Dampness can occur either from condensation within a building or water penetrating into the building, particularly in a wall. The way some insulated walls are made causes problems in areas where the wall is exposed to a lot of bad weather, for example, building on the west coast of the Scottish Highlands. Some PIR insulation can be used with a 25 mm cavity. Some modern forms of PIR insulation don’t have a cavity at all although they usually have a patterned area on the back in order to let the water drain away. A 50 mm cavity is needed under exposed conditions. The wall consists of bricks, air cavity, insulation and inner blocks. Making the cavity wider simply gives any water penetrating the wall somewhere to go. You are unlikely to achieve the thermal fabric efficiency required by SAP 2012 with 50 mm of insulation. It is possible to make the cavity 150 mm and use 100 mm of PIR insulation.
Water Penetration in Exposed Areas