Cavity Wall Insulation


By Dermot Moore

Cavity Wall Insulation is used in  70% of UK properties (Dec 2020), making it a key topic in developing UK construction. 

A Brief History

The building booms of the 1920s and 30s brought cavity walls into the mainstream. Cavity walls were intended as preventative measures against dampness and heat loss, but not always with great success.

Key factors:

Main Wall 

A cavity wall was and remains cheaper than a solid stone wall. However, favouring more inexpensive materials comes with the downside of later problems. CWI efforts in the 1980s have since brought issues concerning selecting cheap materials to the foreground.

‘Cavity walled buildings, particularly those built with softer, lime-based mortars and renders are ‘breathing’ structures in that they exchange moisture readily with the indoor and outdoor environment.’  

Therefore, choosing an insulation that will mitigate this moisture transfer is necessary. In addition, the exterior wall also has the potential to obstruct or support CWI’s effectiveness. Poor wall structure, unbreathable insulation and haphazard instalment increase the risk of dampness, mould, structural disintegration and heat loss.

Wall Ties

Most cavity walls were initially reinforced using stone and brick ties. But these have been superseded by steel ties. The longevity of these ties should be considered in the CWI instalment. The two common types used from the 1940s till the 1980s were strip and wire ties. These ties are guaranteed to corrode. 

When to Install
  1. Environment/Region –  Assess essential factors: dampness, frost, wind, dryness, heat exposure etc.
  2. Material Resilience – Which material is most resilient in the specific climate?
  3. New or Old – Is it new construction or requires retrofit/renovation?


In damp, humid regions like the UK, CWI faces a greater risk of mould growth, most harmful to breathability and indoor comfort. Climate change also makes for evermore erratic heat fluctuations and storms. The effect of climate change increases risks to the structural integrity of the external wall. These factors will affect the R-Valuewhich measures how well a structure (wall, insulation etc.) can resist heat flow. A higher R-value means space is more insulated. But this does not highlight material breathability or porosity, which leads to point no.2.

Material resilience is related mainly to thermal and hydrometric parameters. This includes; conduction, convection, thermal transmittance (U-value), thermal bridging/bypass and resistance to vapour diffusion

Achieving Equilibrium?   

Hemp has long proven to hit the mark. Hemp’s permeability makes it an excellent option for CWI. Hemp insulation allows low levels of conduction and convection. Small amounts are required to prevent air stagnation and condensation within walls. Conventional foam insulation is not breathable, which encourages condensation and mould. Hemp insulation has a low U-value of 0.039 – 0.040. The lower the U-value, the less heat is lost.

Thermal bridges pose a significant economic and safety risk, causing heat loss, dampness and structural damage. The project team should discuss the approach to this problem. If thermal bridges are avoided, hemp’s resistance to vapour diffusion becomes clear, similar to mineral wool. 

CWI requires careful consideration when installing. Unlike EWI, retrofitting cavity walls with insulation involves altering building features. Corroded wire ties must be identified before the CWI instalment. Removal of the ties after insulation placement risks the creation of thermal bridges. Porous wall structure, unbreathable insulation and haphazard instalment increase the risk of dampness, mould, structural disintegration and heat loss. These risks can be easily avoided with a new build. But additional considerations are required if hemp insulation is installed behind a porous wall in a retrofit. 

To conclude

Cavity wall history reveals that creating a healthy, comfortable indoor space is not simple. However, the many trials and errors with this technique mean there has been a great reflection on how best to use cavity walls and CWI, and biomaterials can now take a central position on stage.

To find out more, take a look at our wall build-up examples on the Bio Wall shop page.