Question 1

What is external wall insulation (EWI)?

Accepted Answer

EWI involves fixing insulation boards to the outside of walls, covered with a protective render or cladding system. It wraps the building in a thermal blanket, eliminating thermal bridges and protecting the structure from weather damage while improving appearance.

Question 2

How thick does EWI need to be?

Accepted Answer

To achieve the Part L target of 0.28 W/m2K on solid walls, typically 90-100mm of EPS or 70-80mm of phenolic is needed. The render adds approximately 10mm. Total build-up is usually 100-120mm projecting from the original wall face.

Question 3

Do I need planning permission for EWI?

Accepted Answer

Often yes. EWI changes the external appearance of the building and may require planning permission, especially in conservation areas, on listed buildings, or when projecting over boundaries. Always check with your local planning authority before starting work.

Question 4

How much does EWI cost?

Accepted Answer

Professional EWI costs 100-150 pounds per m2 for EPS render systems, or 120-180 pounds for mineral wool (fire-rated). A typical semi-detached house (80m2 walls) costs 8000-15000 pounds. Grants through ECO4 may cover significant costs for eligible households.

Question 5

Is EWI better than internal wall insulation?

Accepted Answer

EWI generally performs better as it eliminates thermal bridging, protects the building structure, and preserves internal space. However, it costs more, changes external appearance, and may need planning permission. IWI is better for listed buildings or where EWI is not possible.

Question 6

What finish options are available?

Accepted Answer

Common finishes include silicone render (most durable, self-cleaning), acrylic render (economical), mineral render (breathable), and brick slip systems (matches existing brickwork). Colours range from smooth white to textured heritage finishes.

Question 7

How long does EWI last?

Accepted Answer

Quality EWI systems with silicone render last 30-40 years with minimal maintenance. Acrylic renders may need recoating after 15-20 years. The insulation itself is lifetime-of-building. Choose reputable systems with at least 25-year warranties.

Question 8

What happens at windows and doors?

Accepted Answer

Window and door reveals are extended with insulation to maintain thermal continuity. Reveals typically get 20-40mm insulation. Windows may need repositioning or new external sills. This adds 10-20% to project cost but is essential for performance.

Question 9

Can EWI be installed in winter?

Accepted Answer

Most EWI systems require temperatures above 5 degrees C for render application. Winter installation is possible in milder weather or with heated enclosures but may take longer. Summer installation is generally faster and more economical.

Question 10

Will EWI affect my building insurance?

Accepted Answer

Inform your insurer about EWI installation. Most insurers welcome it as it protects the building. Ensure your installer provides certification and warranties. Some insurers may reduce premiums due to improved fire resistance (mineral wool systems).

Wall Area	EPS + Render	Mineral Wool	Property Type
50m2	GBP 5000-7500	GBP 6000-9000	Small terrace
80m2	GBP 8000-12000	GBP 9600-14400	Semi-detached
120m2	GBP 12000-18000	GBP 14400-21600	Detached
160m2	GBP 16000-24000	GBP 19200-28800	Large property

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External Wall Insulation Calculator (EWI)

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Tips for External Wall Insulation

EWI Cost Estimates

Common Mistakes to Avoid

Not checking planning requirements

Poor surface preparation

Inadequate fixings

Ignoring detailing

DIY installation

Understanding External Wall Insulation

EWI System Components

Example: 1930s Semi-Detached (80m2 wall area)

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