Green building will be at the heart of the UK’s action on climate change following the United Nations Climate Change conference in Paris last week.

Developers are already under pressure to reduce carbon dioxide emissions in new homes in line with the EU target for 2020 but something that is often overlooked is the impact of our existing homes.

Given that the majority of housing stock in use by 2020 will already have been built it is essential to turn our attention towards improving the energy efficiency of these homes. However, that is easier said than done with many retrofitting projects leading to a high risk of unintended consequences, often moisture related.

Colin King, director of BRE highlights the issues we need to be aware of during retrofitting and how to minimise these risks.

Challenge 1: Consistency

The overriding issue with current working practices is an inconsistent approach to assessment, checking and quality control. This is chiefly due to a lack of knowledge and understanding in a number of key areas:

  1. Initial surveying and assessment – to identify the best insulation system
  2. Design - of the insulation system and its ability to deal with non-standard junctions and detailing.
  3. Best practice – the workforce’s understanding of the importance of following key principles of best practice.

As such it is important to develop a greater understanding in the following four areas of a retrofit project:

  1. Building elements  must be surveyed before insulation works are undertaken
  2. Impact of cold or thermal bridging and the importance of minimising this
  3. Importance of attention to detail and specification on site
  4. Impact of saturation on; conductivity, condensation risk, temperature gradients, cold spots and convective looping.

One way to achieve this is through targeted and informative ‘toolbox talks’ directed at the operatives on site to ensure they understand the principles of best practice before each project. Any key elements of the insulation system should be clearly explained to installers.

There needs to be more robust and evidenced quality control by well-trained staff, with communication between site installation and quality control to ensure good practice is reinforced and bad practice not ignored. The creation of more robust details would also help to reduce over-reliance on sealants and workmanship.

Attention to detail is particularly vital when undertaking externally fixed external wall insulation, or over cladding to ensure cold bridges are not built into the structure. The crucial elements are:

  • At floor slab/plinth, eaves and roof line details where the roof line has not been extended
  • Where there are two-dimensional junctions around openings
  • Where the thickness of the chosen insulation results in a step where none existed before.
  • When installing external features such as incoming service mains, utility boxes, satellite dishes, telephone junction boxes and other services that abut the wall.

Challenge 2: Losing heat

If the effect of thermal bridging is not taken into consideration when calculating potential heat loss from a building, overall heat loss could be underestimated. A secondary factor is the development of physical problems such as mould growth caused by condensation at the point of the bridge.

To limit the risk of surface condensation at junctions, the temperature factor (the difference in temperature between the internal surface and its surrounding surfaces) should be greater than or equal to the critical value (fCRsi) of 0.75, as quoted in the BRE document FB 61.

When assessing the effects of thermal bridging at a junction, it is important to understand the impact on the temperature factor. Latest research indicates that when reveals and penetrations are not insulated, the temperature factor and subsequent cold bridge is actually worse than before insulation was applied, contributing to significant heat loss.

Cold bridging is the cause of 19% of heat loss through the building envelope so it should not be ignored.

There is an understandable caution around this area, with the desire not to bridge the damp-proof membrane or course. However, suppliers and designers are looking to introduce details that can address cold bridging without compromising these crucial elements.

Addressing these unintended consequences is an essential part of meeting our carbon reduction targets. Only by understanding the risks and designing them out will it be possible to achieve the predicted savings, and ignoring the risks posed by this poor detailing comes at a cost that can far outweigh any uplift in capital cost at the start of a project.

Colin King
Director of BRE