Wind Driven Rain Exposure Zones Explained

Despite the repeated claims of CWI installers and their guarantors that cavity wall insulation can not allow the transfer of moisture across across the space between the inner and outer wall of a property, this has now been discredited by the Building Research Establishment (BRE).

BRE are a world leading, scientific research centre, dedicated to improving the built environment through innovation and technology.

In their publication BRE Good Building Guide 44 part 2 , they state;

“There can be an increased risk of rain penetration if a cavity is fully filled with insulation, ie moisture is able to transfer from the outer to the inner leaves resulting in areas of dampness on internal finishes. Rainwater, under certain driving rain conditions, can penetrate the outer leaf of masonry leading to wetting of the cavity insulation, a reduced thermal performance and damage to internal finishes.”

The guide provides detailed and technical information regarding what types of insulation are to be used in buildings in the four variants of exposure zones.

The Exposure Zone Guide acts as a template to determine the potential measurement of wind driven rain that a property is likely to be subjected to.

In summary, Zones 1 and 2 spread from the East of the UK moving through zones 3 and 4 as you reach the West coast. Zones 3 and 4 include all of Wales, West Scotland, the North West coast, Cornwall, Somerset and Dorset.

The BRE guide suggests that for a typical British house, with face brickwork and 50mm cavities, CWI should only be installed in to properties located in zones 1 and 2.

This doesn’t act as a hard and fast rule. Tall properties in an inland city centre or homes built on a hillside can be as equally at risk.

Below is a copy of the British Standard Rain Index map, which should have been taken in to consideration prior to determining if a property was suitable for CWI installation and which materials/treatments to use.british-standard-rain-index