Description
Frontrock Plus 50 mm is the thinnest stone wool slab Renders World stocks, supplied six slabs to a pack covering 3.6 m², and it sits in the mineral wool insulation range as the detail-zone board rather than a main-wall thickness. Every figure below comes from the Frontrock Plus Declaration of Performance, fetched for this revision.
Fitting 50 mm Stone Wool at Reveals, Soffits and Lintels
The Rockwool Plus 50mm mineral slab declares a thermal resistance of 1.40 m²K/W under EN 13162, which places it at reveals, soffits and junction details where opening geometry caps the depth rather than on the main elevation. Around a window, the depth available between frame and outer face of the main insulation is usually the binding constraint, and 50 mm is what fits.
That gives the board a specific job on a rendered wall: keeping the insulation layer continuous where it would otherwise stop. The thermal benefit at a reveal is modest by area, and the reason to bother is continuity — an uninsulated reveal is the coldest strip of a newly insulated elevation.
- Window and door reveals: the return between frame and main board line, where the render detailing at openings sets out the bead and mesh sequence that follows.
- Soffits and lintel undersides: overhead positions, where a 16.3 kg pack is a realistic one-person lift onto the scaffold lift.
- Junctions and closures: the transitions where the main insulation meets a different material or stops at a boundary.
- Vapour-open solid walls: pre-1919 brick, where the Victorian solid-wall retrofit sequence covers the elevation as a whole.
Why Specifiers Put a 50 mm Slab Around Openings
Five declared properties do the work here, each one taken from the Frontrock Plus DoP and stated at that document's scope — the slab, not the wall it ends up in.
- Euroclass A1 declared for the slab: the highest reaction-to-fire classification, which keeps the material specification consistent right around the opening rather than changing at the reveal.
- Vapour diffusion resistance factor of 1: about as open as a solid material gets, so a reveal built this way carries on drying outward — ROCKWOOL's EWI datasheet describes stone wool as having a vapour resistance similar to still air.
- Declared water absorption, short and long term: the DoP declares both WS and WL(P), which is worth having on a detail that catches run-off from the opening above it.
- Compressive strength CS(10)20, or 20 kPa: comfortable for a fixed-and-rendered reveal, and higher than the ≥ 10 kPa ROCKWOOL declares for its UK External Wall Dual Density Slab — specify to 10 kPa if your supplied batch turns out to be that product instead.
- Dimensional stability DS(70,90): the slab holds its size through heat and humidity cycling, so the reveal-to-main-wall joint stays closed and the base coat above it stays undisturbed.
One durability note sits outside the DoP and is worth the specifier's attention: ROCKWOOL's RW Slabs datasheet cites Danish Technological Institute testing from 2023 on a 65-year-old stone wool sample recovered from a Copenhagen airport hangar, whose thermal, mechanical and fire characteristics had not diminished. That is a material-level finding about ROCKWOOL stone wool, not a declared property of this slab.
Declared Specifications From the Frontrock Plus DoP
| Property | Declared value | Scope |
|---|---|---|
| Thermal conductivity (λD) | 0.035 W/mK | FRONTROCK PLUS, clause 4.2.1 |
| Thermal resistance at 50 mm | 1.40 m²K/W | DoP Table 2, 50 mm row |
| Reaction to fire | Euroclass A1 | The slab, clause 4.2.6 |
| Water vapour diffusion resistance (μ) | 1 | MU1, clause 4.3.8 |
| Compressive strength | 20 kPa | CS(10)20, clause 4.3.3 |
| Tensile strength perpendicular | 10 kPa | TR10, clause 4.3.4 |
| Dimensional stability | DS(70,90) | Clause 4.2.7 |
| Thickness tolerance class | T5 | Clause 4.2.3 |
All of the above rests on DoP RW-CEE-DoP-0238/BCM/23/w1, issued by ROCKWOOL Polska at Cigacice under EN 13162:2012+A1:2015, with notified body 1023. Confirm the DoP travelling with your delivery matches, because a Frontrock slab from a different plant declares separately.
Use 1.40 m²K/W in any U-value calculation, not 1.43. Dividing 50 mm by λ 0.035 gives 1.43, and that figure appears widely, but the declared table on the DoP reads 1.40 at 50 mm — the declared value is the one a building control submission can be held to. A U-value belongs to the whole build-up in any case, and the U-value calculation method works through it layer by layer.
How to Install the Slab: Sequence, Adhesive and Fixings
Stone wool needs a cementitious adhesive because the open fibre structure gives polyurethane foam nothing to key into. Roker U grey adhesive is the wool-compatible product in the range and serves as both the bonding coat and the reinforcement base coat, which is one bagged material covering two operations on a narrow reveal.
- Bond with a full bed at reveals: a narrow strip has little margin for dot-and-dab, and full contact across the back of the slab is what keeps it from drumming under the base coat.
- Set the main board line first: deciding the reveal geometry before the main run means the opening dictates how the thicker boards meet it, rather than being cut to suit afterwards.
- Butt the joints tight: ROCKWOOL's EWI datasheet notes that fibres of adjacent slabs knit together when tightly butt jointed, closing the gaps that would otherwise leak heat.
- Take fixing density from the specification: select length and washer from the insulation fixing accessories range, with the count set by the system holder's specification and a project wind-load calculation rather than a rule of thumb.
- Reinforce the reveal corners: diagonal mesh patches at the corners of openings address the most common crack location on a rendered facade.
Consumption figures for the adhesive are not published here, because no Roker U data sheet was retrieved this session — the EPS adhesives basecoats collection carries the product detail. Store packs under waterproof cover in the meantime, as ROCKWOOL specifies indoor or covered storage, which also keeps the pack weight down for the lift.
Plus 50 mm Against Plus 80 mm: Which Thickness Goes Where
| Specification | Plus 50 mm (this slab) | Plus 80 mm |
|---|---|---|
| Variant | Frontrock Plus 50 mm | Frontrock Plus 80 mm |
| Declared thermal resistance | 1.40 m²K/W | 2.25 m²K/W |
| Slabs per pack | 6 | 5 |
| Pack coverage | 3.6 m² | 3.0 m² |
| Cost per square metre | £12.19 | £19.33 |
| Where it goes | Reveals, soffits, junctions | Main domestic elevations |
Both slabs share the same DoP and therefore the same λ, fire class and vapour figures — thickness is the only variable between them. For the dual-density Frontrock Super range, and for the material choice against foam boards, the mineral wool against polystyrene comparison sets out the trade-offs.
Ordering, Pack Coverage and Cost per Square Metre
The slab is £43.90 per pack of six, read from the Renders World product page in August 2026, and available at the time of that reading. Each pack covers 3.6 m², so £43.90 ÷ 3.6 m² = £12.19/m² — the lowest square-metre rate in the mineral wool range, which is a function of thickness rather than a discount.
Reveal work is measured in linear metres, so convert before ordering. A 300 mm reveal running 60 linear metres gives 60 × 0.3 = 18 m²; at 3.6 m² per pack that is 18 ÷ 3.6 = 5 packs, and adding 10% for cuts brings it to 19.8 ÷ 3.6 = 5.5, so 6 packs. Reveal cutting generates more offcut than main-wall boarding, which is why the allowance is worth taking.
Is This the Right Thickness for Your Opening Details?
- Specify it where 50 mm is the maximum depth the reveal, soffit or junction geometry allows and the insulation layer needs to stay continuous around the opening.
- Step up to 80 mm for main elevations, using the Plus 80 mm slab at 2.25 m²K/W declared, which shares this slab's DoP and handling.
- Add the adhesive at the same time — Roker U grey bonds the slab and forms the reinforcement layer, and ordering both together keeps the reveal detail on one product.
- Check the moisture profile first on a solid wall, using the dew-point and condensation risk method, since reveals are where interstitial questions usually surface.
FAQ — Coverage, Fire Class, Adhesive and Ordering
How much does one pack cover?
Six slabs covering 3.6 m² laid flat. For reveal runs, multiply the linear metres by the reveal depth to get the area first, then divide by 3.6 and add an allowance for cuts before rounding up.
Which adhesive works with stone wool?
A cementitious adhesive formulated for mineral wool fibres, such as Roker U grey, which bonds the slab and doubles as the reinforcement base coat. Polyurethane foam adhesives are made for closed-cell boards and are not the route for stone wool.
Does the A1 classification apply to the finished wall?
The DoP declares Euroclass A1 for the slab itself, which is what this page publishes. A rendered external wall is assessed as a system, typically under current Approved Document B guidance, and the facade fire requirements briefing covers where that distinction becomes the specification driver.
Can 50 mm carry the main wall for Part L?
At 1.40 m²K/W declared, this thickness is specified alongside thicker main-wall slabs rather than in place of them. Its role is keeping the detail zones insulated and non-combustible while the main elevation carries the thermal load, and the target U-value is settled by a build-up calculation.
What if the slabs are rained on before rendering?
ROCKWOOL's EWI datasheet notes a high resistance to wind and rain during installation, with a water-repelling agent added during manufacture and randomly oriented water-repellent fibres resisting ingress. Keeping packs covered on site is still good practice, mainly because dry packs are lighter to handle overhead.
Is stone wool a sustainable specification?
ROCKWOOL states its stone wool contains no gases with ozone depletion or global warming potential, and its UK Rockcycle service takes back clean off-cuts and unused material from construction sites for recycling back into new products. Reveal work generates offcuts, so the take-back route is worth setting up at the start of a programme.
Technical Documentation — Frontrock Plus DoP and ROCKWOOL EWI Datasheet