Product News


Delivering Performance in Full Fill Cavity Wall Installations with Celotex

April 27th 2022


Masonry cavity walls have been a common method of construction for many years. The aim of having two leaves of masonry separated by a clear cavity was to prevent the possibility of moisture coming from the outside to the inside of a building.



Thermaclass Full Fill Cavity

Over time, this cavity became the location for thermal insulation to be installed to reduce heat loss from the building. Traditionally, rigid insulation boards were only used to partially fill the cavity ensuring that the clear air space was retained to prevent moisture ingress. With advances in manufacturing, rigid insulation boards could be used to fully fill the cavity with the aim to significantly reduce the requirement for the clear cavity.

A true full fill cavity insulation board is typically thought of as one that is in contact with both leaves of the construction at the same time, although some products require a small cavity of up to 10mm to be maintained to deliver their performance. In addition to this, products often require the use of taping insulation boards together to maintain integrity of performance, generally resulting in installations taking longer or costing more to deliver the overall benefit.




With a low thermal conductivity of 0.021 W/m.K and availability in a range of thicknesses, Celotex Thermaclass Cavity Wall 21 enables low U-values to be achieved ensuring high levels of thermal performance whilst keeping overall wall widths to a minimum. The reflective facer of the boards gives an added low emissivity benefit to the overall thermal performance of the construction when used in conjunction with the 10mm air cavity.

Unlike many products in this category, Celotex Thermaclass Cavity Wall 21 offers the benefit of being able to be used in both a true full fill installation or with a 10mm residual cavity. This gives benefits to specifiers and designers such as design flexibility and choice of approach. In certain applications the 10mm cavity may make it easier for builders to install or, where required, accommodate for mortar squeeze.

Another significant benefit of Thermaclass Cavity Wall 21 is that it does not require board joints to be taped, saving both time and additional cost to the install. The tongue and groove edge profile helps to keep the boards in line and acts as the barrier to water ingress by preventing water from tracking between the board joints. Thermaclass Cavity Wall 21 has been tested and assessed by the BBA (a third party certification body) as fit for use in full fill masonry cavity wall applications when installed, used and maintained as set out in the certificate.

As with all construction products, 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 to your construction or application, including requirements in relation to fire and applicable height restrictions.

Thermaclass Cavity Wall 21 is available in three thicknesses (90mm, 115mm, 140mm) from your local CCF branch, or you can order online HERE.

For further technical details please see the manufacturers Product Downloads & Specifications HERE.