What is the full EWI system build-up?

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.

What U-value should I target for EWI?

For retrofit projects, Approved Document L (2021) recommends a target U-value of 0.30 W/m²K for existing walls where it is technically and economically feasible. New-build walls must achieve 0.26 W/m²K or better. The Energy Company Obligation (ECO) scheme and Green Deal typically require 0.30 W/m²K. Lower targets (0.20-0.25) may be required for Passivhaus or EnerPHit standards.

Why do edges need more fixings?

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.

Can I use EPS on buildings over 18m tall?

Since December 2018, Regulation 7(2) of the Building Regulations (England) bans the use of combustible materials (including EPS, with a Euroclass E fire rating) in the external wall build-up of buildings over 18m. Only materials rated A1 or A2 (non-combustible) are permitted — such as mineral wool. This rule applies to all residential buildings, hospitals, and care homes over 18m.

How long does an EWI system last?

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.

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External Wall Insulation (EWI) System Calculator

Calculate the full EWI system: insulation boards by U-value target, adhesive, mechanical fixings, basecoat, mesh, primer, and topcoat.

Wall Area

Net area after subtracting openings

Insulation Material

Grey EPS best value; mineral wool for 18m+ buildings

Existing Wall U-Value

Solid brick ~1.5, cavity unfilled ~1.0

Target U-Value

Part L 2021: 0.30 for existing walls

Topcoat Finish

Silicone is most durable and self-cleaning

Insulation Price Per m² (optional)

Per m² of insulation board

How We Calculate This

This calculator sizes every component of an External Wall Insulation (EWI) system, from insulation thickness through to the decorative topcoat. It calculates the required insulation thickness from your existing and target U-values, then quantities all render system materials using manufacturer consumption rates.

The formula

Insulation thickness = (1/U_target - 1/U_existing) × Lambda × 1000

For example, upgrading a solid brick wall (U=1.5) to 0.30 W/m²K with grey EPS (lambda 0.032): thickness = (3.33 - 0.67) × 0.032 × 1000 = 85mm, rounded up to 90mm. With phenolic (lambda 0.021), only 60mm is needed for the same performance.

Material quantities (adhesive, basecoat, mesh, primer, topcoat) are calculated at industry-standard consumption rates per m² with your chosen wastage factor. Mechanical fixings use a higher density at building edges per BS EN 1991-1-4 wind load zones.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.