Cavity wall boards are an insulation material made up a polyisocyanurate (PIR) core with low emissivity foil facings bonded to each side. The board are generally 1200mm x 450mm with thermal conductivity of 0.022W/mK. The high-quality materials are designed to be applied to cavity wall masonry wall applications between the inner leaf and outer leaf as the wall is being constructed. Celotex cavity wall boards are CFC and HCFC-free. The boards use a blend of blowing agents that have zero ozone depletion potential (zero ODP) and low global warming potential (GWP) and reliable energy savings that are long-lasting for all buildings. Celotex has manufactured two-cavity products – Thermaclass 21, and Cavity wall CW4000.

When using Celotex Cavity Wall Products, you need to satisfy yourself that the use of the product meets all relevant national Building Regulations and guidance as well as local, national, and other applicable standards relevant for your construction or application, including requirements in relation to fire and applicable height restrictions.

Cavity Wall Insulation Features:

  • Cavity wall insulation boards improve the thermal quality of the property. This means a more temperature-controlled environment with cooler summers and warmer winters. Thermal quality means fewer drafts, less heat escaping the property and more comfortable living and working space.
  • Thicknesses are available ranging from 40mm – 140mm.
  • High-Performance Facer – Low emissivity foil facing provides improved thermal performance in sealed cavity air spaces, improving the achievable U-value.
  • By preventing heat and energy escaping the property you will save money on your energy bills as less energy will be needed to heat your property.
  • With less heat escaping, it will have a positive effect on global warming and climate change, making your property more energy-efficient and sustainable.
  • Built with the protection from weather conditions in mind.
  • When installed correctly, cavity wall insulation boards will have a lifespan that lasts for as long as the building itself.
  • Cavity wall insulation boards are lightweight and easy to cut, handle, install, and transport.
  • Building height restrictions occur.
  • Options of tongue and groove edge profile or squared edges are available.


See below for a breakdown of Cavity Wall Products by Celotex

How do you install Celotex Cavity Wall insulation?

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

Install wall ties into the inner leaf at 600mm apart horizontal centres. Place the cavity tie clips (wall ties) 900mm apart. Continue building the inner leaf up to the next wall tie course. (450mm above the previous inner leaf). The next course of wall ties is positioned at frequent horizontal centres. Extra wall ties should be positioned at jambs for all openings. The next course of blockwork should then be installed to ensure the ties are secure and in place.

The first row of insulation boards should then be installed between the two rows of wall ties (fleece must face outwards to cavity), ensure each insulation board is maintained tight against the inner leaf and joints are tightly butted to get the best quality of insulation.
Secure each board at a minimum of three points. Additional ties can also be added to satisfy structural requirements such as around window frames or doors or to ensure adequate retention of the Cavity Wall boards or cut pieces.

Apply a foil tape to all horizontal and vertical joints of the cavity wall board edges butting together. These surfaces should be clean, dry, and free from grease, dust, and dirt prior to application.

Next, build the outer leaf (outer brickwork) to the level of the top of the cavity wall boards and repeat the process. (Cavity of 10mm or less). If finishing at a gable, Cavity Wall insulation boards should be continued 200mm beyond the height of the toper storey ceiling and a cavity tray installed above the insulation.

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. 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 continue this course throughout the entire Thermaclass 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.

What is the difference between Celotex Thermaclass 21 and Celotex cavity wall insulation boards?

Cavity wall boards have a straight edge and provide thicknesses from 40mm to 100mm whereas Thermaclass has a tongue and groove edge and provide larger thickness between 90mm and 140mm. The Celotex cavity wall boards are design for partial fill applications and Thermaclass boards are designed for full fill applications.

How do you cut Celotex Cavity Wall insulation?

Cavity wall insulation can be cut by measuring and marking the boards, scoring one side of the board with a sharp knife, and snapping over a hard edge. The boards can also be cut using a fine-toothed saw.

Leave a Reply

Your email address will not be published.