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 75mm thick in 1200 x 455mm slabs, it is built into the cavity as the wall is constructed, and is the thickness used where the cavity is narrower than modern new build standard — on extensions matching an existing wall, garages and outbuildings, and refurbishment or rebuilding work on older properties.

The 32 designation refers to the slab's thermal conductivity of 0.032 W/mK, which at 75mm gives an R-value of around 2.34 m²K/W. In a narrow cavity, that conductivity rating matters more than it does in a deep one: with less depth available, the quality of the insulation is the only variable left, and a lower-performing product in a 75mm cavity may not reach the required U-value at all.

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 take up 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. 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.

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