External wall insulation (EWI) is one of the most effective ways to improve a building’s thermal performance. By wrapping your home in a layer of insulation and finishing it with render or cladding, you can dramatically reduce heat loss, cut energy bills and improve comfort. If you’re considering EWI, you might wonder what the process involves and how to prepare. Having recently helped a relative navigate their insulation project in Berkshire, I know the planning and preparation can seem daunting. This guide breaks the process down into manageable steps to help you make informed decisions.
Why install external wall insulation?
According to retrofit experts, over a third of a building’s heat is lost through the walls; EWI is the most effective method of retaining this heat. It keeps the building dry, improves indoor air quality and can transform the appearance of a house. For older homes built before 1930, EWI can be especially beneficial. It reduces cold bridging, increases airtightness and can even improve sound insulation. However, EWI changes the look of a building and may not be suitable for listed properties or principal elevations in conservation areas. In such cases, internal wall insulation might be a better choice.
Step 1: Assess your property and objectives
Start by considering your property type and goals. Solid-wall houses from the Victorian or Edwardian periods respond well to EWI, while mid-20th-century properties with uninsulated cavities may require combined approaches. Think about the level of insulation you need, your budget and how important aesthetic changes are. It’s wise to obtain a professional survey to evaluate existing wall conditions, damp issues and structural constraints.
Step 2: Check planning and building regulations
In many cases, EWI falls under permitted development, but there are exceptions. If your home is in a conservation area, has special architectural features or is listed, you may need planning permission. Building regulations require that EWI systems meet fire safety and energy-efficiency standards. Contact your local authority and discuss your plans with them; they can advise you on the approvals required.
Step 3: Choose your insulation materials
EWI systems use different types of insulation boards. Closed-cell materials like expanded polystyrene (EPS), polyisocyanurate (PIR) and phenolic foam offer high thermal performance but are less vapour-permeable. Open-cell options such as woodfibre or cork are breathable and eco-friendly but require thicker boards to achieve the same U-value.
Consider factors like thermal efficiency, breathability, environmental impact and cost. A professional installer can help you compare options and recommend a thickness that fits under your roof eaves and around windows.
Step 4: Select your finish
Render is the most common finish for EWI systems. Modern silicone renders are flexible, water-repellent and pre-coloured, meaning they require little maintenance. Traditional lime or mineral renders are breathable and suited to older homes but may need regular upkeep. Cladding materials such as timber, zinc or brick slips are alternatives that can give your home a distinctive look. Whatever you choose, ensure it is compatible with the insulation material and local planning requirements.
Step 5: Arrange finance and schedule the work
EWI can be a significant investment. Check whether you’re eligible for grants or funding schemes aimed at energy efficiency. Once you have financing in place, schedule the work for a time when weather conditions are favourable (spring and summer are ideal). Ensure you have contingency for any unforeseen repairs discovered during preparation, such as damaged render or rotten timber.
Step 6: Prepare your property
Preparation is crucial to a successful installation. Professional installers begin by protecting working surfaces: covering patios, gardens and paths with plastic and masking windows with adhesive film. Old render and loose paint are removed, and any damaged wall sections are repaired. Boilers, external electrical fittings and drain pipes may need to be moved to accommodate the insulation’s thickness. The walls are cleaned and primed with a stabilising solution to create a surface that adhesive can bond to. If there are features like satellite dishes, these are temporarily removed and refitted after completion.
Step 7: Installation process
The installation starts with a starter track fixed horizontally just above the damp-proof course to support the first row of insulation boards. The boards are then adhered to the wall with mortar and overlapped at corners for strength. Where necessary, boards are cut to fit around windows and other features; any gaps are filled with expanding foam. Once the adhesive cures, mechanical fixings are drilled through the boards and into the wall to secure them. These fixings help hold the boards flat and prevent bowing. A base coat of adhesive is applied over the boards, and a glass-fibre mesh is pressed into it to prevent cracking. For certain boards, multiple layers of base coat may be applied. Once the base coat dries, the chosen render or cladding is applied in one or more layers to provide a weatherproof and decorative finish.
Step 8: Aftercare and maintenance
EWI systems are designed to last for decades, but they still require periodic checks. Inspect the render annually for cracks or damage and clean off any dirt or algae growth. Make sure gutters and downpipes function correctly to prevent water running down the walls. Avoid drilling into the insulation; if you need to fit external fixtures, consult your installer to ensure proper weatherproofing. Inside, maintain good ventilation to prevent moisture build-up; although EWI moves the dew point outward and helps prevent condensation, good airflow is still important.
Potential pitfalls and how to avoid them
Poor design or installation can lead to issues such as trapped moisture and mould. To avoid this, work with an experienced installer who understands vapour control, detailing around openings and roof eaves modifications. Ensure that insulation thickness and materials are compatible with your home and that all external features (pipes, vents, cables) are correctly extended or relocated. A thorough survey and detailed specification reduce the likelihood of mistakes.
Conclusion
External wall insulation offers substantial benefits in terms of energy savings, comfort and environmental performance. By understanding the steps involved—assessing your property, checking regulations, selecting suitable materials and finishes, preparing surfaces and following a careful installation process—you can plan a successful retrofit. While the project requires investment and preparation, the rewards are long-lasting: lower energy bills, a warmer home and improved property value. If you’re unsure where to start, consult local specialists like GO2 Property Services, who can guide you through every stage, from survey to completion.