Adding wall insulation

If you’re planning to insulate the exterior walls of your home, read this advice from expert property renovator Michael Holmes.

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What is external wall insulation (EWI)?

A layer of insulation is wrapped around the external walls of your home to make it warmer and more energy-efficient. Compared to cavity wall or internal wall insulation, it is easier to achieve a continuous layer around the building, making EWI one of the most efficient options for renovation or new builds.

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What are its benefits over other types of insulation?

Fitting EWI will not disrupt day-to-day life in your home and, unlike internal wall insulation, it does not reduce the internal area or damage internal decoration. It can also help solve some damp and condensation problems, plus it can potentially improve the external look of a property.

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How will EWI change the appearance of my home?

EWI adds a totally new layer around the walls of a property. The most common finish is render, but other options include brick slips, timber weatherboarding, hung tiles or cladding panels. EWI can be combined with a remodelling scheme to give a building a completely new look. It increases the thickness of the walls, so the roof eaves and verges often have to be extended to cover this; the reveals of window and door openings will also have to be clad, and window sills may need to be replaced or deepened. Gutters, downpipes and external soil pipes will need to be removed and refitted.

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Do I need planning permission?

There is no clear answer to this and the only way to be certain is to get a Certificate of Lawful Development from the local planning authority, for which there will be a fee of around £165. A change of external cladding materials constitutes permitted development and so does not need a planning application. However, as EWI extends the building by 100-200mm all round, it could be viewed as an extension, which would require permission.

Some properties, including those in designated areas (conservation areas, National Parks, Areas of Natural Beauty) have restricted permitted development rights – and in this instance planning permission would always be required.

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Which material used for EWI is the best?

The better the insulation value, the thinner the layer required, so phenolic foam is usually the best choice. However, it is more expensive than urethane boards, mineral wool bats or polystyrene (EPS) boards.

For traditional houses with solid walls (no cavity), it is essential that a breathable insulation material and finish is used to ensure the walls can still breathe to avoid causing damp.

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About how much is EWI likely to cost?

It ranges from £65-£75 per m², making EWI relatively expensive compared to cavity wall insulation. The Energy Saving Trust ( estimates the average cost of £9,400-£13,000 for a typical three-bedroom house of around 90m² on a supply and fit basis (it is a specialist trade). Generally, the annual saving is £475, so the payback is 20-25 years at today’s energy prices.

All costs and estimates correct at time of publishing