Built to last? Why retrofit must be done right

As the UK races to improve the energy efficiency of its ageing housing stock, retrofit measures such as cavity wall, external wall, and spray foam insulation offer potential benefits -- but can also bring unitended consequnces, including damp, mould, and structural damage. This article by the Property Care Association (PCA) explores why careful planning and expert oversight are essential to ensure retrofits deliver on their promises without creating new problems

The UK has a big problem with its housing stock. Research shows that, at an average of just 818 square feet, the UK is home to one of the smallest average property sizes in the world. It also has amongst the oldest housing in Europe, with 78 per cent of homes having been built before 1980, compared with an EU average of 61 per cent, and 38 per cent of the UK’s housing stock being built before 1946, compared with an EU average of 18 per cent. Many would perceive a house constructed in the 1980s as a modern property but in reality, these properties are now 40 years old and well below today’s performance standards. Small, overcrowded and unthermally efficient homes leads to damp and cold conditions.       

A recent briefing note published by BRE (Building Research Establishment) in February provided an update on Progress on energy efficiency in England and priorities for the Warm Homes Plan. It highlights how over half of homes are now EPC C or better, but also estimates that in 2023, the NHS was spending over half a billion pounds a year treating cold-related ill health caused by EPC F and G banded homes. The report concludes that “While UK homeowners have made major progress in improving the energy efficiency of their homes over the last twenty years, we must go further and faster”.     

Our homes need to be warm with an ideal thermal comfort around 16-19 degrees. So clearly there is a huge success story here but this only appears to tell half a story and there have been unintended consequences of retrofit too and most frequently this is damp and mould.

Cavity Wall Insulation (CWI)  

Since roughly 1920, Cavity wall construction has been the predominant form of construction in the UK. Tried and tested, it generally does a fantastic job of preventing damp issues.     

Modern properties include insulation within this void to help improve the thermal performance of the structure. In a quest to replicate this and reduce the heating bill of older cavity wall buildings, many have been retrofitted with insulation. Retrofit CWI typically comes in three types, blown fibre, Expanded Polystyrene Spheres and less commonly Urea Formaldehyde Foam, made by mixing two chemical components which form a foam that expands to fill the cavity.     

However, installing retrofit insulation into the void fundamentally changes the design of the wall and can have unintended consequences. Colin King, a building expert and researcher, has extensively studied the effects of CWI, particularly in older homes. His research suggests that while CWI can be beneficial in the right conditions, it has often been misapplied, particularly in homes not suited for it.     

While CWI can improve insulation, in some cases, it fails to perform as expected due to gaps, poor workmanship, or degraded materials.     

In homes exposed to wind-driven rain, especially in wetter regions, CWI can allow moisture to penetrate the cavity, leading to damp and mould inside the property.

External wall insulation (EWI)

EWI is widely used to improve energy efficiency by adding an insulating layer to the outside of buildings. While it can reduce heat loss and lower energy bills, it also comes with significant risks if not installed correctly. One major issue is trapped moisture, which can lead to damp and mould growth, especially in older buildings designed with vapour-permeable materials. Poor installation can also cause thermal bridging, where gaps or weak points in the insulation allow cold spots to form, reducing effectiveness. In some cases, EWI has been found to degrade external walls, particularly if the insulation prevents them from drying out properly after exposure to rain. Additionally, the aesthetic impact of EWI can be controversial, particularly in historic areas where it alters the appearance of traditional brick or stone facades. While EWI can be beneficial when properly applied, many homeowners have faced costly issues due to poor workmanship, inadequate ventilation, and incompatibility with certain building types.

Spray foam insulation

Mortgage and equity release lenders have now become nervous. Recent reports suggest that a quarter of the UK’s biggest mortgage providers will not lend on homes with spray foam installed. Many others may lend but will consider each application on a case-by-case basis and may still be subject to the provision of certain documentation. Whilst not all installation is problematic, the concerns around the application of spray foam have led to opportunistic removal companies, which often employ pressure selling techniques, to encourage the removal of the insulation.     

Why are lenders so nervous? Many property professionals had concerns that the encapsulation of timber had the potential to lead to timber decay and in February 2024, HSE released a document Spray foam insulation applied to timber sloped roofs in dwellings - Modelling of moisture risk for retrofitted spray foam insulation in existing dwellings.     

Whilst there was some good news with the report stating, “the modelling indicates that risks are low when an open cell (moisture permeable) insulant is applied in line with guidance described in British Standard BS 5250:2021.” In simple terms, this means you need a Vapour Control Layer and/or air gap.       

However, there was bad news too, “The highest risk assessed is when spray foam insulation is applied directly onto the roof covering. This leads to high risks under all modelled scenarios.” It also identifies that the modelling identified higher levels of risk in some scenarios when a sprayed foam is applied to high resistance underlay such as a bitumen felt. Essentially the study found in certain scenarios there was an elevated risk of moisture and timber decay due to the presence of spray foam insulation.     

Currently around 250,000 UK homes have spray foam insulation, which can be difficult and expensive to remove once applied.

Conclusion

The UK’s ageing housing stock needs better energy efficiency, but retrofit solutions like CWI, EWI, and spray foam insulation have led to damp, mould, and structural issues when poorly installed. While insulation can cut energy costs, bad workmanship and incompatibility with older homes have caused unintended damage. Mortgage lenders are increasingly wary, especially of spray foam, due to its risks. Improving energy efficiency is crucial, but it must be done carefully, with expert oversight, to avoid costly mistakes and ensure homes remain both warm and durable.

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