External Wall Insulation

EWI often significantly alters a building's character and appearance, even if that building is already rendered. Decorative architectural features, such as cornicing, string courses and window surrounds, may be affected. The deepening of window and door reveals, and the adjustments to eaves line, can also change a building's appearance. To minimise the risks of creating cold bridges at the reveals, windows and doors may need to be relocated further forward in the overall wall thickness. However, this is not always feasible and practicable in a historic building.

EWI is unlikely to be acceptable in a listed building. The external appearance of a listed building is an important component of its special interest. However, exceptions may apply where a building's exterior has been severely compromised (for example, by inappropriate renders or unsympathetic alterations). Installing EWI in a listed building will typically constitute an alteration and so will require listed building consent.

Whether a building is listed or not, planning permission will be necessary for any changes to its external appearance. Under certain circumstances, EWI may be classed as permitted development, but the Local Planning Authority should always be consulted before any work commences, with that authority determining if permission is required. Building Regulations approval may apply depending on the extent of changes proposed to the thermal element.

For advice on permissions, such as listed building consent, refer to Historic England Advice Note 18: Adapting Historic Buildings for Energy and Carbon Efficiency.

There are three main methods for EWI:

• mechanically fixing insulation boards
• applying insulating render
• installing a framed-out system

One of the most common EWI approaches is mechanically fixing insulation boards to the outer face of an external wall. Usually, a protective render is applied directly to the insulation, but alternative surface finishes may include timber or slate cladding on battens, both of which often provide more robust protection against rain ingress. To reduce air movement and leakage, junctions should have no gaps and penetrations should be sealed, with a parge coat applied to avoid air gaps.

Insulating render directly applied to the external surface of a wall is another option. Typically consisting of a lime base, with natural fibres such as hemp, perlite or cork, insulating renders may be more suitable where a lower profile finish is required. With the correct materials and application, insulating renders can improve thermal performance while managing moisture balance.

A further option is to use a timber or metal framed system, with insulation between and over the studs, finished with cladding or rendering. Ventilation and drainage cavities can be incorporated into the build-up for a more robust assembly.

The increased wall depth created by adding an external render or insulation system will often necessitate other work, such as adapting roof and wall junctions, and window and door openings. Rainwater downpipes and any services fixed to the outside of the building may also have to be repositioned. The effectiveness of EWI can be improved by maintenance and repair, resilient rainwater systems, and traditional building details that direct rainwater away from the building and its openings, such as roof overhangs or hood mouldings.

It is important that the insulation and protective finish installed externally are compatible with the existing wall construction. They should not cause moisture to accumulate nor hinder the wall's drying capacity. Generally, compatibility is achieved by using permeable insulating materials and finishes. Specific requirements of permeability in relation to a building and its context will vary from case to case. While it is important to protect external insulation from rain, this should not be done in any way that will trap moisture, in either liquid or vapour form, within the wall.

Learn more: Properties of Traditional Building Construction.

Well-designed and well-installed EWI should robustly reduce heat loss to the external environment, while maximising thermal continuity and reducing risks of thermal bridging. It should also protect a building's existing fabric from water ingress and external cold temperatures in winter. EWI should not isolate the internal environment from the innate and beneficial hygrothermal buffering potential of solid walls.

When there are adequate opportunities for nighttime purge ventilation, a solid wall construction with EWI can help regulate the internal temperature in relation to external fluctuations, improving thermal comfort and potentially reducing the energy demand from both heating and cooling.

Learn more: Properties of Traditional Building Construction.

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

Most EWI systems that carry manufacturers' guarantees or warranties require the use of air and vapour control layers (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 walls. 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 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 EWI 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 EWI on the internal environment and the occupants' health and wellbeing.

For traditional buildings, 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. However, EWI materials will need to be protected from both weather and mechanical damage (impact or abrasion) to differing degrees. Suitable moisture-permeable finishes include lime renders and rain-screen cladding (weather boarding, tile hanging and so on) with lapped joints.

Some EWI 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. They can, however, be used in circumstances where other types of external insulation would be detrimental to the character of a historic building.

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