Any damp issues already existing in walls must be addressed before considering wall insulation. Resolving damp issues will result in improvements to the efficiency of the walls. If there is still a significant amount of heat loss once these issues have been resolved, wall insulation may be appropriate, but this must be carefully considered.

The hygrothermal properties, which is the movement of heat and water vapour through the building fabric, must be understood to determine if wall insulation is appropriate. The orientation and exposure of the walls will also impact the type, location and material of insulation being considered. Additionally, the specification, detailed design and installation is paramount to ensure there are no performance issues once installed.

External wall insulation

Typically, external wall insulation is applied to the external walls, with protective cladding over. This results in an increase to the depth to the wall. As such, detailed design needs to be carefully considered with adaptations to the roof overhang and wall junctions. There will also need to be consideration to the window reveals and sills, door surrounds, ground level and the repositioning of rainwater goods.  There is a thermal bridging risk (though this is reduced comparative to internal wall insulation); as such design needs to be considered around door and window surrounds, under the roof parapet and at ground level.

Whilst technically, there can be relatively less risk from external wall insulation (compared to internal insulation), it will affect the appearance of the building. If the building is listed or in a conservation area, it can have a harmful impact on the building’s heritage significance. If the building has decorative detailing or is particularly characterful, external insulation will result in a loss of significance. Even if the building is rendered, there will be a notable change due to the increase in depth. As such, this intervention may not be permitted or consented.

External insulation, where achievable, is often most appropriate for detached buildings. Semi-detached buildings and terraced properties will have difficulty installing external insulation due to party walls and how the junctions are dealt with. If it is not possible to insulate the whole building, priority should be given to exposed north walls.

If external insulation can be achieved, the materials used are an important consideration. Natural materials such as wood fibre boards, wood wool boards and cork boards are usually more appropriate for an historic building’s existing fabric and qualities. However, due to the weight of some wood fibre boards, a structural engineer (a CARE Engineer is recommended for historic buildings) may need to be appointed to advise on the additional loads to the external walls. Synthetic materials such as modern closed-cell foam, other plastic-based insulations and the use of protective finishes are often not appropriate for an historic building, due to the lack of vapour permeability.

Lime render can have benefits for energy efficiency, particularly as it is able to assist in moisture movement. Where this can be achieved, thermal properties can be increased by lime render mixes containing natural materials, including hemp and cork.  BRE have issued information papers for natural fibre insulation and hemp lime. Whilst they were authored in 2011, and there have been further developments in the products since then, they are still able to give a broad introduction to the materials.

Lime render

Lime is one of the oldest building materials in the world. It is produced by heating limestone in a kiln, leaving only calcium oxide, known as quicklime. Lime render is made from a specific mix ratio of lime, water and sand. The lime used in preparation will tend to be non-hydraulic lime or hydrated lime. This may also be called slaked lime, high calcium lime, air lime, or lime putty. 

Lime renders were traditionally applied to give protection to walls constructed of traditional materials such as stone or porous brick, or to walls in exposed location facing driving winds. They help by acting like a sponge, absorbing moisture then allowing it to evaporate rather soak into the wall. Some traditional buildings would have been rendered originally and they tend to suffer from penetrating damp if the lime render is removed or replaced with a cement-rich render.

Lime render is considered carbon neutral as the carbon dioxide emitted during the creation of quicklime is reabsorbed by the lime render over years as it chemically converts back into limestone.

In contrast, the manufacture of cement accounts for 8% of human global emissions. When compared to the aviation industry which makes up 2% of CO2 emissions, there is a clear environmental impact.

Internal wall insulation

Internal wall insulation is often considered when external insulation is not possible due to the potential impact on a building’s heritage significance. Whilst this may be thought of as a more achievable option, internal wall insulation results in more technical risk. There is also more disruption to the building and there will be a reduction in room space (a particular consideration for smaller properties). Adaptation of services and electrical cabling, and the relocation of skirting boards, architraves, and other joinery, where it is able to be reused, will also be necessary.

The increased technical risk from internal wall insulation is due to the amounts of junctions and interfaces, and therefore, the increased potential for thermal bridging. This includes around windows, door surrounds, ceilings, and floors. This is a particular consideration when the building is part of a terrace or semi-detached; any internal insulation along a party wall can result in the build-up of moisture in the adjacent property.

Understanding the moisture movement in walls is important, and ensuring that new insulation does not affect it, is vital for performance and to avoid negative impacts to the building, and occupant health. Installation of all types of insulation needs to be well executed with correct and well managed details, but this is particularly amplified for internal insultation. This is due to the thermal bridging potential, as well as hygrothermal considerations, ventilation, and overheating risks. 

As with external insulation, natural materials such as wood fibre boards, wood wool boards, cork boards and wool are usually most appropriate for a building’s existing fabric and qualities. These systems come in a range of thicknesses that can be more appropriate for indoor spaces. A combination of materials, such as using slimline insulation around window reveals may be appropriate, however, as well as ensuring the materials are suitable for the existing fabric, there will need to be consideration of how the different insulations react with each other, and whether they are compatible.

Lime render can also have benefits for energy efficiency. Where this can be achieved, thermal properties can be increased by mixes with natural materials, including hemp and cork. 

Synthetic materials such as modern closed-cell foam, or other plastic-based insulations are often not appropriate for traditional buildings, due to the lack of vapour permeability. For closed-cell insulation, impermeable vapour systems, moisture is generally dealt with by vapour control layer. However, there are technical risks with the control layer as it needs to be installed precise and well to function correctly. Any gaps will result in increased risk. Occupants will also need to be notified on the use and maintenance of the system applied. Any fixings, changes to electrical services or general damage from wear and tear can result in penetrations to the vapour control layer resulting in a build-up of condensation and moisture in those locations.

Cavity wall insulation

Cavity walls started to be constructed from the late nineteenth century and if a building dates after 1919, it is more likely to be constructed with a cavity wall. Due to the experimental stage of these cavity walls, they can vary in thickness and the use of ties. Insulation to cavity walls can be an option for these buildings that will have minimal impact on their character and external appearance.

As with all changes to building fabric, there is a technical risk, due to hygrothermal considerations.  Being able to manage the risks and monitor the insulation is key.  Any failings in the material will be difficult to identify due to the fact the cavity insultation is concealed. Further information on the risks, particularly due to moisture, have been highlighted by the IHBC and BRE: Solid wall heat losses and the potential for energy saving

Monitoring

For all types of insulation, a program of ongoing, post installation monitoring is key to ensuring that there are no performance issues. Embedding moisture meters into the walls as part of the installation process will assist with this and is the best way to manage the monitoring process.

Dealing with issues

If the building has already had wall insulation installed and is experiencing performance issues or deterioration, further information on how to deal with this can be found here: Energy Saving Trust: Insulation problems, Department for Business, Energy & Industrial Strategy: Cavity wall insulation (CWI): consumer guide to issues arising from installations, Property Care Association: Cavity wall insulation,

Further information on insulating walls

Solid Walls

Historic England’s Insulating solid walls

BRE Reducing thermal bridging at junctions when designing and installing solid wall insulation

STBA and Bristol City Council, A Bristolian’s Guide To Solid Wall Insulation (Whilst the guidance is specific to typical buildings within Bristol, this is comprehensive guidance that also covers solid walls and therefore, the principles are still relevant)

Energy Saving Trust, Solid wall insulation

Department for Business, Energy & Industrial Strategy, Retrofit internal wall insulation: best practice

STBA, External Wall insulation

STBA, Internal Wall insulation

STBA, Frame infill insulation

Timber-framed walls

Historic England’s Insulating timber-framed walls

Robert Demaus, Insulation in Timber-framed Buildings

STBA

Cavity Walls

Historic England’s Early cavity walls

Energy Saving Trust, Cavity wall insulation

STBA Cavity Wall Insulation