External Wall Insulation (EWI) Calculator
All system layers calculated per m² - from board adhesive to topcoat render
Standard: 1200x600 mm (0.72 m² per board)
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
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
UK Market PricesFills in the material cost fields with current UK market prices. You can review and edit each price before applying.
Results
Layer 1 - Insulation Boards & Fixings
Net Wall Area
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m² (less openings)
Insulation Boards
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inc. waste
Mechanical Fixings
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total
Adhesive
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bags
Layer 2 - Base Coat & Mesh
Base Coat
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bags
Reinforcement Mesh
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m² (inc. overlaps)
Primer
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litres
Layer 3 - Topcoat Render
Topcoat
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bags / tubs
Topcoat (kg)
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total kg
Type
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finish
Wall Breakdown
| Wall | Gross | Openings | Net | Boards | Fixings | Base Coat (kg) | Mesh (m²) |
|---|
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Frequently Asked Questions
How much external wall insulation do I need per square metre?
A standard 1200 x 600 mm EWI board covers 0.72 m². Add 10% waste. Mechanical fixings: 6-8 per board. Reinforcing mesh: wall area + 10% for overlaps. Basecoat render: approximately 4-6 kg/m². Topcoat: approximately 2-4 kg/m².
What thickness of EWI board do I need?
Part L requires a maximum 0.30 W/m²K for existing external walls in England. A standard 225 mm solid brick wall (U-value ~2.1) needs approximately 100 mm EPS to achieve 0.30 W/m²K. PAS 2035 retrofits targeting 0.15-0.20 W/m²K need 130-200 mm EPS or thinner PIR boards.
What are the main types of EWI board?
EPS: most cost-effective, lambda ~0.032-0.038 W/mK. Mineral wool: non-combustible, better acoustic performance, lambda ~0.033-0.040 W/mK. Phenolic/PIR: thinner for same U-value, lambda ~0.020-0.023 W/mK. Woodfibre: breathable, vapour-open, ideal for solid-wall buildings where moisture management is critical.
Do I need planning permission for external wall insulation?
Usually permitted development if the insulation projects under 200 mm and matches existing appearance. Planning permission required for: listed buildings, Conservation Areas, or if projecting beyond the boundary. Building Regulations approval always required under Parts L, B, and C.
What Building Regulations apply to external wall insulation?
Part L: maximum 0.30 W/m²K for existing walls in England. Part B: buildings over 18 m require non-combustible materials (Class A1 or A2) throughout the EWI system. Part C: render finish must be weather-resistant. PAS 2035 funded schemes require a Retrofit Coordinator to design and inspect the work.
What waste allowance should I use for EWI boards?
Simple rectangular building: 10%. Complex shapes, many windows, gable ends: 15%. Plan the cutting pattern to minimise waste. All boards must be from the same certified system - mixing manufacturers invalidates the fire classification and warranty. Check all boards bear the system manufacturer's label.