Exterior Insulation Calculator — EIFS, ETICS & Boards
Calculate insulation board area and mechanical fixing count for external wall insulation (EIFS/ETICS). Includes guidance on U-value targets, board thicknesses and fire class requirements for different building types.
How the exterior insulation calculator works
- 1Facade area = wall area − openings (m²)
- 2Board count: area ÷ board face area, plus 5% waste for cuts
- 3Mechanical fixings: 6–10 fixings per m² depending on substrate and exposure
- 4Adhesive: area × adhesive coverage (typically 4–6 kg/m² for basecoat application)
- 5Reinforcing mesh: area + 10% for laps at corners and openings
- 6U-value: 1 ÷ (existing wall R + insulation R + surface resistances)
Worked example
House: 120 m² facade area. 120 mm EPS insulation (λ = 0.038 W/mK). 8 mechanical fixings per m².
Boards: 120 × 1.05 = 126 m². Fixings: 120 × 8 = 960. Insulation R-value: 0.12 ÷ 0.038 = 3.16 m²K/W. Assuming 300 mm cavity brick wall (R ≈ 0.5): total R ≈ 3.8 → U-value ≈ 0.26 W/m²K.
Frequently asked questions
What thickness of external insulation do I need?
To achieve a U-value of 0.30 W/m²K on a standard cavity wall, typically 80–100 mm EPS or mineral wool is sufficient. To reach 0.18 W/m²K (Passivhaus-adjacent), 150–200 mm is needed. Use the calculator to check your specific wall construction.
What is EIFS and how does it differ from ETICS?
EIFS (Exterior Insulation and Finish System) is the US/Canadian term; ETICS (External Thermal Composite Insulation System) is the European term. Both refer to the same system: insulation board bonded and mechanically fixed to the wall, covered with reinforced render.
Can I use EPS or mineral wool for external insulation?
EPS (expanded polystyrene) is the most common — lightweight, easy to cut, good R-value and low cost. Mineral wool is non-combustible and handles fire and moisture better than EPS. Where combustible insulation is restricted depends on your local fire rules — for example, in the UK post-Grenfell legislation effectively requires non-combustible insulation on residential buildings over 18 m, but that threshold is UK-specific. Check your local requirements.
How many mechanical fixings do I need per m² of insulation?
A typical figure is around 6 fixings/m² for standard residential and 8–10/m² in high wind-exposure zones, but the actual count comes from a wind-load calculation and the system's fixing pattern — follow the manufacturer's table. Corner boards and panels next to openings usually need extra fixings.
Do I need planning permission for external wall insulation?
In the UK, external insulation on houses is often permitted development, but not on listed buildings, in conservation areas or where the appearance changes significantly. Always check with your local planning authority before starting.