Warm, Dry, Full Fill Cavity Insulation

By Andrew Hale: Glass Product Manager – New Build (Knauf Insulation Northern Europe)

Thermal Performance

With builders working to a wide variety of Building Regulations, the choice of cavity wall insulation techniques and materials has never been greater. What’s more, with some regulations requiring different back stop U-values across the UK, being sure of what is right and what is not right can be problematic.

The LABC and National House Building Council (NHBC) state that a 50mm residual cavity is maintained in external walls in areas of very severe exposure (to wind driven rain), where fair-faced masonry is proposed as the external finish. However, it is common that the outer skin of the cavity wall of homes built in such areas are designed with a further layer of cladding or render to match the existing local architecture. This allows the cavity to be fully filled with insulation and so makes compliance with the Total Fabric Energy Efficiency (TFEE) targets, set out in ADL1A of the Building Regulations for England, easier to achieve.

Walls - External Masonry Cavity

Walls – External Masonry Cavity

Fully filling the cavity with glass mineral wool should still be the primary choice of insulation for any builder. Not only is the insulation easy to install, it remains possible to meet very low U-values with either 100mm or 125mm cavities as detailed below:

Lightweight Aircrete Block  with Brick External Leaf

Target U-value (W/m2K)

Earthwool DriTherm Cavity Slab 32 Ultimate

Inner leaf Finish


100mm Plasterboard on dabs



Plasterboard on dabs

0.17 150mm

Plasterboard on dabs

Rain Penetration

From the early Victorian period, builders and architects began to experiment with ‘hollow’ or cavity walls in order to avoid the effects of wind driven rain, which caused damp interior walls. Cavity walls became more popular during the 1930’s as builders recognised the economic advantages in building them. Then during the 1970’s energy crisis, cavities started to be insulated to reduce the heat loss through the external walls of a property.

The initial cavity size of 50mm set out by both the NHBC and LABC, has now expanded to 100mm or more in new build homes. This is not associated with damp penetration, but instead to increase the thermal performance of a wall by accommodating thicker cavity wall insulation solutions. Fully filling a cavity carries a negligible risk of damp penetration.

Earthwool DriTherm Cavity Slabs

Knauf Insulation’s Earthwool DriTherm Cavity Slabs

Knauf Insulation’s glass mineral wool insulation solutions have undergone and passed rigorous rain penetration tests, set out by the British Board of Agrément. These examinations last for three weeks with increasing levels of intensity of wind driven rain being applied to an insulated cavity wall. At the end of the test, material is removed from the wall to check no water has tracked to the inner leaf – this is in addition to regular visual checks. All in all, this rigorous test simulates the equivalent of two years of rain in London.

This performance has given us such confidence that we have been providing a 50 year moisture guarantee on our Earthwool DriTherm Cavity Slabs product range, for over 40 years and continue to do so.

The LABC Technical Manual and NHBC’s standards reflect this confidence and provide suitable wall constructions with full fill cavity wall insulation (Chapter 7 and Appendix 7.1 respectively). These standards detail that a cavity of 100mm fully filled with insulation can be used in very severe exposure zones, in conjunction with render or impervious cladding. This cavity width requirement decreases to 75mm and 50mm in the other exposure zones and can be used with fair faced masonry without the cladding.

Architects and builders can confidently fully fill cavity wall constructions with Knauf Insulation’s Earthwool DriTherm Cavity Wall insulation.

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