Rockwool External Wall DD Slab
Dual Density Rockwool Insulation for External Walls
A non-combustible Rockwool insulation slab developed for external wall insulation systems. This dual density stone wool board provides a dense outer face for base coat application while preventing overdriven fixings, and its breathable open-cell structure allows water vapour to pass freely through the construction.
Dual density non-combustible insulation for EWI systems
Rockwool External Wall DD (Dual Density) Slab is a stone wool insulation slab — a type of Rockwool insulation — developed for use in external wall insulation systems. Manufactured using a dual density process, the outer layer of the slab has a higher density than the body of the board — providing a solid face for base coat application and helping to prevent overdriven mechanical fixings, while the more resilient underside accommodates minor unevenness in the substrate. The breathable open-cell structure allows water vapour to pass freely through the construction.
Rockwool stone wool is manufactured from volcanic basalt rock, melted in a cupola furnace at temperatures of up to 1,500°C and spun into fine strands. As a material it is non-combustible and achieves the highest Euroclass A1 reaction-to-fire classification to BS EN 13501-1 — meaning it does not burn and will not contribute to any stage of a fire.
When slabs are tightly butted, adjacent boards effectively "knit" together to form a continuous insulating layer, reducing the heat losses that would otherwise occur through gaps between boards.
What is Rockwool insulation?
Rockwool insulation — also written rock wool, and more formally known as stone wool or mineral wool — is a mineral fibre insulation made from volcanic basalt rock. The rock is melted at temperatures of up to 1,500°C and spun into fine fibres, which are then bound and formed into rigid or semi-rigid slabs.
As an insulation material, Rockwool offers a combination of properties that make it particularly well suited to external wall insulation: it is non-combustible with the highest Euroclass A1 fire rating, it is breathable so it allows moisture vapour to escape rather than trapping it, it is water repellent, and it provides excellent thermal and acoustic performance. Because it is made from rock, it does not rot, degrade or settle over time.
For external walls, Rockwool external wall insulation is typically installed as part of a complete EWI system — the insulation slab is fixed to the wall, then finished with a reinforced base coat and a render or brick slip finish to weatherproof and protect it.
Why specify stone wool
Non-Combustible
Euroclass A1 reaction to fire — stone wool does not burn, withstands high temperatures and emits negligible smoke and fumes in the event of a fire.
Dense Outer Surface
A solid, dense outer face provides a sound base for base coat application and helps prevent fixings being overdriven into the slab.
Continuous Insulating Layer
When tightly butted, adjacent slabs knit together to form a continuous layer, reducing heat loss through gaps between boards.
Water Repellent
Treated to repel water — should moisture reach the slab it sheds rather than absorbs, protecting long-term thermal performance.
Breathable
The open-cell structure allows water vapour to pass through, with a vapour resistance similar to that of still air, while water-repellent fibres resist water ingress.
Acoustic Performance
Test evidence demonstrates an improvement to the weighted sound reduction (Rw) of up to 8 dB, substrate-dependent.
Dimensionally Stable
Holds its shape and density over time — it will not slump or compact in the wall the way some lower-density insulants can.
Sustainable Composition
Manufactured from naturally occurring basalt rock and recycled content, with a long, maintenance-free service life.
Straightforward to Install
Semi-rigid and cut cleanly with a serrated blade for a snug fit — no adhesive tapes, staples or nails required to position the board.
Applications
Rockwool External Wall DD Slab is developed for use as the insulation layer within external wall insulation systems, where its dense outer face provides a sound base for the base coat.
What it delivers
Thermal. A low thermal conductivity (typically 0.036–0.038 W/mK depending on slab) delivers efficient heat retention, helping walls achieve target U-values and reducing heating demand.
Fire. As a non-combustible A1 material, stone wool can be used as the insulant in EWI systems specified for buildings of all heights, including those over 11 metres where Regulation 7(2) applies.
Moisture & durability. Water-repellent and dimensionally stable, the slab maintains its thermal and acoustic performance over a long service life without degrading or compacting.
Specification
Indicative technical data for Rockwool External Wall DD Slab. Always refer to the current manufacturer data sheet for full specification, available thicknesses and design values.
Working with the slab
Rockwool External Wall DD Slab is semi-rigid and designed for straightforward handling on site. The dense outer face provides a sound base for base coat application and helps prevent fixings being overdriven.
Always install in accordance with the specified EWI system build-up and the manufacturer's current installation guidance.
Used across every building type
Domestic
Improves energy efficiency, reduces noise and enhances fire safety in homes and residential developments.
Commercial
Reliable thermal insulation, fire resistance and acoustic control for offices, hotels and retail.
Industrial
Withstands high temperatures, providing robust protection in warehousing and manufacturing facilities.
Healthcare & Education
Specified where fire safety, acoustic comfort and durability are critical — including schools and care settings.
Need Rockwool insulation for your project?
Rockwool external wall insulation stocked across our six UK branches. Speak to our team about availability, thicknesses, pricing and the right EWI system for your specification.