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:
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.”
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.”
