Knauf DriTherm Cavity Slab

What Are DriTherm Cavity Slabs?

Knauf Earthwool DriTherm cavity slabs are manufactured from glass mineral wool bound with Knauf’s plant-based ECOSE binder. This means that the finished product feels noticeably softer than conventional fibreglass while remaining free of added formaldehyde.

The number in the product name (32 and 37) state its declared thermal conductivity in W/mK. The “Dri” prefix signals that the fibres have been treated with a water-repellent agent, helping the slab keep its insulating value even if incidental moisture finds its way into the cavity.

Factory cutting at 455mm makes installation straightforward: the slab sits snugly between 450mm wall-tie centres, closing joints with a slight friction fit and minimising air gaps.

Full-Fill Insulation

Knauf Earthwool DriTherm cavity slabs are intended for full-fill installations, meaning the insulation occupies the entire width of the masonry cavity. In a partial-fill approach, a rigid board or slab is fixed against the inner leaf, leaving an air space to the outer brickwork.

With full-fill, the slab spans the entire cavity and rests against both masonry leaves. Because no gaps remain, heat has no easy path through the wall, giving steady, high-level insulation without the need for extra air spaces or secondary barriers.

Benefits

• Straightforward Fit: Each slab matches standard wall-tie spacing, so it slips into place without retaining discs or specialist fixings. That saves time on site and reduces hardware costs.
• Consistent Thermal Layer: The semi-rigid fibres press firmly against adjoining slabs, closing off air pockets that undermine insulation. High thermal resistance helps projects meet or exceed current Building Regulations.
• Lower Environmental Impact: Up to 80% of the glass content is recycled, cutting demand for virgin raw material. The product is rated zero ODP and zero GWP, so it adds no ozone-depleting or greenhouse gases.
• ECOSE Binder: A plant-based binder, 70% less energy-intensive to produce than conventional resins, holds the fibres together. It delivers the same fire, acoustic and thermal performance without added formaldehyde.

Knauf DriTherm 32

Knauf DriTherm 32 cavity slabs take their name from their thermal conductivity rating of 0.032 W/mK. This low value reflects their strong resistance to heat flow. Like others in the range, these semi-rigid slabs are designed to reduce heat loss through external masonry cavity walls.

Each slab measures 1200mm by 455mm. This 455mm width matches standard vertical wall tie centres, making fitting straightforward and helping to maintain tight joints between slabs. 

They're treated with a water-repellent additive and are BBA certified for use in all exposure zones, including those classed as very severe.

DriTherm 32 is noticeably softer and more flexible than traditional fibreglass insulation, making it easier to work with on site. It's also made with Knauf’s ECOSE binder, which improves environmental credentials compared to conventional formulations.

Knauf DriTherm 37

Earthwool DriTherm 37 also takes its name from its thermal performance. With a thermal conductivity value of 0.037 W/mK, these slabs are less thermally efficient than DriTherm 32 cavity slabs, however, they can still be used in buildings that are up to 12m high due to their A1 (non-combustible) rating on the Euroclass scale.

Although DriTherm 37 has a marginally higher conductivity than the 32-grade slab, the installation routine is unchanged. Each full-fill slab slots neatly between wall ties; the edges compress and rebound just enough to butt up tight to the next piece, leaving no stray voids in the cavity.

The mineral wool is non-hygroscopic, so it sheds any incidental moisture and offers no foothold for mould or bacteria.