Description
Rockwool Frontrock Plus 50 mm is the entry-thickness slab in our Plus range — A1-rated stone wool supplied in packs of 6 covering 3.6 m², ready for next-day UK dispatch. It is specified primarily where space-constrained details demand non-combustible insulation in a thin profile: window reveals, soffits, lintels, and junction zones on EWI projects.
What Rockwool Plus 50 mm Does in a UK EWI System
Rockwool Frontrock Plus 50 mm is a single-density stone wool facade slab delivering thermal conductivity of λ 0.035 W/mK, Euroclass A1 reaction-to-fire under EN 13501-1, and an R-value of 1.43 m²K/W at 50 mm — engineered for vapour-permeable EWI build-ups on UK walls. It sits in the broader mineral wool insulation range as the thinnest Plus board, supplied 6 slabs per pack covering 3.6 m². Its job is detail-zone insulation, not main-wall thermal upgrade: where reveal depth, soffit clearance, or junction geometry rules out 80–200 mm boards, the 50 mm slab keeps the whole facade non-combustible without compromising the line of the render finish.
Why Specifiers Choose Rockwool Plus 50 mm for UK Walls
- A1 fire classification across every detail: Stone wool will not ignite, sustain flame, or contribute fuel load — valuable where the project fire strategy typically requires non-combustible insulation to wrap continuously around openings and through junctions.
- Vapour-open at reveals: With a μ value close to still air, the 50 mm slab keeps reveal zones drying outward and avoids the cold-bridge condensation pattern that often appears around windows on retrofitted solid walls.
- Lightweight handling overhead: At roughly 4.5 kg/m², single-handed placement at soffits and lintels is realistic — packs weigh about 16.3 kg, scaffold-friendly for one operative.
- Dimensionally stable through UK seasons: Stone wool does not shrink or expand with temperature, so the slab does not transfer movement into the base coat at reveal-to-main-wall transitions.
- Acoustic uplift through detail zones: The dense fibrous core absorbs airborne sound, supporting overall EWI acoustic performance where reveals and soffits would otherwise act as weak points.
- Recyclable mineral composition: Manufactured from naturally abundant volcanic basalt, supporting BREEAM and circular-economy criteria on regulated projects.
Thermal Specifications — λ, R-Value, Reaction to Fire
| Property | Value | Standard |
|---|---|---|
| Thermal conductivity (λD) | 0.035 W/mK | EN 12667 |
| R-value at 50 mm | 1.43 m²K/W | EN 13162 |
| Reaction to fire | Euroclass A1 | EN 13501-1 |
| Water vapour resistance factor (μ) | ≈ 1 | EN 12086 |
| Specific heat capacity | ≈ 1030 J/kgK | — |
At R 1.43 m²K/W the 50 mm slab alone does not deliver the whole-wall U-values targeted by current Approved Document L guidance — the comparison below shows why this thickness pairs with thicker main-wall boards rather than replacing them.
Physical Specifications — Density, Strength, Dimensions
Physical numbers below set the handling reality on scaffold — pack weight, slab dimensions, and density together explain why this board sits where it does in the range.
| Property | Value |
|---|---|
| Thickness | 50 mm |
| Slab dimensions | 1000 × 600 mm |
| Nominal density | ≈ 90 kg/m³ |
| Tensile strength perpendicular to faces (TR) | ≥ 10 kPa |
| Compressive stress at 10% deformation (CS(10)) | ≥ 10 kPa |
| Slabs per pack | 6 |
| Pack coverage | 3.6 m² |
| Pack weight | ≈ 16.3 kg |
| Weight per m² | ≈ 4.5 kg |
Where Rockwool Plus 50 mm Performs Best — Detail Zones & Build-Ups
The 50 mm thickness is a detail-zone slab. It earns its place where the geometry of the wall — not the U-value target — sets the thickness limit, and where the project fire strategy keeps the insulation layer non-combustible right through to the substrate.
- Window and door reveals: Fits standard frame setbacks without narrowing the visible frame width below acceptable limits, wrapping the opening in A1 material.
- Soffits and lintels: Light enough at 4.5 kg/m² for safe overhead bonding and mechanical fixing without specialist lifting.
- Cavity closures and corner junctions: Maintains fire integrity at the points where the main insulation transitions, including verges and service penetrations.
- Heritage and solid-wall retrofits: Sits comfortably in vapour-open build-ups on pre-1919 brick — see the Victorian solid-wall retrofit guide for whole-elevation detailing context.
How Rockwool Plus 50 mm Fits Into a Full EWI System
Build-up runs vapour-open from substrate to finish. Bond each slab to prepared masonry with Roker U grey adhesive for mineral wool and EPS, a cementitious formulation specified for stone wool fibre and doubling as the reinforcement base coat. Mechanical fixings from the EWI fixing accessories range are applied at a minimum of 8 per m² using 60–90 mm washer plates to distribute load across the slab face.
- Adhesive + base coat: Roker U grey (cementitious, vapour-permeable) — bonding layer and reinforcement layer use the same product.
- Reinforcement mesh: Alkali-resistant fibreglass mesh embedded fully in the wet base coat distributes mechanical loads and resists cracking.
- Mechanical fixings: 60–90 mm disc washer plugs at 8 per m²; stainless-steel pins for coastal or exposed elevations.
- Render finish: Silicone or silicate topcoats keep the system vapour-open from substrate through to surface, in line with the breathing-wall rationale for mineral wool.
How Rockwool Plus 50 mm Compares to Sibling Slabs
| Specification | Plus 50 mm (this product) | Plus 80 mm | Super 80 mm |
|---|---|---|---|
| Range | Frontrock Plus | Frontrock Plus | Frontrock Super |
| Thickness | 50 mm | 80 mm | 80 mm |
| λ (W/mK) | 0.035 | 0.035 | 0.036 |
| R-value (m²K/W) | 1.43 | 2.29 | 2.22 |
| Density | ≈ 90 kg/m³ | ≈ 90 kg/m³ | 80/150 kg/m³ dual |
| Fire rating | A1 | A1 | A1 |
| Pack coverage | 3.6 m² (6 pcs) | 3.0 m² (5 pcs) | 1.8 m² (3 pcs) |
| Typical use | Reveals, soffits, junctions | Standard domestic full-wall EWI | Exposed or impact-prone facades |
For the full mineral-wool-versus-EPS decision walkthrough, our mineral wool vs polystyrene comparison sets out the trade-offs on cost, U-value per millimetre, and fire performance.
How Rockwool Plus 50 mm Performs in UK Weather — Moisture, Frost, Wind Loads
Stone wool is engineered for the wind-driven rain and cyclic freeze-thaw that define UK exposure ratings. The factory hydrophobic treatment sheds rain during the install window, and the open-cell fibre structure dries outward rapidly once the render system is on. Frontrock Plus holds dimensional stability across the typical UK service range of −10 °C to +35 °C, so the slab does not feed thermal movement into the base coat or render at reveal-to-main-wall transitions.
Where wind-driven moisture risk drives detailing decisions — particularly on solid-wall retrofits — the dew-point analysis for external wall insulation walks through the interstitial moisture profile this slab is designed to manage. Detail-zone mineral wool — open-cell, μ ≈ 1 — gives more forgiving moisture behaviour than closed-cell alternatives at reveal and lintel positions, particularly during the install window before render is applied.
Handling and Storage of Rockwool Plus 50 mm on UK Sites
- Cut clean: Use a long-blade insulation knife with the branded face ("THIS SIDE UP") placed downward on a flat board, slicing through the softer back layer first to keep the cut face crisp.
- Orient correctly: Branded face outward toward the render — installing the slab reversed reduces adhesive bond and fixing performance.
- Store dry: Opaque waterproof sheeting on site; saturated packs handle poorly overhead even though the hydrophobic treatment recovers thermal performance after drying.
- Stack flat: Always on a level pallet, away from scaffold edges and protected from mechanical damage to the corners.
Get these four right and the slab arrives at the wall in the same condition it left the pallet.
Pro Tips From UK Installers Using Rockwool Plus 50 mm
A few details separate the cleanest reveal work from average — the points below come from EWI installers on UK retrofits across mixed substrates.
- Plan reveals before the main board run: Decide reveal slab geometry first; it dictates how the 80–200 mm main-wall boards meet the opening, not the other way round.
- Full-bed adhesive at reveals: Use a full bed of Roker U at reveal positions rather than dot-and-dab — the narrow strip needs uniform contact to avoid drumming under base coat.
- Bring fixings into the dense face: Set washer plates flush with the branded face, not recessed into the softer back, to keep the load on the high-density side of the slab.
- Mesh-wrap the reveal corners: Diagonal mesh patches at reveal corners reduce the most common crack location on rendered openings.
Certifications & Compliance — A1, EN 13162, REACH
- EN 13162: Factory-made mineral wool products for buildings — declared λ, dimensional tolerances, and mechanical properties.
- EN 13501-1: Euroclass A1 reaction-to-fire — the highest reaction-to-fire classification, indicating non-combustible material.
- BS EN ISO 9001: Rockwool manufactures under certified quality management (BSI FM 02262), supporting batch-to-batch consistency.
- UK REACH compliant: No hazardous classifications associated with the stone wool composition.
Specifiers routinely cite these certificates as the baseline for non-combustible facade build-ups, and the Building Safety Act 2026 facade-fire briefing sets out where non-combustible classification becomes the specification driver under current Approved Document B guidance.
Is Rockwool Plus 50 mm Right for Your Project?
- Choose this slab if: You need A1-rated stone wool at reveals, soffits, lintels, or junction details where 50 mm is the maximum depth the geometry allows, and the facade specification calls for non-combustible insulation throughout.
- Step up for full-wall coverage: The Rockwool Plus 80 mm slab delivers R 2.29 m²K/W for domestic full-elevation EWI while retaining the same Plus-range handling and A1 rating.
- Exposed or impact-prone elevation? The dual-density Frontrock Super 80 mm provides a hardened 150 kg/m³ outer face for taller buildings and public-facing walls.
- Low-rise project where fire strategy permits combustible insulation? Browse the graphite EPS board range at λ 0.032 W/mK for higher thermal resistance per millimetre at lower material cost.
All values stated are based on manufacturer-declared data from Rockwool and remain subject to confirmation through project-specific assessment. Specifications may be updated by the manufacturer without prior notice — verify against the current Declaration of Performance before specifying.
What to Order Next — Pack Sizes, Lead Times, Compatible Components
Each pack of Rockwool Plus 50 mm covers 3.6 m² (6 slabs at 1000 × 600 mm). For full-system ordering, pair the slab quantity with a matched run of Roker U adhesive and the correct fixing length for the substrate plus slab thickness.
- Adhesive & base coat: Roker U grey adhesive 25 kg — calculate 4–5 kg/m² bonding plus 3–4 kg/m² for the reinforcement layer.
- Mechanical fixings: Select from the EWI fixing accessories collection matched to total embedment (substrate + slab depth).
- Lead time: Full-pack orders confirmed before the daily cut-off ship on the next UK working day.
FAQ — Rockwool Plus 50 mm Coverage, Compatibility, Ordering
How many square metres does one pack of Rockwool Plus 50 mm cover?
Each pack contains 6 slabs of 1000 × 600 mm and covers 3.6 m² laid flat. Order quantities should include an allowance of 5–10% for cuts at reveals, soffits, and junctions, which is where the 50 mm thickness sees most of its use.
Can I use 50 mm Rockwool Plus as the main wall insulation for Part L compliance?
At R 1.43 m²K/W, 50 mm alone does not typically reach the whole-wall U-values targeted by current Approved Document L guidance for main elevations. Its role is at reveals, soffits, lintels, and junctions, alongside 80–200 mm slabs on the main wall. For wall-by-wall calculation, the U-value calculation guide walks through the thickness selection method.
Which adhesive is compatible with Rockwool Plus 50 mm?
Use a cementitious adhesive designed for mineral wool fibres, such as Roker U grey, which bonds the slab to the substrate and serves as the reinforcement base coat in the same product. Polyurethane foam adhesives are not suitable for stone wool because the fibre structure does not provide the closed-cell key foam relies on.
Does Rockwool Plus 50 mm meet A1 fire requirements for relevant buildings?
The slab carries Euroclass A1 reaction-to-fire under EN 13501-1 — the highest classification, indicating non-combustible material. Where current Approved Document B guidance requires A1 or A2-s1,d0 insulation in the external wall construction, this product meets the material-level threshold, though the overall system specification remains a project-specific matter for the building designer.
What happens if the slabs get wet during installation?
Frontrock Plus carries a factory hydrophobic treatment that sheds rain during the install window before the render system is on. The open-cell fibre structure dries outward rapidly, and the slab recovers full declared thermal performance once dry. Saturated packs handle poorly overhead, so site storage under opaque waterproof cover is good practice.
Is Rockwool Plus 50 mm a sustainable specification choice?
The slab is manufactured from naturally abundant volcanic basalt with no blowing agents carrying ozone-depleting or global-warming potential, and off-cuts can be returned to Rockwool's recycling programme. The material composition supports BREEAM credits and closed-loop waste management on regulated UK projects.
Technical Documentation — Rockwool Plus 50 mm TDS & Datasheets