Summary
Most older properties in Letchworth have solid brick walls. Houses with solid walls can have insulation boards applied to the walls either externally, internally, or both (e.g. internally on the front facade and externally on the back, or externally and internally on the same wall – see Hybrid wall insulation). External wall insulation (EWI) is then covered with render, plaster or boarding to protect and conceal the boards.
EWI can significantly improve the energy efficiency of a home but there are a number of considerations to be aware of, notably the cost, complexity, and applicability in regard to the Letchworth Design Principles (see below).
Benefits of EWI over internal wall insulation (IWI) include:
- Retainment of internal floor space.
- Less disruption for homeowners.
- Generally high levels of energy efficiency can be achieved.
Cost/difficulty
HIGH COST – Estimated £16,000-£35,000 for 100–200mm of insulation (110m3). – The Surrey Home Energy Improvement Guide (2025), page 40.
HIGH DIFFICULTY – EWI can be complex to install and it is recommended to seek professional advice from a retrofit coordinator/designer to create a whole house retrofit plan to assess the pros and cons and devise a plan to suit your needs.
Applicability in Letchworth
Generally not applicable, unless certain requirements are met.
The Heritage Character Area Design Principles state:
- External wall cladding can have a negative impact on the appearance and character of a historic building.
- Applying EWI can damage the character of the property by covering up important details and features, and creating deeper window and door reveals.
- EWI will only be supported when:
- the detailing and features of the property, including the brick and render finish, string courses, window and door reveals, are maintained or carefully recreated;
- the house is detached;
- all of the properties in a pair of semi-detached, terrace or group, collectively and simultaneously implement the works.
- External cladding is not acceptable on Homes of Special Interest.
The Modern Character Area Design Principles have similar requirements and in addition state:
- Many modern Character Area homes have cavity walls and insulating the cavity is the best option to improve insulation.
Insights from ASBP
“Vapour open products such as wood fibre insulation and lime render help to retain the ‘breathability’ of the wall fabric, allowing moisture to move through the wall, rather than blocking it. This helps to reduce moisture and condensation risks.
In addition, higher density insulations such as wood fibre provide good thermal mass benefits, helping to absorb, retain and release heat slowly to help keep homes warm in the cooler months and minimise overheating risks in the summer.”
Insights from People Powered Retrofit
“For a property with solid wall construction we would recommend the use of moisture-balancing materials and avoid the use of impermeable plastic foam insulation boards. This is so that the walls can still dry to the outside – though it can also have the benefit of reducing upfront environmental impacts by avoiding petrochemical-based insulation.”
Case study
Coming soon
Links to further information
- Energy Efficiency Guide for Letchworth Garden City residents (2024), page 3
- HCCSP Retrofitting Your House Guide (2024), page 18
- Saltaire Retrofit Handbook (2024), page 37
- Retrofitting Your Home – Cambridge Guide (2022), page 16
- The Surrey Home Energy Improvement Guide (2025), page 51
- A Bristolian’s Guide to Solid Wall Insulation (2015), pages 45-56
Image credits – TBC
This guidance is part of Walls
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