Martin Twamley, Technical Director at Steico UK, explains how fabric first measures like External Wall Insulation (EWI) can help decarbonisation and produce long-term benefits for the building and its occupants.
Putting forward a fabric-first strategy has long been the mantra of the construction industry. Measures such as External Wall Insulation (EWI) focus on improving the energy efficiency of a building’s fabric before adding mechanical systems or renewable technologies. EWI can help create a well-insulated, airtight building envelope, reducing energy demand for heating and cooling at source, rather than relying on additional technologies to compensate for poor construction.
Fabric-first thinking is now being challenged by an alternative point of view, driven by the need to rapidly decarbonise the building stock. It prioritises installing low-carbon heating systems, such as heat pumps, before major fabric upgrades such as EWI, with the aim of accelerating decarbonisation.
The theory is that relying solely on electricity for domestic hot water, space heating, cooling and lighting will eventually mean that a dwelling becomes ‘zero carbon’ once the grid is fully decarbonised.
What is EWI and where is it used?
EWI involves fixing insulation to the outside of a building’s external walls. The insulation is attached using adhesive and mechanical fixings, then finished with a protective render to prevent moisture ingress.
EWI is most commonly used to improve the energy efficiency of existing solid-wall properties, typically made of solid brick or stone, which do not have cavities. However, it can also help new-build projects meet high thermal performance standards, increase usable internal space and reduce construction costs by allowing thinner wall build-ups.
Most EWI systems focus on achieving the required U-value to comply with building regulations. However, in addition to improving thermal efficiency, some EWI systems can help manage moisture and mitigate summer overheating.
Both of these issues, when positively addressed through EWI, can enhance the thermal comfort of occupants. This would not be the case if the fabric were left uninsulated and only a heat pump was installed.
Why is EWI considered to be a good option to improve thermal performance of a dwelling?
EWI wraps the walls of a house in a continuous protective layer. It helps to keep the cold out, keeping the walls and everything inside the house warm.
There are many junctions in a house, such as where walls meet floors and around windows, that can act as weak points in the building’s thermal performance. Heat can escape more easily at these locations, which are known as thermal bridges.
EWI helps to keep these junctions warm, reducing the pathways through which thermal bridging can occur. Because the internal face of the wall remains warmer, the dew point at which condensation forms is pushed outwards towards the external surface of the EWI. As a result, the risk of condensation forming on internal wall surfaces is reduced.
By wrapping the building in a continuous layer of insulation, EWI helps to cover any gaps in the existing fabric, reducing draughts. The weather-resistant layer on the outside of the EWI protects the existing structure from rain and extremes of temperature, which has the potential to extend the life of the building.
How does the choice of insulation affect the performance of an EWI system?
The properties of the insulation used within an EWI system directly influence the benefits delivered to the building fabric and its occupants. The specific type of insulation selected can improve the thermal performance of external walls and significantly affect the overall performance of the system.
Wood-fibre insulation products, such as STEICOprotect L Dry, are just one example.
Moisture management and diffusion open EWI
Insulating a building’s fabric with EWI can change how moisture is managed, making effective moisture control essential. Poor moisture management can result in condensation forming on the surfaces of walls and ceilings, which can lead to damp and mould growth. Meanwhile, moisture trapped within the building fabric, known as interstitial condensation, can cause long-term structural damage.
This is particularly relevant to older solid-wall and heritage properties, which are often constructed from breathable materials. In these cases, diffusion-open insulation, such as wood fibre insulation, is often preferred by EWI specifiers over diffusion-closed alternatives. Being diffusion-open allows moisture to pass through the wall build-up and closely matches the original behaviour of the building fabric.
Diffusion-open EWI systems incorporate breathable external weather-protection layers, ensuring that moisture can escape fully to the outside.
Mitigation of overheating with EWI that adds thermal mass to external walls
Not all EWI types are equal when it comes to mitigating overheating. Adding thermal mass helps to slow the passage of heat from outside to inside on a hot summer’s day. However, heat storage capacity and thermal performance also play an important role. Wood fibre insulation combines high density, high heat storage capacity and low thermal conductivity, and this combination of performance parameters can significantly reduce the risk of summer overheating.
These properties allow EWI systems using wood fibre insulation to absorb and store heat during the day, delaying its transfer indoors so that peak external temperatures do not immediately affect internal conditions.
The summer performance of EWI can be quantified in a similar way to winter heat loss. Parameters such as thermal diffusivity and time lag describe how quickly heat moves through a material. Lower thermal diffusivity results in a longer time lag between external heat peaks and a rise in internal temperature, allowing designers to assess insulation materials used in EWI systems for both winter energy efficiency and summer comfort.
Selecting EWI systems that can help reduce internal temperatures and prevent overheating improves the thermal comfort of occupants. It can also reduce the likelihood that air-conditioning systems will be required, helping to reduce energy bills for the occupants and carbon emissions for the planet.
Embodied carbon contribution of EWI
Where an EWI project has a focus on sustainability, the embodied carbon contribution of the products within the system can become an important performance metric.
Although insulation can help to contribute to the energy efficiency of a building, the energy used in its manufacture means that it comes with its own embodied carbon footprint. This can be measured by Environmental Product Declarations (EPDs), and these EPDs can be used as a way to specify one EWI insulation solution over another.
How do the benefits of EWI support the drive for decarbonisation?
When decarbonisation is presented as an alternative priority to fabric-first measures, the thermal improvements delivered by EWI should not be underestimated. By reducing heat loss, EWI lowers the amount of energy needed to maintain a comfortable indoor temperature. This reduced energy demand leads directly to lower carbon emissions, supporting rather than undermining decarbonisation goals.
Some may argue that if the energy used for heating comes from a zero-carbon source, energy efficiency becomes less important. From a purely theoretical decarbonisation standpoint, this argument has merit. However, the reality is that the electricity grid is not yet fully decarbonised and many households cannot afford technologies such as solar PV and battery storage to supply carbon-free energy for heat pumps. Decarbonisation is unlikely to be the main driver for homeowners to consider External Wall Insulation. An inefficient building fabric still means higher energy consumption and increased bills, regardless of the carbon intensity of the energy supply. Knowing that the energy is net zero offers little comfort when costs remain high. The main motivation for homeowners to consider external wall insulation is more likely to be focussed on the delivery of tangible benefits, such as reducing energy bills and improving thermal comfort within the home.
As discussed, EWI is a fabric-first measure that can offer benefits beyond improved energy efficiency and thermal performance. It can enhance the condition of the building fabric while significantly improving thermal comfort for occupants. The same cannot be said if priority is given to decarbonisation through the installation of low-carbon heating systems.
A more effective approach is for the two strategies to work in tandem. By first reducing energy demand through improvements to the building fabric, homes become more affordable to run while remaining aligned with long-term decarbonisation objectives. Any low-carbon systems subsequently installed can then be correctly sized to meet the reduced energy requirements.
