Insulated Render Board Calculator — EPS & Phenolic Systems

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.

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.

The required thickness depends on your target U-value and the insulation material. To upgrade a solid brick wall from 1.5 W/m²K to the Part L target of 0.30 W/m²K, the added resistance must be (1/0.30) − (1/1.5) = 2.67 m²K/W. EPS (lambda 0.038) therefore needs about 101mm — round up to a 110mm board. Phenolic (lambda 0.021) achieves the same improvement with about 56mm — round up to 60mm, nearly half the thickness. Our calculator sizes the board automatically from your existing and target U-values. (An 80mm EPS or 50mm phenolic board would only reach roughly 0.36 / 0.33 W/m²K, missing the 0.30 target.)

Yes. External wall insulation (EWI) is controlled by Approved Document L (Conservation of Fuel and Power) and must meet Part B (Fire Safety) requirements. Regulation 7(2) statutorily bans combustible materials in the external walls of relevant buildings with a storey at or above 18m — every material in the external wall must achieve Euroclass A1 or A2-s1,d0 (classified to BS EN 13501-1). Since the June 2022 ADB amendment (in force 1 December 2022), Approved Document B Volume 1 also recommends that insulation and filler in the external walls of any residential building with a storey above 11m achieve at least A2-s1,d0. In practice this limits combustible EPS (Euroclass E) systems to residential buildings under 11m; between 11m and 18m use mineral wool or an A1/A2-rated system.

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.