Product Description:

Knauf DriTherm Cavity Slab 32 is a water-repellent glass mineral wool slab designed for full-fill insulation of external masonry cavity walls. Supplied at 150mm thick in 1200 x 455mm slabs covering 2.18m² per pack, it is built into the cavity as the wall is constructed, on new build housing, extensions, and outbuildings.

At 150mm, this is the thickness specified where a demanding U-value has to be met and the cavity has been designed deep enough to take it. The 32 designation refers to the slab's thermal conductivity of 0.032 W/mK, which at this depth gives an R-value of around 4.69 m²K/W — a substantial improvement over a 100mm cavity and the kind of performance now expected on new build work under current Building Regulations.

Full-fill insulation works only if the cavity is genuinely full, which is where mineral wool slab has the advantage over rigid board. The slabs are semi-rigid and slightly compressible, so they push tightly against the masonry on both sides, close up around wall ties, and absorb the small irregularities in blockwork and mortar squeeze that leave gaps behind a rigid board. A water-repellent additive runs through the slab, so moisture crossing the outer leaf in driving rain is shed rather than absorbed, keeping the insulation dry and its performance intact. Slabs should be built in as the inner leaf rises, fitted tightly with no gaps at joints, and kept clear of mortar droppings, which would otherwise bridge the cavity.