What is an insulated render board system?

An insulated render board (IRB) system consists of rigid insulation boards (EPS, phenolic, or mineral wool) adhesive-bonded and mechanically fixed to the external wall, then finished with a reinforced render system. It improves thermal performance without affecting internal floor space. Common UK systems include Weber therm, Sto Therm, and K-Rend Insulated Render systems.

How many mechanical fixings per m² are needed?

The standard fixing density is 6 fixings per m² for most UK applications up to 3 storeys. At building corners and edges (within 1m of corners), this increases to 8-10 per m². For high-rise or exposed locations, wind load calculations per BS EN 1991-1-4 may require higher densities. Our calculator uses a tuneable default of 6/m² which you can adjust in Advanced Options.

What thickness of insulation do I need?

The required thickness depends on your target U-value and the insulation material. EPS (lambda 0.038) needs approximately 80mm to upgrade a solid brick wall from 1.5 W/m²K to 0.30 W/m²K. Phenolic (lambda 0.021) achieves the same improvement with only 50mm — nearly half the thickness. Our calculator automatically sizes the insulation based on your existing and target U-values.

Does an insulated render board need Building Regulations approval?

Yes. External wall insulation (EWI) is controlled by Approved Document L (Conservation of Fuel and Power) and must meet Part B (Fire Safety) requirements. Since the Grenfell Tower fire, Regulation 7 restricts the use of combustible materials on buildings over 18m. EPS systems are generally limited to buildings under 18m; mineral wool or A1/A2 rated systems are required for taller buildings.

What is the basecoat and mesh layer for?

The reinforced basecoat layer (4-5mm thick with embedded alkali-resistant fibreglass mesh) serves as the structural render layer. It distributes impact loads, resists cracking, and provides a keyed surface for the topcoat. The mesh must overlap by at least 100mm at all joints. This layer is critical — without it, the render topcoat would crack over the insulation board joints.

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Insulated Render Board Calculator — EPS & Phenolic Systems

Calculate insulation board quantities, mechanical fixings, basecoat, mesh, and topcoat for insulated render board systems.

Wall Area

Net area after subtracting openings

Insulation Type

Phenolic is thinnest; EPS is most economical

Existing U-Value

Solid brick ~1.5, cavity unfilled ~1.0

Target U-Value

Part L 2021 recommends 0.30 for existing walls

Topcoat Type

Silicone is self-cleaning; mineral is breathable

Price Per m² of Board (optional)

Per m² of insulation board

How We Calculate This

This calculator sizes all components of an insulated render board system, from the insulation boards through to the finished topcoat. It calculates the required insulation thickness from your existing and target U-values using the thermal resistance formula.

The formula

Insulation thickness = (R_target - R_existing) × Lambda × 1000

Where R = 1/U-value (thermal resistance in m²K/W) and Lambda is the thermal conductivity of the insulation material. For example, to upgrade from U=1.5 to U=0.30 with EPS (lambda 0.038): R_needed = (1/0.30) - (1/1.5) = 2.67 m²K/W, so thickness = 2.67 × 0.038 × 1000 = 101mm, rounded up to 110mm.

Material quantities (adhesive, basecoat, mesh, primer, topcoat) are then calculated by multiplying the wall area by manufacturer-published consumption rates plus your chosen wastage factor.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.