External Wall Insulation (EWI) System Calculator

A complete External Wall Insulation (EWI) system has seven layers: (1) adhesive mortar applied to the wall, (2) insulation boards pressed into adhesive, (3) mechanical fixings through the boards into the wall, (4) reinforcing basecoat render, (5) alkali-resistant fibreglass mesh embedded in the basecoat, (6) key coat/primer, and (7) decorative topcoat render. Each layer serves a specific structural or weathering function.

For retrofit projects, Approved Document L Volume 1 (2021 edition, incorporating 2023 amendments) Table 4.3 sets a limiting U-value of 0.30 W/m²K for external (or internal) wall insulation in existing dwellings, where it is technically and economically feasible. New-build walls must achieve 0.26 W/m²K or better (AD L Vol 1, Table 4.1). Grant-funded retrofits delivered under the Energy Company Obligation (ECO4) and PAS 2035 typically design to 0.30 W/m²K. Lower targets (0.20-0.25) may be required for Passivhaus or EnerPHit standards.

Wind pressure on a building is highest at edges and corners (typically 1.5-2x the field pressure per BS EN 1991-1-4). The edge zone is normally defined as within 1m of all building corners, parapets, and roof edges. Standard fixing density is 6 per m² in the field area, increasing to 8-10 per m² in edge zones. Exposed or high-rise buildings may need even more — a wind load calculation should confirm.

There are two thresholds to check. STATUTORY: since 21 December 2018, Regulation 7(2) of the Building Regulations (England) (SI 2018/1230) bans combustible materials — including EPS (Euroclass E) — in the external wall of relevant buildings with a storey at least 18m above ground, requiring A1 or A2-s1,d0 (non-combustible) products such as mineral wool. The buildings in scope are those containing dwellings, an institution, student accommodation or rooms for residential purposes, extended on 1 December 2022 to hotels, hostels and boarding houses. GUIDANCE: from 1 December 2022, Approved Document B also recommends A1 or A2-s1,d0 external-wall materials (including insulation) for all residential buildings above 11m, even though the statutory ban itself was not lowered to 11m. The safe default is therefore mineral wool (A1) for any residential building over 11m; EPS is generally only suitable below 11m. Always confirm with your local building control body.

A properly installed EWI system should last 25-30+ years before the topcoat needs refreshing. The insulation and basecoat layers are protected and typically last the life of the building. Silicone topcoats are self-cleaning and maintain their colour well. Acrylic topcoats may need recoating after 15-20 years. Regular maintenance (clearing vegetation, repairing cracks) extends the system life significantly. For the topcoat render, see our Rendering Calculator.