Putting forward a fabric first strategy has long been the mantra of the construction industry and for good reason. Fabric first 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. By creating a well-insulated, airtight building envelope, measures such as EWI can help to reduce 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 that is 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, to accelerate decarbonisation.
The theory being that by solely relying on electricity for domestic hot water, space heating, cooling and lighting will mean that eventually a dwelling will become ‘zero carbon’ when the grid is fully decarbonised.
So, is a fabric first strategy in some way compromising the chances of the UK achieving zero carbon? Should existing dwellings have electric heat pumps rather than worry about the performance of the building fabric? In this piece we explore this topic in detail and explain how fabric first measures like EWI can help decarbonisation and produce long term benefits for the building and its occupants.
Before we start, let's refresh our memory on what we mean by External Wall Insulation.
What is External Wall Insulation and where is it used?
External Wall Insulation involves fixing insulation to the outside of a building’s external walls, unlike cavity wall insulation which fills the gap between inner and outer wall layers. The insulation is attached to the external wall 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 be specified for new-build projects. As well as helping new buildings to meet high thermal performance standards such as Passive House, EWI increases usable internal space, and can 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 External Wall Insulation systems can play a part in managing moisture and mitigating summer overheating.
These are both issues that when positively addressed by EWI can enhance the thermal comfort of the occupants. This would not be the case if the fabric was 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?
The analogy that is often used to describe the benefits of externally insulating a house is to regard the insulation as a kind of tea cosy. In the same way that a tea cosy is placed over a tea pot to keep both the tea and the pot warm, External Wall Insulation wraps up the walls of a house in a continuous protective layer. It helps to keep the cold out and 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 act as weak points in the building’s thermal performance. Heat can escape more easily at these locations, which are known as thermal bridges.
External Wall Insulation 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 also helps to cover any gaps in the existing fabric. This reduces draughts and contributes to a more comfortable internal environment for occupants. The weather resistant layer on the outside of the EWI protects the existing structure from rain and extremes of temperature, and this 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 insulation used within an EWI system directly influence the benefits delivered to both the building fabric and its occupants. While the primary role of the insulation is to improve the thermal performance of the external walls, the specific type of insulation selected can significantly affect the overall performance of the system.
Moisture management and diffusion open External Wall Insulation
Insulating a building’s fabric with External Wall Insulation can change how moisture is managed, making effective moisture control essential. Poor moisture management can result in condensation on the surfaces of walls and ceilings that can lead to damp and mould growth. While moisture trapped within the building fabric, known as interstitial condensation, can cause long-term structural damage.
This is particularly relevant in 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 rather than diffusion closed. Being diffusion open, the External Wall Insulation 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 outer weather protection layers to ensure that moisture can fully escape to the outside.
Mitigation of overheating with EWI that adds thermal mass to external walls
Not all External Wall Insulation types are equal when it comes to helping to mitigate overheating. Adding thermal mass helps to slow the passage of the heat from outside to inside on a hot summer’s day. However, the heat storage capacity and thermal performance also play a role. Wood fibre insulation combines high density, high heat storage capacity and low thermal conductivity and this blend 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 peak external temperatures do not immediately affect internal conditions.
The summer performance of External Wall Insulation can be quantified in a similar way to winter heat loss. Parameters such as thermal diffusivity and time delay describe how quickly heat moves through a material. Lower thermal diffusivity results in a longer time lag between external heat peaks and internal temperature rise, allowing designers to assess insulation materials used in EWI systems for both winter efficiency and summer comfort.
Selecting EWI systems that can help to reduce the internal temperature and prevent overheating increases the thermal comfort of the occupants and also reduces 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 in this blog, External Wall Insulation is a fabric-first measure that, when specifying appropriate insulation types, 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, as is often the case in improving the built environment, is for the two strategies to work in tandem. By first reducing energy demand through improvements to the building fabric, such as External Wall Insulation, 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, resulting in dwellings that are closer to perhaps the ultimate goal of being both low energy and low carbon.
Where can I find out more information about wood fibre insulation solutions for External Wall Insulation?
STEICO makes wood fibre insulation made from at least 80% natural wood sourced from PEFC certified* forestry. STEICOprotect L dry is a render carrying, diffusion open insulation board ideal for External Wall Insulation. It can be combined with a number of render systems to deliver an EWI solution that can help to support the energy efficiency of buildings.
To discuss how our EWI products can support a fabric first approach to your next construction project, please get in contact here and our expert team who will be happy to help.
*PEFC-04-31-2550