What is external wall insulation (EWI) and how does it work?

External wall insulation (EWI) involves fixing rigid insulation boards to the outside of a building's walls, then covering them with reinforced render. The system typically consists of insulation boards fixed with adhesive and mechanical fixings, a glass fibre mesh embedded in a base coat, and a decorative top coat or render. EWI is particularly suited to solid-wall properties (pre-1920s) that have no cavity to fill.

Which EWI system should I choose: EPS, mineral wool or phenolic?

EPS (expanded polystyrene) is the cheapest and lightest option, with a thermal conductivity of around 0.032-0.034 W/mK. Mineral wool (e.g. Rockwool) has excellent fire resistance (A1/A2 rated) and acoustic performance, with conductivity of ~0.035 W/mK. Phenolic boards (e.g. Kingspan K5) have the best thermal performance (~0.021 W/mK) so you can use thinner boards, but they are the most expensive. For buildings above 18m, mineral wool is required under fire safety regulations.

What thickness of EWI do I need to meet UK Building Regulations?

Under Part L of the Building Regulations (2021 update), external walls should achieve a U-value of 0.30 W/m²K for new builds and 0.30 W/m²K for conversions. For existing buildings undergoing renovation, a U-value of 0.55 W/m²K or better is typically required. For a solid brick wall, you generally need 90-100mm of EPS, 80-90mm of mineral wool, or 50-60mm of phenolic insulation to achieve 0.30 W/m²K.

Do I need to insulate window and door reveals with EWI?

Yes, window and door reveals must be insulated to prevent thermal bridging at the junctions. Typically, thinner insulation boards (20-30mm) are used on the reveals as space is limited. The reveal insulation should overlap with the main wall insulation to maintain continuity. This calculator accounts for reveal areas by calculating the perimeter of each opening multiplied by the insulation depth.

How much does EWI cost in the UK?

EWI typically costs £80-£150 per m² fully installed, depending on the insulation type and finish. For a typical semi-detached house with 70-90 m² of external wall, the total cost is usually £6,000-£15,000. EPS systems are cheapest, mineral wool mid-range, and phenolic most expensive. Grants may be available through the ECO scheme, the Great British Insulation Scheme, or local authority programmes.

External Wall Insulation (EWI) Calculator — Boards, Adhesive & Render

Calculate insulation boards, adhesive, mesh, base coat, top coat, starter track and corner beads for external wall insulation systems in the UK.

Wall Dimensions

Total Wall Area

Gross external wall area to be insulated

Wall Base Length (optional)

Total length along base of walls, for starter track

EWI System

Insulation Type

EPS is cheapest; phenolic is thinnest

Insulation Thickness

100mm typical for Part L compliance

Openings & Accessories

Number of Windows

Each deducts 1.2 m² + adds reveal returns

Number of Doors

Each deducts 1.8 m² + adds reveal returns

External Corners

Number of external wall corners

Include wastage allowance

Adds 10% extra for cutting and fitting

Wastage Percentage

10% is standard for EWI installations

How We Calculate This

This calculator estimates all the materials needed for a complete external wall insulation (EWI) system, from insulation boards through to the finished render coat.

How it calculates

Net wall area = Gross wall area − (Windows × 1.2 m²) − (Doors × 1.8 m²)

Reveal area = (Window perimeter × insulation depth) + (Door perimeter × insulation depth). This accounts for the return insulation around each opening.

Insulation boards: Standard 1200×600mm boards (0.72 m² each). Total boards = (Net wall area + reveal area) ÷ 0.72

Material rates

Adhesive (fixing): ~5.5 kg/m² for dot-and-dab or full-bed fixing
Mesh: 1.1 m² per m² of wall (allowing 100mm overlaps at joints)
Base coat: ~3.5 kg/m² to embed the mesh
Top coat/render: ~3.0 kg/m² for the finished decorative layer

Accessories

Starter track runs along the base of all rendered walls to support the first row of insulation boards and provide a straight edge. Corner beads protect and reinforce external corners. Both are essential for a professional finish and long-term durability.

Frequently Asked Questions

Related Calculators

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Cavity Wall Insulation Calculator

Estimate cavity wall insulation quantities and potential savings.

Loft Insulation Calculator

Calculate loft insulation rolls needed for your loft area.

Last updated: February 2026

All calculations are estimates. Verify with your supplier.