Summary
Most older properties in Letchworth have solid brick walls but houses built post-1920 may have cavity walls*. Houses with cavity walls can have insulation injected into the cavity via holes in the external brick layer.
Cavity wall construction (CWI) is a cost effective way of significantly improving the energy efficiency of a home but there are a number of considerations to be aware of, notably in regard to the quality of installation. There has been multiple instances of CWI failure over the years due to improper installation, leading to damp and mould issues, wall tie degradation and ‘cold spots’. As with any retrofit measure, make sure to employ a competent and accredited installer to reduce the risks of problems occurring.
Benefits of cavity wall construction (CWI) over other methods of insulation such as external or internal wall insulation include:
- Lower cost.
- Little disruption for homeowners.
- Preservation of the existing external facade which is important where it is important to preserve the heritage/character of the area (see further information below on Letchworth Design Principles).
*How do I know if I have a cavity wall?
It’s not always easy to tell if the walls of your house contain a cavity, particularly if the walls are covered externally by render. If the bricks are visible externally, the absence of what appear to be half bricks in a repeated pattern over the wall, suggests that it has a cavity. However, the presence of such half bricks is not a conclusive indication of a solid wall as some modern cavity-walled buildings in Letchworth were built with half bricks to give the appearance of a solid wall! It is important that you know what type of wall you have before installing any wall insulation. If you are in any doubt consult a surveyor.
Guidance suggests that you can tell by measuring the width of the wall. If it is more than 260mm thick, is it probably a cavity wall.
Cost/difficulty
MEDIUM COST
MEDIUM DIFFICULTY – This is not a DIY measure and cavity wall insulation should always be carried out by a certified installer who can provide a guarantee from CIGA (Cavity Insulation Guarantee Agency).
Applicability in Letchworth
The Modern Character Area Design Principles state:
- Many Modern Character Area homes have cavity walls and insulating the cavity is the best option to improve insulation.
Homes in the Heritage Character Area generally do not have cavity walls so no specific restrictions are referenced in the Design Principles.
Insights from ASBP
“There is no natural insulation option for cavity wall insulation and materials such as mineral wool, polystyrene beads and urea formaldehyde foam are commonly used. These materials generally have higher ’embodied carbon’ and some are manufactured from petrochemicals. As a minimum aim to use materials with high levels of recycled content.
If you are keen to follow a more ‘natural’ approach then consider insulating internally or externally with a natural insulation system such as wood fibre board with lime render/plaster, whilst being mindful of the restrictions set by the Letchworth Design Principles.”
Links to further information
- Energy Efficiency Guide for Letchworth Garden City residents (2024), page 3
- HCCSP Retrofitting Your House Guide (2024), page 16
- Retrofitting Your Home – Cambridge Guide (2022), page 15
- The Surrey Home Energy Improvement Guide (2025), page 51
This guidance is part of Walls
See more guidance notes on this topic below: