Thermal Bridging – what’s the value of a y-value

Thermal bridging of construction elements has become a very important part of the compliance of housing in the UK, but you would never know this if you worked for a contractor, developer or architect designing, planning or building the said housing. Despite this being an important factor since 2010, the level of knowledge and understanding in the industry is still very low.

For new readers, thermal bridging is expressed as a ‘y-value’ or amount of heat loss stemming from the aggregated thermal bridges in a design. The thermal bridges occur at every junction in the building, every wall-to-wall connection, wall-to ground connection, wall-to-roof connection, all jambs, cills and heads, and all balconies, projections or bays. Each junction has its own ‘psi-value’ and these are aggregated together to make a ‘y-value’. In order to assess the performance of the building, all the junctions have to be measured and a value assigned to them.

This calculation is carried out using specialised software that calculates the rate of heat movement, or heat flux, through the junction.

To summarise the regulations, there are three options for compliance under UK regulations regarding thermal bridging.

  • take a default value
  • use accredited details or other approved calculations
  • use calculated details where individual junctions are calculated by a SAP or other qualified assessor

The impact of this can be easily illustrated. If we take a typical mid floor apartment, where there is a simple envelope, and two large openings in the façade, one for a bedroom and one for a living room. Where all else is unchanged, the difference between the default value and a calculated value based on accredited details is 0.5% saving of the DER, but when the lintels are changed to a thermally broken type, the saving increases to 3%. If this is the impact on a single mid floor apartment with a very simple envelope, the impact will be greater in a detached dwelling with many more openings.

This demonstrates that changing the manufacturer for a single junction can change the overall performance by 3%. This is the type of change that contractors are used to making on the fly as they make purchasing decisions about the project. They are not used to being tied to a more detailed performance specification that makes such a difference to the compliance of their project.

This lack of knowledge was typified on a project recently where the main contractor substituted one timber frame manufacturer for another, without checking whether the new manufacturer could match the thermal bridging characteristics of the original. The SAP Assessor had used the psi-values of the original manufacturer in the design stage SAP. The replacement manufacturer not meet the specification and they could not supply any information on the thermal bridging of their product at all. They had never been asked for it on previous projects.

This baffles me. Timber frame construction is a naturally thermally efficient construction method, for a company to sell such a system without providing calculated thermal bridging information is to hamstring their own performance for the sake of a few thousand pounds.

The companies who are paying attention to the changes in the regulations are taking the steps to have their psi-values of their products tested in typical constructions and they are using that information to make sales to contractors earlier in the process than usual, bringing themselves into the design conversation at an early stage of the project. By doing so they are getting themselves and their products written into the performance specification for the project. Whether it is thermally broken lintels, high performance windows, better performing insulants, or aerated blockwork, there are a lot of products on the market now which rely on this focus on thermal bridging.

The concern I have is that there is a lot of evidence in the industry that site teams and purchasing teams are not picking up on this need to watch the performance of the envelope, and they are substituting products to save money without paying attention to the psi-values of the materials they are buying. To be fair to them , the design teams need to be more explicit in the information that is provided to them so that it is more obvious to them what products need to be installed to be compliant. It is not reasonable to expect buyers to read through a SAP calculation, so a schedule of psi-values needs to be provided to them with an explanation as to how they were derived and why.

In the long run this focus on the fabric of dwellings can only be a good thing, as this is the best place to find savings in energy that are likely to be sustained in the long term life of the building.



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