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).

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The 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

The Thermaclass 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.

Safety Certifications:
BBA approved – certificate 16/5343
Euroclass E Reaction to Fire classification, according to EN 13501-1
manufactured under Quality Management Standard – ISO 9001:2015
CE Marked (20.DOPTCCW-01) to BS EN 13165

More Information:
Brand: Celotex Thermawall
Length: 1190mm (1205mm including tongue)
Width: 450mm (465mm including tongue)
Thickness: 90mm, 115mm, 140mm
Material: Polyisocyanurate
Facings: Low emissivity foil
Thermal Conductivity: 0.021W/mK


Thickness (mm)Thermal Conductivity (W/mK)Thermal Resistance (m2K/W)Length (mm)Width (mm)Pieces Per Pallet
  • When installing, some boards will feature the branded foil facer facing outward and some the unbranded foil facer facing outward. This is not an issue as both sides of the board provide the same performance level.
  • This is a guide only and a professional builder or architect should be consulted before you begin insulation your property. Insulation installed incorrectly can be a cost time and money,
  • Care should be taken to avoid any damage to the board during installation. Avoid piercing the foil facing of the insulation board with the wall tie. This is a potential point of moisture entry. Any damaged boards should be replaced.

Thermawall Cavity Boards 21 Installation Guide

Step 1: Build up the first section of the wall inner leaf (concrete blocks).

Step 2: Install the first row of wall ties at a minimum of one course of blockwork below the DPC – Damp Proof Course.

Step 3: 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.

Step 4: 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.

Step 5: 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.

Step 6: 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.

Step 7: 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.

Step 8: 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 dropping on the top edge of the insulation.

Step 9: 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.