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External wall insulation EWI calculator
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External Wall Insulation (EWI) Calculator

Calculate all layers of an EWI system - insulation boards, mechanical fixings, adhesive, base coat, reinforcement mesh, primer, and topcoat render. Supports multiple walls with opening deductions.

External Wall Insulation (EWI) Calculator

All system layers calculated per m² - from board adhesive to topcoat render

The most widely used EWI insulation. Economical, good thermal performance, lightweight. λ ≈ 0.038 W/mK. Standard board 1200×600 mm.
Standard: 1200×600 mm (0.72 m² per board)
Standard for most EWI systems. Adhesive bonds the board; mechanical fixings (plastic disc anchors) provide wind-uplift resistance. Min. 6 fixings/m².
Dab & bead: ~4 kg/m² · Full coverage: ~6 kg/m²
Standard: 6/m² central, 8-10/m² at perimeter zones and corners
Typically 4-6 kg/m² applied in two passes with mesh embedded
Min. 100 mm overlap at all mesh joints
Flexible, water-repellent, breathable. Most widely specified for EWI. Applied at 1.5-3 mm grain size. Typically 2.5-4 kg/m².
10%
10% standard; 15% for complex elevations with many reveals, bay windows, or curved walls

Wall Sections

Job Costing (optional)

Per 25 kg bag
Per 25 kg bag
Per 25 kg bag

Results

Layer 1 - Insulation Boards & Fixings

Net Wall Area
-
m² (less openings)
Insulation Boards
-
inc. waste
Mechanical Fixings
-
total
Adhesive
-
bags

Layer 2 - Base Coat & Mesh

Base Coat
-
bags
Reinforcement Mesh
-
m² (inc. overlaps)
Primer
-
litres

Layer 3 - Topcoat Render

Topcoat
-
bags / tubs
Topcoat (kg)
-
total kg
Type
-
finish

Wall Breakdown

Wall Gross Openings Net Boards Fixings Base Coat (kg) Mesh (m²)

Insulation Types

  • EPS (Expanded Polystyrene) - Most widely used. λ ≈ 0.038 W/mK. Lightweight, easy to cut, takes adhesive well. Combustible - fire barrier boards required at openings.
  • Mineral Wool - Non-combustible (A1/A2 rated). λ ≈ 0.035 W/mK. Required in buildings over 11 m or where fire risk is a concern. Slightly heavier, needs full-coverage adhesive.
  • PIR / PUR Board - Higher performance. λ ≈ 0.022 W/mK. Thinner boards for same U-value. Combustible - restricted on high-rise.
  • Phenolic Board - Best performance. λ ≈ 0.018 W/mK. Most expensive. Used where thickness is highly constrained.
  • Wood Fibre - Breathable, vapour-open, sustainable. λ ≈ 0.038-0.042 W/mK. Suited to solid stone and traditional buildings where vapour control is critical.

System Layers

  1. Wall prep - Clean, sound substrate. Repair cracks. Apply primer if high suction.
  2. Adhesive - Dab and bead or full-coverage. Bonds insulation to wall.
  3. Insulation boards - Staggered joints, bonded and mechanically fixed.
  4. Mechanical fixings - Plastic disc anchors (e.g. Ejot, Hilti). Min. 6/m², more at reveals and corners.
  5. Base coat (first pass) - Polymer-modified render, 3-4 mm thick.
  6. Reinforcement mesh - Alkali-resistant fibreglass mesh embedded in base coat. 100 mm overlaps.
  7. Base coat (second pass) - Trowel flush, cover mesh completely.
  8. Primer - Applied when base coat is cured (typically 24-48 hrs).
  9. Topcoat render - Silicone, acrylic, or mineral. Final decorative layer.

Thickness & U-Values

Approximate U-values for EPS on 215 mm brick cavity wall (existing U ≈ 0.28 W/m²K):

  • 60 mm EPS → ≈ 0.35 W/m²K
  • 90 mm EPS → ≈ 0.26 W/m²K
  • 100 mm EPS → ≈ 0.23 W/m²K
  • 120 mm EPS → ≈ 0.20 W/m²K
  • 150 mm EPS → ≈ 0.17 W/m²K

Part L 2021 target for existing dwellings: ≤ 0.30 W/m²K where technically and economically feasible.

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