The Regs: Three key considerations for successful retrofits

There is finally an awareness that we need to tackle the energy performance of our existing building stock and retrofitting energy efficiency measures (EEMs) is an increasingly common commission for architects. The focus is often on improving the U-value of existing walls and roofs and replacing doors and windows. All very positive but such schemes face a number of challenges. When retrofitting buildings of traditional construction there are three key considerations to ensure success and avoid the classic pitfalls: cold bridging, materials and ventilation. These are very much interlinked and even the best-intentioned schemes can end up with issues.

Cold bridges

A cold bridge is caused when a material within the building structure transfers heat at a significantly higher rate than the materials surrounding it. A typical example is the internal reveals of windows and doors, where, due to the depth of the frame, the wall thickness cannot be easily increased.

It can be tempting to leave this area uninsulated, but not only does that let heat out, it also increases the risk of condensation forming, due to the temperature differential. Therefore it is best to add at least a thin layer of insulation around the reveal itself. While this will not stop the heat loss completely, it will slow it down and reduce the risk of moisture condensing on the cooler surfaces.

Counter-intuitively, in some cases having less insulation and getting the cold bridge right can not only reduce the risk of condensation, but actually results in less heat loss than having thick insulation and poor cold bridges. Cold bridging cannot be eliminated entirely, but the time and expense spent on detailing these areas correctly is cost-effective, compared with dealing with condensation.

Materials

Traditional homes were designed to ‘breathe’, whereas many of the current products used in retrofit are impermeable. For example, some external wall insulation products are designed to keep moisture out of the building but can have the unforeseen consequence of trapping moisture created inside the building within its solid walls, meaning damp can develop.

So the materials specified need to be carefully tailored to the context of each building. In older buildings, this might mean choosing wood fibre or hemp-based products as an alternative to foam-based insulation. However, there’s no point investing in wood fibre and then covering it with an inappropriate material such as a conventional sand-and-cement render. Lime render is a breathable option that could suit this kind of external wall insulation project. Remember: every layer added should complement the others. It’s also important to do a condensation risk analysis to make sure there is no risk of interstitial condensation in the build-up.

Ventilation

Many existing dwellings are ventilated through air infiltration, rather than purposeful ventilation, and this helps them stay free from condensation. But EEMs can reduce infiltration and cause the dwelling to become under-ventilated. The new Part F of the Building Regulations now requires an assessment to be made to ensure that retrofit work does not reduce ventilation below an acceptable level.

Architects now have to demonstrate this as part of the Building Control application and can use the simple method shown in the table below. It requires the architect to write down all the energy efficiency measures that are proposed. In addition, they also need to consider measures that reduce ventilation – such as removing a chimney. Each of these is classed as major or minor and, depending on how many of each there are, you may be required to install additional natural or even mechanical ventilation to offset the effects.

To find out more see BSI standards PAS 2030 (installation, commissioning, and handover of retrofits) and PAS 2035 (retrofitting dwellings including how to assess for retrofit, identify improvement options, design and specify EEMs, and monitor projects). These also cover a comprehensive range of EEMs to improve insulation of fabric and airtightness; plus heating and cooling systems with smart controls, including low or zero-carbon technologies.

Geoff Wilkinson is managing director of approved inspectors Wilkinson Construction Consultants. www.thebuildinginspector.org