External or Internal Wall Insulation – Which is the best solution for the thermal upgrade of existing solid masonry walls?

By Steve Duke: Sales Director (Refurbishment & Contracting) – Knauf Insulation Northern Europe

There is an ongoing debate about which is the best solution for the thermal upgrade of existing solid masonry walls. Which is the optimum solution, external wall insulation (EWI) or internal wall insulation (IWI)? The answer could be both, neither, or the installation of both systems as a hybrid solution. The truth is that insulating solid walls can be complicated and the relative merits of the available options should be considered on a project by project basis.

The new Energy Company Obligation (ECO) scheme will bring much more attention to this issue as it focuses heavily on the delivery of solid wall insulation measures. Therefore, it is anticipated that there will be a significant increase in demand for solid wall insulation (SWI) over the coming years.

The Green Deal, offers a new way of financing these types of measures, with a loan tied to the property rather than the individual, although it remains to be seen how successful this will be without any further nudges from the Government (such as stamp duty incentives for properties that have been upgraded).

Most people tend to think of SWI as being external wall insulation (EWI) and either over look, or are unaware of the internal wall insulation option. Knauf Insulation provides systems for both options.  A range of EWI solutions are available as part of its ThermoShell EWI system and incorporate either expanded polystyrene (EPS) board or rock mineral wool slabs, which are mechanically fixed to the substrate  (with supplementary adhesive), mesh-reinforced render coat and render finish, and are  suitable for use on new or existing domestic and non-domestic buildings. It is fair to say that EWI is the most widely used solution and the EPS system is often considered to be the entry-level solution that offers a good thermal upgrade, but is inferior to  rock mineral wool solutions due to the fact that rock mineral wool is:

  • Non-combustible
  • Has excellent acoustic performance
  • Is a breathable insulation, which is particularly important when upgrading existing buildings

Will the impact that the Green Deal is expected to have over the next few years – lead to the preference of one system over another?

Internal Wall Insulation (IWI) is now being used more frequently as it offers a number of advantages over EWI systems. It is generally more cost effective, there is no requirement for scaffolding, it can be installed in all weather conditions and can be installed on a room by room basis.

The ThermoShell IWI system also provides acoustic benefits as it incorporates a glass mineral wool slab which provides a high level of sound absorption. Knauf Insulation recently carried out one of the largest IWI installation projects in the UK with housing association, NG Homes, in Glasgow. One tenant commented that they were impressed by how much quieter their home was, as the system had significantly reduced the level of road traffic noise that could be heard within the property. A number of comments were also made about the flats warming up much more quickly when the heating was switched on.

It is important to recognise that there are advantages and disadvantages for both IWI and EWI systems. This is highlighted when there are restrictions on what can or cannot be installed on an existing property. For instance, an ideal solution for terraced houses is to install a “hybrid” solution comprising of IWI to the front elevation and EWI to the rear elevation. If we look at this issue from the perspective of planning departments and the local population, will the importance and benefit of maintaining the appearance of the built environment lead to this solution becoming the norm?

The new ECO scheme also allows funding to be claimed by social housing providers when installing insulation measures in empty properties, provided that a tenant is able to move into the property before the notification deadline for the measure

Knauf Insulation has a dedicated ThermoShell Training Academy in Birmingham which enables existing trades to up skill in order that they can install our innovative ThermoShell IWI and EWI systems.  Government initiatives are placing an increased emphasis on upgrading the thermal performance of the existing 6.6 million solid wall properties in the UK. Therefore, Knauf Insulation’s academy is an ideal route for tradesmen of all disciplines who are looking to take advantage of the commercial opportunities created by the Green Deal. We also run an approved contractor scheme to manage the quality of installation, with all installing technicians being inspected on a regular basis to maintain high standards of installation.

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