Internal Wall Insulation

Significant internal features such as plaster cornices, picture rails, skirting boards and door architraves, will typically be affected by IWI. They will likely be either concealed or disturbed to accommodate the insulation. Historic details can rarely be replicated completely faithfully, and the authenticity of the building may, therefore, be compromised.

The effect of revised room proportions on their design needs to be considered. Changes to the internal character and appearance of a room can be compounded by the need to extend insulation back from the external wall onto party walls, other internal walls, floors and ceilings to reduce the risk of thermal bridging.

Listed building consent is almost always needed for IWI in a listed building. For further advice, refer to Historic England Advice Note 18: Adapting Historic Buildings for Energy and Carbon Efficiency.

There are three main approaches to IWI:

• adhering directly onto the wall
• installing a framed-out system
• applying insulating plaster

IWI is best adhered directly to the inner face of an external wall using a permeable lime-based parge coat, with a permeable finish also applied to the room side. Existing walls should be stripped of all impermeable finishes, including paint, and any materials that may support mould growth, such as wallpaper, before IWI is applied.

Alternatively, insulating materials may be installed between a timber or metal frame fixed to the wall. In such cases, it is prudent to also fix thin insulation boards to the inner face of the frame and a permeable finish on the room side, to minimise thermal bridging and to ensure no air or moisture can find its way behind the insulation build-up (to minimise interstitial condensation risk). Occasionally, if a building is particularly exposed to driving rain, the timber structure and insulation may be erected as a separate inner leaf with a ventilated cavity between the insulation and the existing wall. Both these installation methods need to be approached with caution, particularly regarding thermal bridging, thermal bypass, moisture movement and condensation risks.

Another approach is to use an insulating plaster, applied in layers directly onto the interior wall surface. Insulating plasters are typically lime based and they include materials such as aerated lime, cork, hemp, perlite or silica-based compounds. They can facilitate higher levels of airtightness, sealing junctions such as joist ends. There are some concerns about wet applied systems incorporating natural fibre insulation in damp and drying conditions.

A well-installed air and vapour control layer (AVCL) can help to improve airtightness and control air movement and vapour diffusion in IWI assemblies. Wet systems such as lime/earth plasters or insulating lime plasters may have the added benefit of regulating both water vapour and liquid water transport, further reducing the risk of moisture accumulation and mould within the wall system.

It may be possible to use a combination of these three approaches, such as installing an insulating plaster finish over a natural fibre IWI system.

Care needs to be taken with the design and installation of IWI to avoid thermal bridges, particularly at the reveals of windows and doors and wall/floor junctions. It is also often necessary to relocate services (radiators and associated pipe runs, electric power points and light switches) and adjust skirting boards and door architraves. IWI may reduce the floor area of a room or corridor, sometimes to the extent that it cannot be used as before.

It is important that the insulation and protective finish installed internally are compatible with the existing wall construction. They should not cause moisture to accumulate nor hinder the wall's drying capacity. Generally, this is achieved by using permeable insulating materials and finishes. Specific requirements of permeability in relation to the building and its context will vary from case to case. In all cases, it is essential to carefully consider how to prevent vapour from the warm internal air condensing on the cold side of the insulation, on the external solid wall or within the insulation wall material itself.

Recent research by the Department for Energy Security and Net Zero (DESNZ) 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 shows 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 0.3W/(m²·K), the [interstitial condensation] risk was reduced while still receiving two-thirds of the energy savings".

Many of the potential moisture risks associated with IWI can be reduced considerably if the external facade is in good condition and protected from water ingress (for example, by applying appropriate lime render or weather-screen cladding). The use of hydrophobic surface treatments is not recommended for buildings of traditional construction. Research is under way to better understand the impact of hydrophobic surface treatments, but current evidence does not support their use. Where wind-driven rain exposure is high and/or solid walls are thin, IWI may not be suitable because materials may degrade in the face of moisture accumulation or lose their thermal insulating properties when saturated.

In extreme exposure cases, a ventilated cavity between the IWI and the original wall can prevent the passage of wind-driven rain to the inner layer of the assembly. This cavity should be ventilated to the outside to prevent condensation within the cavity and to help any water that penetrates the thickness of the wall to evaporate. However, there is a risk that there will be insufficient air movement within the cavity and that adding vents may alter the building's character or appearance. Any such cavity should not be ventilated to the inside of the building, as the air movement would simply cause thermal bypass of the insulation. This would render the IWI ineffective and risk warm moist air condensing on the external wall where it cannot be seen.

Learn more: Properties of Traditional Building Construction.

Installing IWI may reduce wintertime heat losses to the external environment, but it may also isolate the internal environment from the benefits of the innate hygrothermal buffering properties of solid walls, potentially increasing the risk and severity of summertime overheating. These benefits and risks need to be carefully assessed in relation to the thickness of the solid wall construction. Existing heat transfer properties (thermal resistance and conductivity, mass, inertia and diffusivity) of the external walls and those of any proposed insulation materials should also be thoroughly considered.

Opportunities for nighttime purge ventilation need to be identified to ensure the building can effectively regulate the internal temperature in relation to external fluctuations. Nighttime purge ventilation will improve thermal comfort and potentially reduce the overall energy demand from both heating and cooling.

Separating the building's existing fabric from the internal heat gains by installing IWI will increase the risk of the building being excessively cooled in winter. In turn, this may increase the risk of moisture accumulating and freeze/thaw damage.

Learn more: Properties of Traditional Building Construction.

Adding any insulation to a building will likely impact the air exchange pathways.

Most IWI systems that carry manufacturers' guarantees or warranties require the use of AVCLs. However, effective vapour control is very difficult to achieve in practical terms, particularly when using AVCL membranes in an existing building . It is challenging to install an AVCL without leaving pathways where air may leak, for example at joints and perimeter junctions, or where joists or services penetrate the wall. AVCL membranes can also be easily damaged during construction works or by building users. The longevity and performance of tapes and adhesives used in such systems are currently under investigation.

Another effective way to control air and vapour in a building of traditional construction is to apply a continuous permeable internal plaster, such as lime. This method creates a more continuous system that cannot be undermined by the failure of tapes or adhesives. Permeable internal plaster is less easily damaged during construction works or by building users, and is more easily maintained.

Learn more: Properties of Traditional Building Construction.

In all cases, it is important to understand the likely effects of IWI proposals at the design stage. The aim is to avoid damage to both new and historic building fabric, and to establish the potential impact of IWI on the internal environment and the occupants' health and wellbeing.

For a building of traditional construction, the need to use compatible materials ordinarily precludes the use of modern closed-cell foam and other plastic-based insulations. Protective finishes that block moisture movement (in either liquid or vapour form) are also inappropriate.

New insulation products are continually being developed, particularly those that have a very minimal thickness (about 5–10mm). The benefits of these products as far as reducing overall energy consumption can sometimes be quite small, and relatively expensive to achieve. Furthermore, their appropriateness regarding permeability and both liquid and vapour transport is still being investigated.

Some IWI materials can be applied as a single coat, such as certain insulated lime renders. These are generally water vapour permeable, but their performance in relation to liquid water movement is currently being researched. Certain insulated lime renders can, however, be used in circumstances where other types of internal insulation would be detrimental to the character of a historic building or where exposure to wind-driven rain or flooding precludes the use of natural fibre insulation.

IWI has the potential to make a room feel more comfortable by reducing radiant heat loss, raising the surface temperature of the walls and so reducing the risk of condensation occurring on decorated surfaces.

Learn more: Risks of Energy Efficiency Interventions in Buildings of Traditional Construction.