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External Wall Insulation (EWI) Calculator — Boards, Adhesive & Render
Calculate insulation boards, adhesive, mechanical fixings, mesh, base coat, top coat, starter track and corner beads for external wall insulation systems in the UK.
Wall Dimensions
Gross external wall area to be insulated
Total length along base of walls, for starter track
EWI System
Openings & Accessories
Each deducts 1.2 m² + adds reveal returns
Each deducts 1.8 m² + adds reveal returns
Number of external wall corners
Adds 10% extra for cutting and fitting
10% is standard for EWI installations
EWI-220 TDS ~5 kg/m² fixing; full-bed on uneven walls higher (default 5.5)
EWI-220 reinforced layer ~4 kg/m² (≈6mm bed) to embed mesh
Silicone render ~2.4 kg/m² at 1.5mm grain (EWI-075 TDS); coarser uses more
Standard: 25 kg bags
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How We Calculate This
This calculator estimates the main materials for an external wall insulation (EWI) system, from insulation boards through to the finished render coat. It includes an indicative mechanical-fixing count, but the exact fixing density (and primer, beads and trims) should always be confirmed against your chosen system supplier’s specification and a wind-load assessment.
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. This single figure already includes the reveal boards — they are not an additional quantity to order on top.
Material rates
Default rates follow EWI Pro product data sheets (EWI-220 adhesive/basecoat, EWI-075 silicone render); they vary by system and substrate, so all are adjustable under Advanced options.
- Adhesive (fixing): ~5 kg/m² (EWI-220 TDS); full-bed on uneven solid walls runs higher
- Mechanical fixings: ~6 plugs/m² for EPS/phenolic, ~8/m² for mineral wool (field zone) — denser at edges and corners; confirm by a wind-load calc to BS EN 1991-1-4
- Mesh: 1.1 m² per m² of wall (allowing 100mm overlaps at joints)
- Base coat: ~4 kg/m² (EWI-220 reinforced layer, ≈6mm bed) to embed the mesh
- Top coat/render: ~2.4 kg/m² for 1.5mm-grain silicone render (EWI-075 TDS); coarser grain uses more
Accessories
Mechanical fixings(plastic-sleeved plugs) secure every board to the substrate in addition to the adhesive — the count here is a field-zone estimate only. Starter track runs along the base of all rendered walls to support the first row of insulation boards and provide a straight edge; for the best figure enter your actual base run, as the fallback is a rough square-building approximation. Corner beads protect and reinforce external corners. All are essential for a professional finish and long-term durability.
Frequently Asked Questions
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.
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. See our U-Value Calculator to compare the thermal performance of different insulation types.
Under Part L of the Building Regulations (2021 update), new-build external walls have a notional U-value of 0.18 W/m²K (Part L 2021 limiting is 0.26 W/m²K for new dwelling walls). For existing buildings undergoing renovation (including EWI retrofit), a U-value of 0.30 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. Try our Insulation Thickness Calculator for specific recommendations.
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.
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.
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Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.