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

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.