Thermaclass Cavity Wall 21

Thermaclass Cavity Wall 21 is a rigid polyisocyanurate (PIR) insulation solution from Celotex for masonry full fill cavity wall applications. Featuring a tongue and groove edge profile, Thermaclass Cavity Wall 21 has an excellent thermal performance of 0.021 W/m.K (“lambda 21”) which can help to achieve lower U-values without increasing the overall wall thickness. For wall constructions featuring a 10mm residual cavity, the low-emissivity foil facing increases the thermal performance in sealed cavity air spaces, improving the achievable U-value. Thermaclass Cavity Wall 21 has been assessed by the British Board of Agrément (BBA) as fit for use in defined full fill cavity wall applications if installed, used and maintained as set out in the certificate.

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Watch the video below to discover how Thermaclass Cavity Wall 21 could benefit your project:

Rigid full fill masonry cavity wall insulation

Thermaclass Cavity Wall 21 can be installed with a residual cavity of up to 10mm. The nominal air space helps with ease of install for bricklayers and accommodates any mortar squeeze. However, if required, the product can be used as a full fill solution with no residual cavity.

High thermal performance with Thermaclass insulation

With a thermal conductivity of 0.021 W/m.K, the target U-value may be achievable with a thinner insulation solution. This can allow a reduced overall wall thickness, to maximise the inside space of a building.

Product features of Thermaclass Cavity Wall 21 at a glance:

  • Rigid PIR full fill masonry cavity wall solution with low lambda of 0.021 W/m.K
  • Engineered tongue and groove board edges maximise thermal performance, resist water penetration, and remove requirement for taping joints*
  • Low emissivity foil facing provides improved thermal performance in sealed cavity air spaces, improving the achievable U-values**
  • 90mm, 115mm and 140mm thicknesses to suit common cavity widths
  • Boards are sized to fit standard cavity wall tie spacing. They are light and rigid to assist with ease of transportation, handling and fitting.
  • BBA certified for full fill cavity wall applications
  • Manufactured using a blowing agent that is CFC/HCFC free, has zero ozone depletion potential (ODP), and has low global warming potential (GWP)
  • Produced under an ISO 14001:2015 accredited Environmental Management System

* When installed according to our Installation Guide (download below)
** Based on a residual cavity of 10mm

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:

  • BBA certificate
  • Declaration of Performance
  • Health & safety datasheet

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.

Find out more

Visit the product page for further details, from a more in-depth look at the boards in masonry full fill cavity applications in the product install guide, to comprehensive information in the product brochure and datasheet, or to view the Declaration of Performance and BBA certificate.

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Contact us for more information

Thermaclass Cavity Wall 21 is the first insulation solution in the new low-lambda Thermaclass range.

To find out more about how Thermaclass Cavity Wall 21 can benefit your project, contact our Technical Support Team on 01473 820850. (Telephone lines are open from 9am to 5pm, Monday to Friday.)

Alternatively, email:


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