External Wall Insulation

Case Study

Steve, Valerie and their children were adding a highly insulated extension to their 1930s semi in Kendal and wanted to improve the insulation of the rest of the house at the same time. The house already had cavity wall insulation when they bought it but was draughty and cold in the winter. As their existing pebbledash needed replacing they decided that they would add external wall insulation to the outside of the house. They consulted with their architects (www.millerclear.co.uk) and an energy efficiency expert, Diane Hubbard (www.greenfootsteps.co.uk) who explained that adding this insulation would only see its full benefit if they improved the airtightness of the house at the same time.


They decided to improve airtightness in two ways: replacing their existing cavity wall insulation with specialist expanding foam insulation, and using airtightness tape around windows and other gaps made by pipes etc. The original cavity wall insulation was removed and replaced and new triple glazed windows were installed – taking about a week in total. The existing pebble dash was not removed, instead it was skimmed over and 150mm of expanded polystyrene insulation attached over the top. This was then covered with self-cleaning silicone render. This also took about a week. Once this was done, new window sills were added to cover the increased depth of render, and new downpipes were fitted.


As the house is outside of the Kendal conservation area no permission was required to change the render and windows.


R&R Cavity Wall Insulation Removal Specialists – 07967 646032 Heatlok Spray Foam Insulation – www.heatlok.co.uk Enlightened Windows – https://enlightenedwindows.co.uk/ The builders and renderers are no longer operating, however a similar recommended company is: www.pegasus-externals.co.uk/


Cavity wall insulation removal: £1700

Heatlok insulation: £2700

External Wall insulation, including installation and render: £6400

Triple-glazed timber and aluminium composite windows, installation and airtightness taping: £11000

Total cost: £21800


Mid-winter gas usage has been cut from an average of 66kWh per day to 32kWh per day. This is a reduction of around 1 tonne of CO2 per year (which is about how much the average car produces in half a year). The payback period is difficult to calculate (as it depends on gas prices), however as the house needed re-rendering some of the price is offset by how much a simple re-pebbledash would have cost, cheaper windows could have also been used. Perhaps most importantly the house is always warm and comfortable in the winter, despite needing less money to heat.