Internal wall insulation – field test

Internal wall insulation blog post

The construction industry received an early Christmas present in mid-December when, after much delay, the Each Home Counts – Bonfield Review was finally published.

The review’s primary objective was to identify ways of ensuring energy efficiency and renewable energy measures fulfil their potential for homeowners. Amongst its findings, the report highlighted that:

While many successful examples of the installation of energy efficiency and renewable energy measures exist, there are also many cases of poor quality work and performance, especially in the area of insulation and fabric upgrades.

It’s unclear how the Government will respond to the key recommendations of the report, but, it is vital that the construction industry acts now to address the issues it raises around workmanship. This is particularly important for internal wall insulation (IWI) applications. The unusual design of many older properties can make them challenging to treat. However, with careful detailing, they can transform a property’s energy performance as extended monitoring programs have shown.

Long-term monitoring

To accurately assess the effectiveness of solid wall insulation applications, long-term monitoring is required. Fortunately, the Energy Savings Trust (EST) conducted just such a programme on 36 so called ‘hard-to-treat’ dwellings between 2009 and 2013. The monitoring was carried out by BSRIA and considered a whole range of factors such as:

  • indoor and outdoor ambient temperatures
  • internal and external wall surface temperatures
  • indoor humidity
  • gas and electricity usage

BSRIA also conducted a series of thermal imaging surveys and took U-value and airtightness measurements.

All of this data was then reviewed by the EST to ensure it is as accurate as possible. We’ll now take a detailed look at the results from two of the internal wall insulation applications. Both were upgraded to a high standard by experienced dry-lining operatives, employed by Mansell Projects Ltd.


The first property is a picturesque two-storey end-terrace with 500 mm thick stone walls. The owners preferred IWI over external wall insulation (EWI) as they wanted to keep the rustic stone façade. The pre-1900 dwelling is also situated in a conservation zone which may have prevented exterior alterations. All the work was completed without the owners having to leave their home. This allowed them to be involved in decisions about both the finish and aesthetics of the alterations.

92.5 mm premium performance phenolic insulation plasterboard was fitted with a range of ancillary fixing components.  Despite the slim build-up, the construction achieved a final U-value of 0.215 W/m2.K – a remarkable 83% improvement on the walls’ original thermal performance.

This change has had a significant impact on how the homeowners heat the property. Gas usage was previously constant through the day, whereas after the IWI application it was rarely used in the morning or afternoon. The long-term EST monitoring programme revealed a 20% saving in normalised annual gas consumption for primary space heating. Whilst additional heating is provided by a dual fuel fire, its contribution could not be measured. As such, the true energy savings could be even greater.


The pre-1900 property in Burnley is a three-storey mid-terrace which also has 500 mm thick stone walls.  Internal wall insulation thermal imageryThe area surrounding the property was part of an ongoing property “face lifting” scheme and so external works were restricted. This made IWI the preferred option.

The same phenolic dry-lining system was installed by Mansell Projects. The insulated plasterboard was mechanically fixed to 25 x 50 mm pre-treated timber battens, lined with 100 mm wide damp proof course strips. It achieved a U-value of 0.18 W/m2·K – an outstanding result which represents an 89% improvement over the original performance.  Airtightness levels were improved by a considerable 57%.Internal wall insulation - detailing

The thermal images clearly reveal how effective the IWI system is. This is particularly true at junction locations where the cold regions appear to have been minimised. These images, combined with the reduced air-leakage, emphasise the positive impact of proper installation practice and good detailing.

Following the installation, normalised annual gas consumption for primary space heating fell by 45%. The study of heating behaviour patterns at the property also showed a reduction in both the intensity and duration of heating events to achieve the desired temperature.

Successful delivery

The monitoring results from both case studies show the clear benefits, both in terms of comfort and energy usage, which can be achieved through well considered IWI installations.

In the next and final blog in this series, we’ll look at a project which has taken the IWI retrofit process even further, achieving full Passivhaus certification.

Missed the other blogs in this series?

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Kingspan Insulation is a market leading manufacturer of optimum, premium and high performance rigid insulation products and insulated systems for building fabric and building services applications.

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