Risks of Energy Efficiency Interventions in Buildings of Traditional Construction

As our climate changes, buildings are being subjected to higher moisture levels both externally and internally. Winter precipitation is projected to increase by around 33%, with more frequent intense rainfall events and shorter but more concentrated wind-driven rain spells, particularly in the winter months. Annual wind-driven rain from the west, southwest and south is also projected to increase significantly. Additionally, as the atmosphere warms, humidity levels are rising because hotter air holds more water vapour molecules.

Wind-driven rain is one of the largest sources of moisture accumulation within the fabric of a building. Moisture may also accumulate when there is a failure, such as a plumbing leak or an overflowing gutter. Significant amounts of liquid water can penetrate and move through building materials and assemblies by capillary flow and/or gravity. Internal moisture content can also reach a very high level depending on how the building is used: for example, if occupants dry their clothes indoors.

When considering energy efficiency interventions, it is crucial to understand and account for the building's:

• External environment and location
• Exposure to solar radiation, rain, wind and wind-driven rain
• Flood risk
• Repair and maintenance needs
• Extent of shading from adjacent buildings or landscape features

These factors are of particular relevance when insulating walls in a building of traditional construction. This is because moisture moves through traditional permeable materials in both liquid and vapour forms. If inappropriate materials or assemblies are used, these can hinder natural drying mechanisms and cause moisture to become trapped within the building fabric. Incompatible materials may also force moisture inward to evaporate into the internal environment.

Some materials have good vapour permeability but poor hygroscopicity (water vapour buffering) or capillarity (liquid moisture transport), which may render them inappropriate for certain uses within buildings of traditional construction. This is because transporting moisture by vapour diffusion through a material's thickness is extremely slow, sometimes almost negligible. Conversely, water vapour transport via unobstructed air pathways (mass air movement driven by differences in vapour pressure or temperatures either side of the building component) and liquid moisture transport by capillarity or gravity are the primary moisture transport mechanisms through a permeable material.

When installing internal or external thermal insulation in a building of traditional construction, using a permeable insulating material that manages both liquid and vapour transport may help control the moisture content within the built assembly. For example, some insulating materials are hygroscopic and will help regulate indoor humidity levels. However, it is important to follow the manufacturer's recommendations, because some materials will fail to maintain their insulating properties if they become saturated: for example, if wind-driven rain penetrates through the thickness of the wall or there is a flood or plumbing leak.

Recent research by the Department for Energy Security and Net Zero (DESNZ) titled Demonstration of Energy Efficiency Potential (DEEP) notes that 'installing IWI (internal wall insulation) always led to an increased moisture risk [in the case studies], even when only a thin layer of insulation was applied'. The research also showed that vapour permeable systems had lower risks of moisture accumulating within the building envelope and that 'by targeting U-values around 0.8W/(m²·K), rather than the Building Regulations limiting U-value of 0.3 W/(m²·K), the [interstitial condensation] risk was reduced while still receiving two-thirds of the energy savings'.

The extent of moisture risk depends on many factors, including:

• The building's exposure
• The type and thickness of materials used (for example, permeable vs impermeable)
• Whether moisture accumulation issues are resolved before insulation is installed
• The detailing of insulation around junctions and penetrations
• Whether a robust hygrothermal assessment is conducted to underpin energy efficiency proposals

In traditional construction, highly capillary active lime renders and limewash coatings were a common protective external finish; for example, on rubble masonry walls and on some timber-framed buildings. This type of external finish reduces rainwater penetration into the wall and aids drying by maximising the evaporative area across the whole surface. However, over time, many such finishes have been replaced with impervious renders, to prevent moisture from penetrating the building fabric. It is now widely understood that renders made of non or low capillary active materials are inappropriate. This is because water can become trapped behind the surface finish, causing the wall to stay saturated. In turn, this can lead to fabric decay, moisture penetrating to the interior and an increase in heat loss potential.

Solid ground floors account for a large proportion of moisture exchange in buildings of traditional construction. When impermeable materials are used, the building's moisture equilibrium is affected and localised concentrations of moisture can occur. For example, if excess groundwater accumulates because of defective drains at the base of abutting walls. When the walls are also finished with impervious materials, excess moisture will rise until it can evaporate.

Energy efficiency measures, such as wall insulation, can adversely affect the thermal continuity of the external envelope if they are not designed and installed appropriately. It is essential that such measures maintain a homogenous surface temperature to minimise thermal bridging and prevent mass air movement into or through the building envelope, to avoid thermal bypass. Adequate ventilation must also be provided.

Thermal bridging

Thermal bridging occurs when an area of the building envelope has a higher heat transfer capacity than its surrounding materials, which causes increased heat transfer with the outside. The surface temperature in that area will be lower than its surroundings. As such, the area may reach dew point temperature if the indoor moisture content is high enough, causing concentrated surface condensation and the potential for mould growth. This typically occurs where:

• There is a break in the thermal insulation layer
• The thermal insulation is penetrated by an element with a higher thermal conductivity, such as pipes, cables or ductwork
• The thermal conductivity changes, such as when different insulation thicknesses or materials with different thermal properties are used, or when the quality of detailing or installation is poor. Such changes may occur in window or door reveals, at junctions of different building phases and constructions, or where window or door frames are in contact with both the internal and external environments

Thermal bypass

Thermal bypass is convective heat transfer, between 2 areas of the building envelope, that bypasses the usual conductive and radiant heat transfer paths.

In retrofit projects, this may occur when internal thermal insulation is installed within studs and there are gaps between the insulation boards, allowing warm indoor air to migrate behind them. When this warm air meets the colder surface of the wall behind the insulation, it may reach dew point temperature and interstitial condensation may then occur, causing hidden mould growth and fabric decay.

Minimising uncontrolled ventilation (air leakage and draughts) through the building envelope will help reduce associated heat losses and will increase the thermal comfort of occupants. However, maximising airtightness may inadvertently reduce indoor air quality, pose a risk to the health of the building's occupants and increase overheating risk.

Without appropriate ventilation, the building envelope may be less able to regulate the indoor moisture content of the air, particularly if internal traditional hygroscopic finishes have been removed or covered (for example, with impermeable paints). This can increase the risk of surface condensation and mould growth.

Additionally, excess heat and particulates may accumulate indoors if ventilation is inadequate. Some insulating materials or systems can release volatile organic compounds, which contribute to indoor air pollution.

When planning any energy efficiency interventions, it is essential that a ventilation strategy is implemented to regulate the internal heat gain and moisture levels, reduce condensation risks and eliminate pollutants.

Inappropriate detailing or the substandard installation of energy efficiency measures can lead to moisture accumulation, thermal bridging or thermal bypass.

For example, when installing external wall insulation, it is essential that continuity of the insulating layer is achieved. Detailing around rainwater goods, roof eaves/verges and any openings must be adequately designed and installed.

Risks of overheating and poor indoor air quality may become more common as the climate changes.

Energy efficiency interventions designed without following a whole building approach can adversely affect the thermal buffering capacity of a traditional building; for example, by separating the internal environment from the inherently beneficial thermal mass and inertia of mass solid walls. Such measures may also hinder a building's capacity to regulate humidity fluctuations; for example, by separating the internal environment from the inherently beneficial hygroscopicity of traditional finishes. If a building's thermal buffering capacity is compromised, this can cause overheating and/or high indoor humidity levels, which can severely affect the health and wellbeing of building users.

Retrofit measures should aim to reduce solar gains (for example, by adding external shading where appropriate to the building) and to use night purge ventilation where possible. The likelihood of overheating can be assessed using dynamic thermal modelling tools. The Good Homes Alliance's Overheating in Retrofit and Existing Homes – Tool and Guidance is a useful resource.

1 in 4 properties is currently at risk of flooding, according to the Environment Agency. If the flood risk is not considered or inappropriate materials are used, energy efficiency interventions may hinder the drying process of a building of traditional construction after flooding. Energy efficiency measures may even need to be removed and replaced after a flood.

Traditional and replacement materials that are used to maintain, repair and retrofit buildings of traditional construction may suffer accelerated degradation due to climate change hazards, such as increased rainfall, temperatures and humidity. Further research is needed to understand how climate change will affect material decay mechanisms and to provide robust evidence to underpin the specification of materials and replacement/maintenance regimes.