Features: November 22nd, 2013

As energy bills continue to rise, upfront investment is insulations produces bigger dividends. This article outlines an effective approach to insulation which reduced the cost of energy by a quarter.

As part of a trial into solid wall insulation by the Energy Saving Trust, Saint-Gobain Isover installed its newly launched Optima Internal Wall Insulation (IWI) system into a three bedroom family home in Bolton, Greater Manchester.

Recent figures have revealed that the installation resulted in a reduction of the wall U-values by 84 per cent, contributing to 26 per cent less fuel being used to heat the house – a saving of £312 per year . As a solid wall property, this work would eligible for significant grant funding under the current Energy Company Obligation (ECO), as well as being a measure suitable for a Green Deal plan.

At the end of 2011, the Energy Saving Trust (EST) commenced the largest in situ solid wall insulation trial ever undertaken in England. By gathering a detailed data set from more than 75 houses across the country, the EST aimed to develop a clearer understanding of building performance in relation to domestic solid wall insulation.

As part of this project, Isover’s Optima IWI system was installed at the Bolton property, in order to better understand the effectiveness of internal solid wall insulation in a variety of areas:
• Reducing energy bills through lower fuel consumption
• Decreasing CO2 emissions, as a result of minimised fuel consumption
• Controlling internal ambient temperatures
• Reducing energy waste through heat leakage

The house

This particular property is a post-1900, three storey, mid-terrace house with solid, natural stone walls. The home has three bedrooms and two bathrooms, as well as a kitchen-diner, living room, study and a utility room. Double glazing is installed throughout and a single storey extension has recently been constructed at the rear of the house.

Because the home has ‘conservation status’, and was constructed of natural stone, external wall insulation (EWI) was not considered suitable for the property and, as such, IWI was decided on as the most appropriate and effective form of insulation by the owners and architects.

Action taken

The Bolton house was fitted with Isover’s Optima IWI system, an insulation solution specifically designed to improve the thermal performance of solid or ‘hard to treat’ walls.

System components were assembled in-line with installation guidelines, insulating all walls within the property, with the exception of the extension (which had been built to more recent Building Regulations with an insulated cavity wall), and the two bathrooms. In addition, Isover’s Protect external wall treatment was applied to the external surface of the masonry wall to control moisture ingress.

Measurement, assessment and results

To accurately assess the effectiveness of Optima IWI, a variety of data from the insulated property was collected and analysed. This included thermal imaging, measurement of the U-value and air permeability. Temperature and humidity sensors were also installed to monitor environmental conditions and smart meters were fitted to track gas consumption.

Heat leakage

Thermal imaging demonstrated a clear improvement in heat leakage from the house, from pre- to post-insulation, as the internal insulation had an effect on reducing internal heat from escaping through the walls. The post-insulation image clearly shows the difference between the insulated and un-insulated property resulting in approximately two degree difference in wall temperature.

Internal thermal images pre- and post-installation show the dramatic effect of insulating the walls in equalising the fabric temperatures (the now-insulated internal walls are a similar temperature to the already insulated ceiling).

Ambient internal room temperature versus internal wall surface temperature

Pre-installation, the difference was as high as 5°C, due to heat leaking from the walls. This, along with convection currents within the room, will likely have contributed to drafts and a feeling of cold in the house.

However, post-insulation, this difference was vastly reduced, with little or no difference observed over the measurement period.

Comparison of energy performance

As external temperature has a large impact on the amount of gas used to heat the house, data collected from sensors measuring environmental conditions was taken into consideration when analysing the figures. Comparing energy consumption data collected pre and post-insulation, the trial found that yearly gas use decreased from 26685 kWh to 19748 kWh, a significant 26 per cent reduction. This equates to a 1271 kg reduction in CO2 emissions from the property and a saving for the homeowner of £312 over the course of a year. With energy prices rising significantly, these savings will only improve further over time.

The normalised total gas consumption – a category covering all gas usage including heating, cooking and hot water – fell by 6937 kWh/year.


Significant investment was made during the project to analyse condensation. Measurements of wall surface temperature were taken in all EST trial houses, along with humidity and ambient temperature and this information was used to calculate the dew point of the wall surface and identify any risk of condensation. Condensation could have occurred before insulation was installed due to cold external walls, but was unlikely to occur post-insulation, unless poor ventilation and humidity had allowed it to build up. In this property, no risk of condensation was identified after Optima IWI, Vario membrane system and Isover Protect had been installed.

About Isover: Isover, part of the Saint-Gobain group, is one of the UK’s leading manufacturers and suppliers of mineral wool insulation products and systems.

About Saint-Gobain: Saint-Gobain, the world leader in the habitat and construction markets, designs, manufactures and distributes building materials.