During the past two decades retrofit activity has yielded mixed results – the numerous incarnations of government retrofit grant schemes including Energy Company Obligation 4 (ECO4) which was due to close in March but has been extended until at 31st December 2026, and the Great British Insulation Scheme are now deemed largely unsuccessful in the context of their objectives.

The National Audit Office recently concluded that “98% of homes with external wall insulation under ECO require major work to correct major issues that will cause…damp and mould.” Insufficient audit and monitoring of these schemes were identified as a root cause that allowed contractors to take advantage of the system.

What has become clear is that schemes which propose generic retrofit measures at scale have failed to understand the significant unintended negative consequences that can affect older and traditional buildings. 

When well-intended interventions backfire

Solutions encouraged by government retrofit grant schemes, such as wall insulation and double glazing, are often unsuitable for older buildings. Taking a one-size-fits-all approach can result in unintended negative consequences. Much of the recent reporting on mould in rental properties neglects the part that inappropriate retrofitting has played in the problem.

Modern buildings employ an ‘anorak’ principle, keeping moisture out using waterproof materials such as concrete and cement. Traditional buildings are more like a tweed jacket, using water-permeable materials such as timber and lime mortar that absorb moisture deep into the fabric and dry out slowly while keeping occupants warm.

Careless or poorly specified retrofit measures can cause irreversible damage by disturbing the delicate balance of a building’s performance. This can include overheating, mould growth, penetrating dampness, radon risk, thermal bridging and interstitial condensation.

A wet wall is always going to insulate less well than a dry one, and an insulated wall which is still wet because of water being trapped by use of inappropriate materials may well be colder and more prone to mould than an uninsulated dry wall. Many people involved in the retrofit industry are unaware of these risks, or that Building Regulations exemptions exist for buildings of traditional construction. 

The need for a holistic approach

Historic England’s advice note on adapting historic buildings for energy and carbon efficiency, published in February 2026, recognises the significant challenge in ensuring that improvements are made to ensure a future sustainable use for older buildings while making sure that this is undertaken with the utmost care and accuracy, by a professional team, to eradicate the risk of dampness and mould growth developing as a result of these measures.

Solutions should be designed on a case-by-case basis as each building will present its own unique challenges and opportunities. For listed buildings, the first step must be to assess the heritage significance and the impact differing interventions might have.

Finding the right fit

Not all options will be suitable. The sensitive repair of timber windows and the installation of secondary glazing often provides better acoustic and thermal performance than double glazing, for example. Similarly, the many different forms of insulation including sheep’s wool, polyisocyanurate (PIR), polystyrene beads, and aerogel all require careful assessment using specialist condensation risk analysis software to ensure that the technically correct solution is reached.

Only by taking a holistic approach can we ensure our historic buildings retain their beauty and cultural significance while remaining relevant, resilient and energy-efficient.

Further information

Contact Michael Wright