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.
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.
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.
Wood fibre insulation products, such as STEICOprotect L dry can be used to deliver External Wall Insulation.Combined with a number of render systems, wood fibre insulation delivers an EWI solution that can help support the energy efficiency of buildings.
To find out more information about how wood fibre insulation solutions for EWI can support a fabric first approach, contact STEICO’s expert team.