Thermaclass Cavity Wall 21

Thermaclass Cavity Wall 21 is an easy to install, polyisocyanurate (PIR) insulation board for full fill cavity wall applications, with excellent thermal performance (0.021 W/m.K). Featuring a low emissivity foil facing, 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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Product Features


  • Lightweight 1190 x 450mm boards (installed dimensions) 
  • Tongue and groove edge profile 
  • Low emissivity foil facing 
  • BBA approved – certificate 16/5343 (Full Fill Cavity Walls)


Fire Classification

Euroclass F 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.

Product Specification

Thickness (mm) Length (mm) - Net/installed Width (mm) - Net/installed Lambda Value (W/m.K) R-Value (m².K/W)
90 1190 450 0.021 4.25
115 1190 450 0.021 5.45
140 1190 450 0.021 6.65

Product Benefits


  • Thermal Performance - 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.
  • High Performance Facer - Low emissivity foil facing provides improved thermal performance in sealed cavity air spaces, improving the achievable U-value (based on a residual cavity of 10mm).
  • 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 BBA certification page for full details and restrictions.


Key Considerations

When using Celotex 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:

Celotex Thermaclass Cavity Wall 21 should not be used in the external walls of buildings over 11 metres in height. Recent changes to Building Regulations mean that only non-combustible insulation or insulation of limited combustibility should be used in buildings of that height.