Knauf DriTherm Cavity Slab

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Knauf DriTherm Cavity Wall Insulation Slabs are made from glass mineral wool and are either semi-rigid or rigid, depending on the specification. Designed for use in masonry cavity walls, they deliver strong thermal performance while also helping to manage moisture with their water-repellent treatment.

These slabs are non-combustible and carry a Euroclass A1 fire rating (the highest level available). They’re suitable for use in all exposure zones on buildings up to 12 metres in height and can also be specified for multi-storey projects up to 25 metres, depending on the system.

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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.

Frequently Asked Knauf DriTherm Cavity Slab Questions

Are Knauf DriTherm Cavity Slabs Fireproof?

Knauf Insulation DriTherm 32 and Knauf Insulation DriTherm 37 slabs are rated A1 on the Euroclass scale. The European Reaction to Fire classification system (Euroclasses) is the EU common standard for assessing the qualities of building materials in the event of a fire.

Insulation materials are measured against up to five tests and given a rating from A1 (non-combustible) to F (combustible). With an A1 rating, these slabs achieve the highest possible certification on the Euroclass scale. This means they provide excellent protection against fire and minimise the risk of fire spreading.

Is Knauf Cavity Wall Insulation Waterproof?

Although Knauf Insulation DriTherm 32 and DriTherm 37 cavity slabs are water-resistant (thanks to their water repellent additive), this does not mean that they are waterproof. Whilst the slabs are able to resist the penetration of water to a certain degree, they are not completely impenetrable.

Both the DriTherm cavity slab 32 and DriTherm cavity slab 37 have a water vapour resistivity of 5.00MNs/g.m. Earthwool slabs are non-hygroscopic and are therefore approved for use in all exposure zones, as they will not absorb moisture. Another welcome benefit of the water-resistant binder used in Knauf slabs is that it eliminates the chance of rot, vermin, or fungus.

For this reason, we highly recommend Knauf slabs for people who live in high-rainfall areas. People choose Knauf Earthwool slabs when they need full-fill insulation that has a long lifespan. 

How to Cut Knauf Insulation DriTherm Cavity Slabs?

Knauf Earthwool wool cavity slabs are very light and malleable. You can simply use a sharp serrated knife to slice through Dritherm 32 and DriTherm 37. 

Always ensure you take proper safety precautions. Before you cut and fit into masonry cavity external walls, we recommend wearing goggles, aprons, and even protective glasses. If you are concerned about safety precautions, always consult with a professional.

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