1 · Substrate primer

Existing facade surveyed — brick, block, concrete, or render. Cleaned, repaired, and primed with STO-specified primer where the substrate requires. Failed pointing repointed; failed render hacked back to substrate. Spalled concrete made good. Lintels, sills, and DPCs all checked before any board fixing.

2 · Adhesive bed

Cement-based adhesive applied to the back of each board on a perimeter-and-dab pattern. Provides initial fix and pulls the heavy mineral wool board flat to the substrate. Full coverage on uneven substrates where mechanical fixings alone wouldn't hold the board weight (mineral wool at 130 kg/m³ is significantly heavier than wood fibre at 110 kg/m³ — yes, the figures favour wood fibre, but rock wool's density distribution loads the fixings harder).

3 · STO Therm Mineral K board

The rock-wool board itself. Cut to size with a fine-tooth saw, butted close at joints, laid in a staggered brickwork pattern. Full boards at openings; cut pieces at reveals. The boards are heavier than wood fibre — facade-grade lifting and handling matters, particularly on mid-rise where scaffold access dominates the labour spec.

4 · Wind-load anchors

Mechanical anchors are mandatory on mineral wool EWI — never adhesive-only on a mid-rise facade. Wind-load calculation per project determines anchor density (typically 8–14 per m² depending on building height and exposure). Stainless steel pin through polymer plug, drilled through the board into the masonry beyond. Anchor heads recessed and base-coat covered.

5 · Base coat with mesh

STO-specified cement-based base coat embeds a glass-fibre reinforcing mesh in the wet coat. Mesh continues around all corners, over every anchor head, and is mitred and overlapped at openings. Provides the crack-resistant carrier layer for the finish render. Base coat cures for 5–7 days before the topcoat.

6 · STO Silco K mineral topcoat

STO Silco K silicate render or similar mineral topcoat. Vapour-open, available in standard render colours, 25-year manufacturer guarantee from STO when installed by a registered applicator. CleverBloom is on STO's registered applicator list. Textured or smooth finish available; smooth on commercial work, textured on residential mid-rise.

Rock-fibre wool

Spun from molten basalt and recycled steel-mill slag at temperatures above 1,500°C. The fibres are matted and bonded into rigid boards at 100–130 kg/m³ density — denser than batt-grade rock wool, dimensionally stable for facade-grade mechanical fixing. STO sources rock wool from Rockwool and Knauf depending on project specification.

70% recycled mineral content

A significant proportion of the rock fibre is recycled steel-mill slag — a by-product of iron and steel production that would otherwise be a waste stream. Each m³ of STO Therm Mineral K incorporates roughly 70 kg of recycled mineral content. EPD (Environmental Product Declaration) published; specifiable into BREEAM and LEED assessments.

Bonded with thermosetting resin

A small percentage of phenolic or melamine-based thermosetting binder holds the fibres rigid for handling and installation. The binder content is low enough (typically 3–5% by weight) that the bulk material remains A1 non-combustible; the binder pyrolises rather than burns above 200°C.

Fire: A1 non-combustible

Rock fibres are inorganic mineral, melt at temperatures above 1,000°C without sustaining combustion. The classification applies to the board as a unit (binder content low enough not to affect A1 status), and to the complete STO Therm Mineral K render system as installed. The defining spec advantage on every over-11 m external wall scope.

BBA Agrément Certificate

British Board of Agrément certification — UK third-party technical approval covering the complete STO Therm Mineral K render system. Includes installation guidance, performance data, durability testing, and fire test references. The certificate that UK building control and fire engineers reference during plan approval and EWS1 review.

ETA European Technical Assessment

European-level technical approval covering thermal performance, mechanical performance, durability, and assembly fire behaviour. ETA + BBA together give the UK specifier both the European reference standard and the UK national assessment — the dual-certification basis for major contractor and freeholder procurement.

25 year STO render guarantee

STO Silco K render system carries a 25-year manufacturer guarantee on the finish layer when installed by an STO-registered applicator. The guarantee covers render adhesion and weathering on the rock-wool substrate. CleverBloom is on STO's registered applicator list.

50+ year fabric design life

STO Therm Mineral K systems from the 1980s European retrofit programmes remain in performant service. With a sound substrate, weather-tight detailing, and a maintained render finish, the boards stay in service for 50+ years. The mineral wool fibre doesn't degrade in normal facade exposure.

Render inspection every 10 years

Standard external-render inspection cadence applies. Weather-exposed elevations checked every 10 years for cracking, render adhesion, and any signs of moisture ingress at sills or verges. Localised render repair is straightforward and doesn't disturb the underlying mineral wool boards.

Sill, drip, and verge details

Sills, drips, and verge details are the failure points of mid-rise EWI. Water shed clear of the render face matters more than the board specification — a single failed sill detail will track moisture into the assembly and create a localised performance failure. Every STO Therm Mineral K scope from CleverBloom uses the manufacturer's matched detail kit (sill caps, drip beads, verge profiles).

Cavity firebreaks (if applicable)

For buildings with cavities behind the EWI line (over-cladding scopes, ventilated rain-screen replacements), horizontal and vertical cavity firebreaks are mandatory per Approved Document B. STO Therm Mineral K is a direct-applied render system (no cavity), so this doesn't apply to typical specifications — but on hybrid scopes with sections of ventilated rain-screen, the firebreak schedule is part of the project scope.

Triple-glazing in proportion

A 150 mm STO Mineral wall sits at ~0.22 W/m²K U-value. Original single-glazed windows on the same elevation are at 4–5 W/m²K. Once the wall is fixed, the windows become the dominant heat-loss path. Triple-glazed replacement units in proportion to the new wall performance — installed flush with the new external face — is the standard mid-rise retrofit pairing.