Celotex Isover

Thermaclass Cavity Wall 21

Thermaclass Cavity Wall 21 is an easy to install, high thermal performance polyisocyanurate (PIR) insulation board for full fill cavity wall applications, with a thermal conductivity of 0.021 W/m.K. It 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.

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The boards have a low emissivity foil facing to each side and feature an engineered tongue and groove joint profile, helping to ensure a continuous layer of insulation to maximise thermal performance and provide protection from wind driven rain.


  • External Walls – Full Fill Masonry Cavity Wall

Product Features

  • Lightweight 1190x450mm boards (installed dimensions)
  • Tongue and groove edge profile
  • Low emissivity foil facing
  • BBA approved – certificate 16/5343 (Full Fill Cavity Walls)

Product Properties

  • Board / Roll size: 1190x450mm (installed) 1205 x 465mm (including tongue)
  • Thickness : 90-140mm
  • Material: PIR

Product Performance

  • Lambda: 0.021 W/m.K
  • Reaction to Fire: Euroclass E Reaction to Fire classification, according to EN 13501-1 (Products with this level of classification are not provided with a rating for smoke emission)

Product Benefits

  • Excellent thermal performance with a thermal conductivity of 0.021 W/m.K, reducing the thickness of insulation required to hit the target U-value.
  • Low emissivity foil facing provides improved thermal performance in sealed cavity air spaces, improving the achievable U-value.
  • Easy to install – The boards are sized to fit standard cavity wall tie spacing. They are light and rigid, assisting with ease of transportation, handling and fitting.
  • BBA certified - Thermaclass Cavity Wall 21 has been assessed by the BBA as fit for use in defined full fill cavity wall applications if installed, used and maintained as set out in the certificate. Please read the certification for full details and restrictions

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Key Considerations

When using Thermaclass Cavity Wall 21, you need to satisfy yourself that 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. Please refer to the following product documents:

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.