Isover Cavity Wall Slab (CWS 36) is a semi-rigid, glass mineral wool insulation slab engineered specifically for thermal insulation in full-fill or partial-fill masonry cavity wall constructions in new builds or extensions.

It is primarily used for:

• Thermal Insulation in Cavity Walls: Inserting between inner blockwork and outer brickwork leaves to reduce heat loss through external walls, helping buildings meet local Building Regulations (such as Part L in the UK).

• Full-Fill and Partial-Fill Cavity Applications: Designed to fit securely between wall ties without slumping, maintaining thermal performance across the entire wall height.

• Fire Protection: Achieving an Euroclass A1 reaction to fire rating (non-combustible), adding crucial fire resistance within external masonry walls.

• Acoustic Sound Reduction: Absorbing airborne sound travelling through external wall structures, reducing outside noise transmission.

• Water Repellency: Treated with silicone water-repellent additives to prevent moisture transmission across the cavity from the outer brick skin to the inner blockwork.

Available for collection from our Rugby branch or local delivery using our own fleet.

Timber products are typically delivered in one of our vans or pickup trucks. Delivery is kerbside and the final offloading position is always subject to safe access and the driver’s discretion.

Please tell us before delivery about:

• Narrow roads or restricted entrances
• Parked vehicles or limited turning space
• Overhead cables, trees or other obstructions
• Soft, sloping or unsuitable ground
• Any other access concerns

Enter your postcode on the product page to check your delivery area. We’ll confirm the delivery date after your order is placed.

What does the "36" in CWS 36 stand for?

The number 36 refers to the thermal conductivity (lambda value) of the insulation, which is 0.036W/mK. A lower lambda value indicates better thermal efficiency, helping walls achieve target U-values with thinner cavity widths.

Can Isover CWS 36 be used for full-fill cavities without a residual air gap?

Yes. Isover CWS 36 is BBA-certified for full-fill cavity applications in external masonry walls. The glass mineral wool fibers are treated with a silicone water-repellent additive, ensuring that water passing through the outer brickwork cannot bridge across the cavity to the inner leaf.

How is CWS 36 secured inside the cavity during bricklaying?

Slabs are installed friction-fitted tightly against the inner blockwork leaf as wall construction progresses. They are secured in place using standard double-drip wall ties fitted with plastic insulation retaining clips (retaining disks) to prevent slabs from bowing or slumping forward.

What fire classification does Isover CWS 36 achieve?

Isover CWS 36 achieves a Euroclass A1 reaction to fire rating—the highest possible non-combustible safety rating. It produces no toxic smoke or flaming droplets, making it fully compliant for use in multi-story residential and commercial buildings.