Summary
Most older properties in Letchworth have solid brick walls. Houses with solid walls can have insulation boards applied to the walls either externally (EWI), internally (IWI), or both in a ‘hybrid’ approach e.g. internally on the front facade and externally on the back, or externally and internally on the same wall. As with EWI and IWI, the insulation board is then covered with render, plaster or boarding to protect and conceal the boards.
Combining external and internal insulation on different elevations can allow the front ‘public’ facing facade to be insulated internally to meet e.g. heritage requirements, whilst rear or side facing facades can be insulated externally, allowing the home to be fully insulated.
This may allow the drawbacks of each approach (EWI/IWI) to be balanced out so that disruption and loss of internal floor space can be minimised. However, a hybrid approach is not without its challenges as combining both insulation options brings added complexity in terms of design and detailing.
Cost/difficulty
HIGH COST
HIGH DIFFICULTY – EWI and IWI can be complex to install and combining both in a hybrid approach brings added complexity in terms of design and detailing. 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
As this is a combination of both external wall insulation and internal wall insulation, please see the relevant pages for further information on their applicability in regard to the Letchworth Design Principles.
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.
Don’t forget, for internal wall insulation even a relatively thin layer of natural fibre insulation (40mm) can provide significant energy savings.”
Case study
Whalley Range retrofit
A retrofit of a semi-detached home in Greater Manchester using both internal and external wall insulation.
Links to further information
- HCCSP Retrofitting Your House Guide (2024), page 16
- A Bristolian’s Guide to Solid Wall Insulation (2015), pages 69
Image credit – Retrofit Works
This guidance is part of Walls
See more guidance notes on this topic below: