Thermal bridging

Modelling bespoke Psi values at a residential development in Brampton, led to a 68% improvement in thermal loss calculations. Photo credit: Vistry Partnerships

When the proposed changes to Part L come into force, using generic or default Psi values is unlikely to be enough to comply.

“The amount of carbon savings from calculating bespoke Psi values will very often be cost-effective,” explains Brandon Wipperfurth, senior sustainability and energy consultant at Darren Evans.

“It usually pays for itself in savings elsewhere in the building. Plus, the more buildings that use these bespoke values, the more effective each individual Psi value becomes.”

There are two types of thermal bridging:

  • Repeating thermal bridges: these are almost always accounted for in U-values themselves – timber-frame stud structures, for example.
  • Non-repeating thermal bridges: are where building elements come together such as walls and floors, walls and roofs and around windows.

Psi values are the numerical values used to measure the amount of heat loss from these junctions around a building. They are a critical factor in passing SAP assessments.

“There are always Psi values that are cost-effective to complete,” says Brandon.

“For example, for £3,000, if you have five units, doing bespoke Psi values is a no brainer. It’s very cost-effective. You can get often get 3-5% carbon savings against generic Accredited Construction Details.”

We work often work with architects to help design or amend the detailing to make the Psi values more efficient and help maximise the SAP benefit.

If we’re involved early enough, we can also identify any buildability or condensation issues that may be relevant with the detail.

“We’ll always find 5-10 Psi values that are worth completing to make the building more energy-efficient,” says Brandon.

“I’ll always make sure you’re not wasting money. For example, if you request 15 details, I might say it’s not worth calculating these three, so we’d only calculate 12. This saves you money and is more efficient. We’ll only calculate the Psi values that matter.”

Top tips

  1. Calculated psi values can be used on subsequent sites Once a set of Psi values are calculated you can use these repeatedly on subsequent sites that use the same construction details, making them extremely cost effective and helping to reduce the pressure on the specification to meet Part L standards
  2. One balcony junction can account for 8% of a building’s carbon footprint For example, if you’re using an E23 balcony with a slab and thermal break, knowing the specifications of the thermal break is very important.“E23 junctions should be calculated every single time without fail”, explains Brandon.“If you use the default E23 Psi value in SAP, this one junction alone can inflate the buildings carbon footprint by upwards of 3-8% depending on its overall length. It’s extremely powerful and important to get this calculated.”
  3. Save material costs by calculating Psi values as early as possible Budgeting for Psi values early on in the process enables you to calculate a more accurate representation of the building’s fabric performance, which helps to prevent overspecification.For example, if you can achieve 5-8% of carbon savings from calculating bespoke Psi values, you might be able to save material costs on-site by reducing insulation thicknesses in certain locations around the building.For larger sites psi value calculation costs can be as low as less than £5 per junction per plot when averaged over the number of dwellings across the site.

Watch out for: 

Proposed changes to Part L mean that Accredited Construction Details will soon no longer be allowed in England and Wales. “This means there’s likely to be an additional 3-4 Psi values worth completing as part of the SAP Calculation process,” says Brandon. “Remember, once the detail is done, you can potentially apply it across multiple sites. You purchase the intellectual property on the detail itself, not where it is. This enables you to achieve economies of scale. The more units benefit from bespoke Psi values, the more cost effective the Psi values become.”
Get in touch

Next steps

  1. If you would like to chat with Brandon or one of the Darren Evans team, don’t hesitate to get in touch.
  2. We’ll always advise you on the Psi values for your project to ensure your calculations are as thermally efficient as possible.
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