mm x 465mm/450mm
  • Materials: Polyisocyanurate (PIR), Foil,
  • Thermal Conductivity: 0.021W/mK

Thermawall Cavity Boards 21 Installation Guide

Build up the first section of the walls inner leaf (concrete blocks).
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 of the board. When planning the installation, the position should be decided for example, whether the tongue should appear on the ‘top and left’ or ‘top and right’ of the boards and maintain this defined orientation throughout the installation.
Before installing the Thermaclass Cavity Wall 21 insulation board: a. Identify where the wall ties will land on the tongue side of the board. b. Using a self-retracting knife, cut the facer then use a sharp specialist insulation saw to make the final small incision in the tongue profile, with a tilt to follow the downward facing direction from the inner leaf to the outer. c. The incision should not be more than 15mm deep. d. 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 all board edges are sealed.
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 construct the cavity wall and install insulation following the previous steps, ensuring excess mortar is removed from the cavity face and a cavity board is used to prevent mortar from dropping on the top edge of the insulation.
If the 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.

Thermaclass FAQs

How do you cut thermaclass boards?

The best and easiest way to cut Thermaclass boards is to measure and mark the board to size and then score the insulation with a sharp knife, snap the board over a hard edge and score the other side. A fine-toothed saw can also be used to cut the boards.

Why does Thermaclass have tongue and groove edges?

The tongue and groove edge minimises gaps between boards and ensures the continuity of insulation. The edges also provide extra protection from wind and wind-driven rain, maximising the thermal conductivity. These edges will also mean the use of joint tape and foil tap on boards joins is no longer necessary.

Additional information

Dimensions1190 × 450 mm




Material Type

Polyisocyanurate (PIR)

Pieces Per Pack


Sales Unit


Unit Coverage


Manufacturers Reference


Thermal Conductivity


Fire Rating

Euroclass E

Edge Type



Low Emissivity Foil

Weight (kg/m2)



7-10 Working Days

4 reviews for 90mm Celotex Thermaclass Cavity Wall 21 (1190mm x 450mm) – 6 Sheets (3.21m2)

  1. Oliver

    Outstanding insulation quality!

  2. Matthew

    Products arrived within days of ordering, and it’s made a noticeable difference in my projects. The service was excellent, and the website is intuitive to navigate.

  3. Samuel

    Used instead of Cavity wall insulation, good alternative.

  4. Smiths Jones Lofts

    Been in the building game for 38 years and never used Celotex Thermaclass was well worth the money. Nice size cavity boards! shame they dont do smaller pack sizes contacted Celotex direct and was recommended to go through insulation board.co.uk

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