Description
Renders World stocks the 80 mm ROCKWOOL FRONTROCK PLUS slab as the full-wall thickness in our mineral wool insulation range, supplied five slabs to a pack covering 3.0 m². Every performance figure below is quoted from the FRONTROCK PLUS Declaration of Performance and ROCKWOOL's published parameter table for that product, both fetched for this revision.
Where the Rockwool Plus 80mm Mineral Slab Fits on a UK Wall
The Rockwool Plus 80mm mineral slab declares a thermal resistance of 2.25 m²K/W at 80 mm under EN 13162, which puts it on full domestic elevations where a non-combustible layer has to run continuously across the wall. Thinner slabs handle the returns; this one handles the field.
- Whole-elevation boarding: detached, semi-detached and terraced work where the same stone wool slab runs from base track to verge without a change of material.
- Solid-wall retrofit: pre-1919 brick, where the Victorian solid-wall retrofit sequence sets out how the elevation is worked as a whole.
- Vapour-open build-ups: a declared diffusion resistance factor of 1 keeps the insulation layer about as open as a solid material gets.
- Rendered facades needing a non-combustible layer: the slab itself is declared Euroclass A1, which is a material credential rather than a finished-wall one.
What that buys on a programme is consistency of specification: one slab type across the field, one adhesive, one base coat, and reveals picked up by the 50 mm board from the same range.
Declared λ, Thermal Resistance and Fire Class for the Slab
Ten characteristics are declared for FRONTROCK PLUS, and each belongs to the slab as manufactured — not to the rendered wall it ends up inside. The scope column below is copied from the document that carries the value.
| Property | Declared value | Scope |
|---|---|---|
| Thermal conductivity (λD) | 0.035 W/mK | FRONTROCK PLUS, EN 12667 |
| Thermal resistance at 80 mm | 2.25 m²K/W | DoP Table 2, 80 mm row |
| Reaction to fire | Euroclass A1 | The slab, EN 13501-1 |
| Compressive stress at 10% deformation | 20 kPa, CS(10)20 | EN 826 |
| Tensile strength perpendicular to faces | 10 kPa, TR10 | EN 1607 |
| Water vapour diffusion resistance factor | 1, MU1 | EN 12086 |
| Short-term water absorption | 1.0 kg/m², WS | EN 1609 |
| Long-term water absorption | 3.0 kg/m², WL(P) | EN 12087 |
| Dimensional stability | DS(70,90) | EN 1604, 70 °C and 90% RH |
| Thickness tolerance | T5 | EN 823 |
One figure is worth a purchasing instruction. Dividing 80 mm by λ 0.035 gives 2.29 m²K/W, and that number circulates widely, but the declared table reads 2.25 — use the declared value in any calculation a building control officer may read, and treat 2.29 as arithmetic rather than performance. The same applies to compressive strength: this slab declares 20 kPa, whereas ROCKWOOL's UK External Wall Dual Density Slab declares ≥ 10 kPa, so confirm which product is on the pallet before specifying to either.
The A1 classification is declared for the slab under EN 13501-1 and does not transfer to a wall finished in an organic render, which classifies on its own evidence. For the wider specification picture, the 2026 facade fire requirements briefing covers where a non-combustible layer is a regulatory driver and where it is a project preference. Two figures previously shown here — a 90 kg/m³ density and a specific heat capacity — have come off the page, because neither document declares them.
How to Fit the 80 mm Slab to Masonry
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 bonding coat and reinforcement base coat.
- Bond on a full bed: full contact across the back of the slab is what stops drumming under the base coat on long unbroken runs.
- Butt the joints tight: ROCKWOOL's EWI System Holders datasheet notes that the fibres of adjacent slabs knit together when tightly butt jointed, closing the gaps that would otherwise leak heat.
- Stagger the courses: the same datasheet specifies a brick bond pattern to avoid thermal bridging in its double-layer instructions, and the logic holds on a single layer.
- Take the fixing count from the specification: length, washer and density come from the system holder and a project wind-load calculation, which is why no per-square-metre figure is printed here — choose the hardware from the insulation fixing accessories range once that number is issued.
- Store under cover: the datasheet asks for slabs indoors or under waterproof covering, and dry packs board faster.
Mesh and base coat follow the same day where the weather allows, using the cementitious products in the EPS adhesives basecoats collection. With a diffusion resistance factor of 1, the slab dries outward through the render, and the condensation risk method for external insulation shows how that behaves across a full build-up.
Plus 80 mm Against the Slabs Either Side of It
Three slabs cover the domestic range, and the declared numbers separate them cleanly.
| Slab | Declared λ and R | When to choose it |
|---|---|---|
| Frontrock Plus 50 mm | 0.035 W/mK · R 1.40 at 50 mm | Reveals, soffits and lintel returns |
| Frontrock Plus 80 mm | 0.035 W/mK · R 2.25 at 80 mm | Full elevations on standard domestic work |
| Frontrock Super 80 mm | 0.036 W/mK · R 2.20 at 80 mm | Where the system specification calls for the Super build |
At equal thickness the Plus slab declares the slightly better pair of figures, which is the opposite of what most buyers expect from a premium-sounding name. What separates the two ranges mechanically is not stated here, because neither Declaration of Performance declares a density — both declare CS(10)20 and TR10 — so the Super slab is described on its own page rather than characterised from this one.
Pack Coverage, Price per Square Metre and What to Order With It
A pack holds five slabs and covers 3.0 m², which works out at 0.6 m² of face area per slab (3.0 m² ÷ 5 = 0.6). The pack price on the product page reads £58.00, August 2026, so the insulation layer alone costs £58.00 ÷ 3.0 m² = £19.33/m² in materials.
- Pack count: 100 m² ÷ 3.0 m² per pack = 33.3, rounded to 34 packs, +10% for cuts at reveals and returns = 37 packs.
- Adhesive and base coat: ordered against the same area from the cementitious range, with consumption taken from the adhesive's own sheet rather than this page.
- Fixings: length and count set by the specification, then picked from the accessories range.
- Reveals: the 50 mm slab in the same range keeps the material consistent around openings.
Delivery at Renders World runs by courier for parcels up to 25 kg and by pallet up to 1,200 kg, next working day or two days on the economy option, and all deliveries are kerbside — a site-wide policy rather than a figure attached to this pack. Pack weight is not published here this session, because the store's weight field and the previously published figure disagree.
Choosing the Right Stone Wool Slab for the Job
- Choose this slab where a full elevation needs one non-combustible thickness at a declared R of 2.25 m²K/W and the pack maths above suits the programme.
- Step down to 50 mm where opening geometry caps the depth, using the Frontrock Plus 50 mm board at a declared 1.40 m²K/W.
- Consider the Super build where the system holder specifies it for the elevation; both Super thicknesses sit in the same stone wool slab range.
- Weigh stone wool against polystyrene before committing, since the trade-off is fire classification and vapour openness against thermal resistance per millimetre and cost — the mineral wool and polystyrene comparison works through it.
FAQ — Ordering, Compatibility and Compliance for the 80 mm Slab
How many packs cover a 100 m² elevation?
Thirty-four packs cover the bare area, from 100 m² ÷ 3.0 m² per pack = 33.3, and 37 packs is the realistic order once 10% is added for cuts at reveals, returns and parapets. Ordering the allowance up front avoids a part-pack top-up mid-programme.
Is the Super slab better at the same thickness?
On declared thermal figures, no: Plus declares λ 0.035 W/mK and R 2.25 m²K/W at 80 mm against the Super slab's 0.036 W/mK and 2.20 m²K/W. Both declare Euroclass A1, CS(10)20 and TR10, so the choice comes down to what the system specification names rather than to a published performance gap.
Which adhesive and render suit this slab?
A cementitious, stone-wool-compatible adhesive is the requirement, Roker U grey being the one in this range, and a vapour-open finish keeps the diffusion path continuous through to the surface. Foam adhesives are formulated for closed-cell boards and are not the route here.
Can a U-value be claimed from this slab alone?
A U-value belongs to the whole build-up — substrate, adhesive, insulation, base coat and finish — so the declared 2.25 m²K/W is one layer's contribution to that calculation and nothing more. The U-value calculation method for wall insulation thickness shows how the layers are added.
Does Euroclass A1 apply to the finished wall?
Classification belongs to what was tested, and here that is the slab under EN 13501-1. A rendered external wall carries its own fire evidence, assessed as an assembly under current Approved Document B guidance.
What is worth checking on delivery?
Match the Declaration of Performance travelling with the pallet against RW-CEE-DoP-0238/BCM/23/w1, since ROCKWOOL declares from several plants and a slab from another site declares separately. Confirming it at goods-in keeps the specification file straight.
Technical Documentation for the FRONTROCK PLUS Slab