Polystyrene EPS Grafit 20 mm — Graphite Insulation Board for Soffits and Lintels, λ 0.031 W/mK

Genderka EPS 031 Fasada Extra Plus 20 mm is a graphite-enhanced expanded polystyrene board delivering a declared thermal conductivity of λD 0.031 W/mK and thermal resistance of R 0.60 m²K/W, purpose-built for soffit insulation, lintel returns, and secondary insulation layers within external wall insulation systems across the UK. Each board measures 1000 × 500 mm and is supplied in packs of 30 boards covering 15.0 m², providing generous material for junction and secondary-layer detailing on domestic and light-commercial projects. Explore the full selection of graphite EPS insulation boards to match every zone of your façade build-up to the right thickness.

Certified to EN 13163:2012+A1:2015 with the classification string T1-L2-W2-Sb2-P5-BS100-DS(N)2-DS(70,-)2-TR100, this board shares the same graphite bead technology and mechanical properties as every variant in the Genderka EPS 031 Fasada Extra Plus family. At 20 mm it bridges the gap between the slimmest reveal board and the thicker primary-wall insulation slabs — thick enough to contribute meaningful R-value at soffits and lintels, yet slim enough to fit within tight depth constraints. For context on how UK energy standards influence insulation thickness choices across the building envelope, see the Future Homes Standard 2026 and its insulation requirements.

Key Benefits and Performance Advantages

Double the Warmth of the Slimmest Board

• Twice the R-Value of 10 mm: At R 0.60 m²K/W, this 20 mm board provides double the thermal resistance of the thinnest variant in the range, making it the natural step up for soffits, lintels, and secondary insulation layers where 10 mm alone falls short of the thermal bridging target.
• Graphite-Enhanced λ 0.031 W/mK: Integrated graphite particles reflect infrared radiation within each cell, delivering approximately 20 % better insulation than standard white EPS of the same thickness — so a 20 mm graphite board outperforms a 25 mm white EPS board in real-world thermal terms.

Built for Junction Detailing

• Soffit and Lintel Coverage: The 20 mm profile fits comfortably above window heads, beneath concrete lintels, and across soffit returns where the available cavity is too narrow for thicker main-wall boards but where a stronger thermal contribution than 10 mm is needed.
• Secondary Equalising Layer: Ideal as a levelling layer bonded over an existing render carrier board or uneven substrate, creating a uniform surface for basecoat application while adding measurable insulation value to the assembly.

Handles and Cuts Cleanly on Site

• Bending Strength BS100 (≥ 100 kPa): Each board resists flexing during handling and cutting, making it practical to work with around complex junction geometry without cracking or snapping — even when pre-cutting batches of strips at ground level.
• Tensile Strength TR100 (≥ 100 kPa): Adhesive bonds securely to the graphite-EPS surface, holding the board firmly in place at exposed soffits and lintels subject to wind pressure throughout the UK heating season.

One Pack Covers an Entire Façade's Detailing

• 30 Boards per Pack (15.0 m²): A single pack provides enough material for soffit and lintel detailing across a typical semi-detached façade. At 15 kg/m³ density each board weighs approximately 0.15 kg, keeping scaffold-platform handling effortless for a single operative.

Drops Straight Into Standard EWI Build-Ups

• Compatible With All Standard EWI Adhesives and Finishes: Bond this board using cement-based or polyurethane foam adhesives from the EPS adhesives and basecoats range, then finish with basecoat, mesh, and render following the same build-up sequence as the main wall area. Where the specification requires mechanical retention, pair with the appropriate fixing plug from the insulation fixing accessories range.

Technical Specifications

Physical Properties

Property	Value
Manufacturer	Genderka Sp. z o.o.
Product Range	EPS 031 Fasada Extra Plus
Thickness	20 mm
Board Dimensions	1000 mm × 500 mm
Coverage per Board	0.50 m²
Boards per Pack	30
Pack Coverage	15.0 m²
Density	15 kg/m³
Colour	Steel grey (graphite)
Compressive Strength (CS)	≥ 70 kPa
Bending Strength (BS)	≥ 100 kPa (BS100)
Tensile Strength (TR)	≥ 100 kPa (TR100)
Dimensional Stability — Normal	± 0.2 % (DS(N)2)
Dimensional Stability — 70 °C	≤ 2 % (DS(70,-)2)
Reaction to Fire	Class E (EN 13501-1)
Standard	EN 13163:2012+A1:2015

Thermal Performance

Property	Value
Declared Thermal Conductivity (λD)	0.031 W/mK
Thermal Resistance (RD) at 20 mm	0.60 m²K/W
Classification String	EPS EN 13163 T1-L2-W2-Sb2-P5-BS100-DS(N)2-DS(70,-)2-TR100

Best Use Scenarios

Ideal Applications

• Soffit Insulation: Line the underside of concrete or timber soffits with 20 mm graphite EPS to close the thermal bridge between external wall insulation and the roof-edge zone, keeping heat inside the building and maintaining interior surface temperatures above the dew point at the ceiling-wall junction.
• Lintel and Ring-Beam Returns: Wrap exposed concrete lintels and structural ring beams with 20 mm boards before rendering to prevent these dense elements acting as thermal short-circuits through the insulated envelope.
• Secondary Equalising Layer: Apply over an uneven render carrier board or existing rough masonry to create a flat, thermally enhanced surface ready for the primary insulation layer or directly for basecoat and mesh.
• Composite Thickness Build-Up: Combine with a thicker primary board — for example, bond 20 mm onto a 100 mm Grafit board at parapets or column edges to achieve 120 mm total insulation depth where a single board of that thickness is unavailable or impractical to fix.

When an Alternative Fits Better

• Narrow Reveals With Minimal Depth: Where only 10 mm of clearance exists between the insulated wall plane and the window frame, the 10 mm Grafit board at R 0.30 m²K/W fits without encroaching on the glazing line.
• Primary Wall Insulation: For the main wall area, the 30 mm Grafit board at R 0.95 m²K/W serves as a minimum cost-effective primary layer, while thicknesses of 80 mm and above deliver full Part L compliance.
• High-Rise or Non-Combustible Requirements: Where Euroclass A1 fire rating is mandated, the mineral wool insulation range provides non-combustible slabs compliant with Building Safety Act façade requirements for buildings above 18 metres.

System Integration

The 20 mm graphite EPS board slots into the broader EWI build-up as a secondary or junction-detailing layer. Apply adhesive from the adhesives and basecoats collection to the full back face of the board using the full-bed buttering method — at 20 mm, full-face coverage maximises the adhesive bond area and prevents voids behind the slim profile. Press firmly onto the prepared soffit, lintel, or substrate surface, ensuring tight butt joints between adjacent boards.

Where the project specification requires mechanical retention at soffits or exposed junctions, the shortest fixing plug in the fixing accessories range provides the supplementary hold needed. Once bonded and (where specified) fixed, apply the reinforcing basecoat with embedded Atlas 150 g/m² fibreglass mesh, wrapping the mesh continuously from the main wall plane around the junction return. The complete layer-by-layer build-up sequence — from substrate preparation through to decorative render finish — is detailed in the EWI system build-up layers guide.

Comparison Table — Genderka EPS Grafit Slim Variants

Property	10 mm Grafit	20 mm Grafit (This Board)	30 mm Grafit
Thickness	10 mm	20 mm	30 mm
λD	0.031 W/mK	0.031 W/mK	0.031 W/mK
RD	0.30 m²K/W	0.60 m²K/W	0.95 m²K/W
Pack Coverage	30.0 m²	15.0 m²	10.0 m²
Boards per Pack	60	30	20
Typical Use	Reveals, narrow returns	Soffits, lintels, secondary layer	Cost-effective entry thickness

UK Climate Performance Context

The 20 mm board's primary role is thermal bridge closure at soffits and lintels — areas that, when left uninsulated, can account for 10–15 % of total fabric heat loss in a typical UK dwelling even when the main wall is fully insulated. Adding 20 mm of graphite EPS with λ 0.031 W/mK to a bare concrete lintel (original localised U-value ≈ 3.5 W/m²K) reduces that localised heat loss by approximately 40–45 %. In SAP calculations, insulated lintels and soffits lower the linear thermal transmittance (psi-value) at wall-to-roof and wall-to-opening junctions, contributing directly to a better overall dwelling energy rating. For a standard three-bedroom semi-detached home, fully detailed soffit and lintel insulation can improve the energy performance certificate by 1–3 SAP points, supporting the step from EPC band D into band C.

Trade Insight: Handling and Storage

At 20 mm the boards are thin enough to flex noticeably under their own weight, so store packs flat on a level surface rather than leaning them against a wall. The steel-grey graphite colour absorbs solar radiation quickly — for best results, keep packs under opaque sheeting or inside a covered storage area until needed, even during overcast weather, because cumulative UV exposure softens the outer bead layer over time. When cutting boards to soffit or lintel dimensions, a sharp Stanley knife with a long blade gives the cleanest through-cut in a single pass; for best edge quality, avoid scoring-and-snapping, which can leave a ragged surface that reduces adhesive contact area. One practical approach that speeds up junction detailing: pre-cut a full batch of soffit and lintel strips to measured dimensions at ground level, label them by opening, then transport the pre-cut pieces to scaffold height in a stackable crate — this minimises cutting at height and keeps working platforms clear.

Certifications and Compliance

• EN 13163:2012+A1:2015: Harmonised European standard for factory-made expanded polystyrene products used in buildings, covering declared thermal conductivity, dimensional tolerances, and mechanical strength properties.
• Reaction to Fire — Euroclass E: Suitable for use within EWI systems on buildings up to 18 m in height when combined with approved fire-barrier detailing at each floor level, in accordance with Approved Document B and Building Safety Act guidance.
• CE Marking and DoP: Each production batch carries a Declaration of Performance referencing the specific Genderka factory of origin (Bydgoszcz, Ostrów Mazowiecka, Oświęcim, Brzeziny, or Wschowa), supporting traceability for Building Control inspections.
• Junction Detailing Compliance: Insulating soffits and lintels with graphite EPS is recommended under Approved Document L and the conventions set out in BR 443 to reduce the linear thermal transmittance at critical junctions, supporting improved overall dwelling fabric energy efficiency.

Technical Disclaimer

All thermal values stated are declared (λD, RD) per EN 13163 and manufacturer technical data sheet (Genderka karta techniczna nr W31FEP, July 2024). Actual installed performance depends on substrate condition, adhesive application method, and the complete EWI system build-up. Confirm current specifications with the manufacturer or your project energy assessor before specifying. Data is subject to manufacturer update; verify at point of order.

Summary

The Genderka EPS Grafit 20 mm board delivers λ 0.031 W/mK and R 0.60 m²K/W in a slim profile purpose-built for soffit insulation, lintel returns, and secondary layers within EWI systems. Each pack of 30 boards covers 15.0 m², providing ample material for junction detailing across a typical domestic project. Need the slimmest possible reveal insulation? The 10 mm Grafit board fits within the tightest reveals at R 0.30 m²K/W. For a cost-effective primary insulation layer on lintels and ceilings, the 30 mm Grafit board steps up to R 0.95 m²K/W — the entry point for meaningful standalone thermal resistance.

Frequently Asked Questions

How many 20 mm boards are in a pack, and how far does one pack go on a typical project?

Each pack contains 30 boards covering 15.0 m². For soffit and lintel detailing on a three-bedroom semi-detached property, a single pack generally provides enough material for all junction zones with margin for cutting waste. Adding 5–10 % to the measured area accounts for trimming around complex profiles and services.

Is graphite EPS recyclable, and what environmental benefit does junction insulation provide?

Graphite EPS is fully recyclable through specialist polystyrene recovery programmes — soffit and lintel off-cuts can be collected on site and returned for granulation and re-expansion into new products. The environmental benefit of insulating junctions is that even small uninsulated areas create disproportionate heat loss; closing these bridges with 20 mm of graphite EPS improves overall system efficiency, lowers heating demand, and reduces the building's lifetime carbon emissions.

Does a 20 mm board need mechanical fixings at soffits, or is adhesive alone sufficient?

For most horizontal soffit applications, a full-bed adhesive bond supplemented by temporary propping until the adhesive cures (typically 24 hours) is sufficient. The board's light weight — approximately 0.15 kg per board — means gravity load on a horizontal adhesive joint is minimal. Where the project specification or Building Control requires mechanical retention, a short fixing plug can be installed after the adhesive has cured, provided the substrate behind the board offers adequate embedment depth.

Can I combine the 20 mm board with thicker boards to achieve a custom total insulation depth?

Combining thicknesses is a common and accepted practice in EWI detailing. Bond the 20 mm board to the face of the thicker primary board using a thin bed of compatible adhesive, then apply the reinforcing basecoat and mesh across the entire composite surface as a single continuous layer. This technique is particularly useful at parapets and column edges where the total required depth falls between standard board thicknesses available in the range.