External wall insulation (EWI) involves fixing insulation boards to the exterior walls of a building, then covering them with decorative render or cladding. This can significantly improve thermal efficiency in buildings with solid masonry walls.
Benefits of External Wall Insulation
- Creates a warmer home by preventing heat loss through walls
- Can reduce heating bills by up to 35%
- Prevents condensation and mould growth on walls
- Weatherproofs exterior walls and enhances durability
- Improves soundproofing and noise insulation
- Minimal loss of internal floor area compared to internal insulation
- Extends the lifespan of walls by protecting from weather damage
- Enhances external appearance of property through new render or cladding
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How EWI Systems Work
There are a few key components in a typical external wall insulation system:
- Insulation boards – Rigid boards made of mineral wool, EPS, XPS or phenolic foam. They come in various thicknesses.
- Mechanical fixings – Special plastic or metal fixings that secure boards to the wall surface.
- Basecoat and reinforcing mesh – Fibreglass mesh embedded in a basecoat render to form the base layer.
- Primer – Helps topcoat render adhere properly.
- Topcoat render – The final decorative and weatherproof finish, available in various textures and colours.
- Beads – Edge profiles to provide neat trim finishes around windows, doors and corners.
Assessing Wall Suitability
EWI can be applied to most structural wall types, but a survey should check for:
- Damp or salt issues that need remediating first.
- Wall condition – surfaces should be sound to fix boards securely.
- Areas of potential risk like parapets or overhangs.
- Guttering and drainage systems – may need adjusting.
- Existing fixtures – assess treatment around windows, pipes, etc.
Installation Process
Installation typically follows these steps:
- Prepare wall surface – repairs, cleaning etc.
- Apply base rails and corner beads.
- Fix insulation boards with adhesive and mechanical fixings.
- Apply basecoat render with reinforcing mesh.
- Apply primer.
- Apply final decorative render or cladding finish.
- Make good details around windows, doors, pipes etc.
The process takes around 2-4 weeks depending on building size and labour crew size. Access scaffolding is erected to reach higher areas. Residents may need to vacate if external access is blocked.
Costs and Considerations
- EWI costs around £100-£150 per m2 installed, although larger projects can benefit from economies of scale.
- Planning permission may be required – check with your local authority.
- Building regulations approval is needed, including for fire safety.
- Scaffolding adds significantly to costs but is essential for safe installation.
- Disruption is moderate during installation due to external works.
- A reputable installer should provide guarantees for materials and workmanship.
- Can qualify for ECO funding if solid walls lack insulation.
Overall, EWI requires a serious investment, but delivers excellent thermal performance that can pay back in energy savings over time. It is one of the most effective ways to insulate challenging solid wall properties.