Celotex Isover

Upgrading Cavity Walls

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Masonry cavity walls as we understand them today have been around for about a century. Early cavity walls did not feature thermal insulation materials; the air space alone was relied upon for additional thermal performance. As national building regulations began to set target U-values for building elements, insulation materials started being used in the cavity - but not to the extent we see now.

In recent years, where assessment has shown it to be a suitable solution, many walls with a clear cavity have been filled retroactively. The performance of such a solution is dependent on the efficiency of the insulation material and the width of the cavity. Similarly, where walls were constructed with insulation, the performance is limited to the standards of the time.

For homeowners looking to improve the performance of their properties - perhaps to achieve compliance with modern building regulations, or perhaps to go beyond that and meet a retrofit standard such as EnerPhit - replacing or adding more insulation material in an already-treated cavity is not possible.

Using an insulated plasterboard product, either adhered to the masonry inner leaf or mechanically fixed to timber battens, represents an ideal option for improving the thermal efficiency of the wall. Lightweight polyisocyanurate (PIR) foam boards from Celotex are easy to handle, and their thermal efficiency means thicknesses can be kept to a minimum, reducing lost floor space.

As part of an holistic approach to upgrading you may also want to consider building fabric, airtightness and controlled ventilation along with improvements in thermal performance. The composite facing on the insulation board, behind the plasterboard layer, can form part of the air and vapour control layer when the tapered edge plasterboards are taped and jointed.

Key Considerations

When using Celotex or ISOVER 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 for your construction or application, including requirements in relation to fire and applicable height restrictions. In addition to the product datasheet, please refer to the following product documents:

Celotex products 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.

The building detail is for illustrative purposes only. It does not constitute advice and should not be relied upon.


Insulation products
Product Name Thickness Range Sizes Lambda
Celotex PL4000 25-65mm (+12.5mm Plasterboard) Width 1200mm, Length 2400mm 0.022 W/m.K
Celotex GA4000 50-100mm Width 1200mm, Length 2400mm 0.022 W/m.K
Celotex TB4000 20-40mm Width 1200mm, Length 2400mm 0.022 W/m.K