A masonry cavity wall showing ISOVER CWS Glass Mineral Wool used to fully fill the cavity, or Celotex CW4000 PIR to partially fill the cavity. This building detail is for illustration purposes only and should not be relied upon.

Polyisocyanurate (PIR), glass mineral wool and blown insulation products may be specified as full and partial fill insulation solutions to improve the U-value of a masonry cavity wall. Choosing between full and partial fill installation and subsequently selecting the correct insulation type is essential in terms of compliance with Building Regulations and is crucial to the energy performance and the carbon footprint of a building. There is a variety of products available for full fill cavity insulation. PIR, glass mineral wool and blown insulation offer different properties and specification will depend on the client’s needs and building performance requirements. Full fill solutions are designed to prevent moisture movement through the cavity, eliminating the need for a clear cavity that is required when partial fill insulation products are installed. The distinctions between cavity insulation products available on the market are often subtle, but it is crucial that the right products for a project are selected to achieve optimum performance of the cavity wall insulation. All cavity wall solutions must have third-party certification, confirming they are fit for purpose. These solutions must also be specified and installed in accordance with the relevant certifications. We will now explore the benefits of each cavity insulation type in the context of full and partial fill installation.

PIR  Glass mineral woolBlown Insulation
Full fill
Partial fillXX

PIR cavity insulation for masonry cavity walls

A masonry cavity wall being insulated with Celotex CW4000 partial fill insulation.

The key advantage of PIR cavity insulation boards, whether for partial or full fill construction, is their low thermal conductivity that translates into a thermal performance that is commonly 50% higher when compared to alternative insulation materials. The addition of an aluminium foil facer helps improve the thermal performance of any air cavity the PIR board faces into. Traditional PIR boards with square edges are frequently used for partial fill insulation with the residual cavity creating an effective barrier to the passage of moisture. As requirements for thermal performance of building envelopes increase in line with the updates to energy efficiency regulations, PIR boards are becoming more prominent as a full fill installation too. By filling the cavity with thermally efficient insulation, the achievable U-value of the masonry cavity wall is improved compared to a partial fill solution, without increasing the overall thickness of the construction. Further improvements can be made to the U-value by choosing a solution with a lower lambda value, such as Celotex Thermaclass Cavity Wall 21 with a thermal conductivity of 0.021 W/m.K. The design of the PIR board’s profile for full fill application can in some cases differ from that of partial fill.  In this case the straight edges are replaced with rebated edge profiles, resulting in interlocking joints that help prevent moisture from penetrating through the insulation layer.

Glass mineral wool insulation for cavity walls

The flexibility of glass mineral wool batts affords the capability to accommodate small variations in cavity width, such as those caused by mortar snots. Glass mineral wool products benefit from the batts being effectively knitted together, offering a continuous thermal envelope. Glass mineral wool products such as those in the ISOVER range can achieve Euroclass A1 Reaction to Fire classification and have excellent acoustic insulation properties. The thermal performance of glass mineral wool is typically around 0.034W/m.K, depending on the product,  and when installed across the whole width of the cavity wall, achieving a thermal performance roughly equal to partial fill PIR cavity wall insulation.

Blown wall insulation for cavity walls

Blown wall insulation is an alternative full fill solution which is pumped into cavities via holes drilled into the outer leaf of the masonry wall which may make it appropriate for existing properties. The suitability of the masonry wall itself and the building location however must be carefully considered. The use of blown insulation is not limited to existing buildings and can commonly be seen in new build housing sites.

How does Thermaclass Cavity Wall 21 change the way we think about cavity way insulation?

Innovation in the design of full fill PIR cavity wall insulation, as seen with  Celotex Thermaclass Cavity Wall 21, means that the U-value of masonry cavity walls can be significantly improved. Suitable for use in all but the most severe exposure zones, Thermaclass Cavity Wall 21 features tongue and groove edge profiles, a low emissivity foil facing, and a thermal conductivity of just 0.021 W/mK that helps to deliver lower U-values compared to other cavity wall build-ups of equivalent thickness. Celotex Thermaclass Cavity Wall 21 is a result of research, innovation and development of insulation technology and product design that can help improve the thermal comfort of buildings. It means the higher thermal performance of the building envelope can be maintained whilst allowing for a thinner design of the cavity wall of a building. You can download the full Thermaclass Cavity Wall 21 product brochure, datasheet and installation guide here.

Celotex Thermaclass CW21, high thermal performance PIR insulation for full fill masonry cavity walls.