Psi Values and Why they Matter

by scott douglas

The word Psi comes from the Greek Alphabet, and the standard pronunciation in the UK, we drop the "P" so it is pronounced "Si", for example, (p)si values.


Heat Loss and Psi Values

Psi Values in your Building are becoming more critical as we work toward the UK's Net Zero Target.

So, what is a Psi Value?

Psi Values measure Heat loss at junctions in your Building. A junction is typically where different construction details meet, and this could be;

  • Ground Floor > External Wall

  • External Wall > Balcony

  • Lintels

  • Sills

  • Windows

  • Door Jambs

Psi Values are different to your U Value.

U Values measure heat loss through the thermal elements as opposed to the Buildings Fabric, such as Windows and Walls.

Heat Loss around Junctions is also known as a non-repeating thermal Bridge.

Thermal Bridges are areas which have Higher Heat Transfer impacting energy performance.

The Psi Value is calculated using Software which measures W/mK ( Watts per metre Kelvin).

This measures the heat flow per unit at a Junction, and like U Values, the lower the reading indicates less heat loss.

Why are Psi Values Important?

  • Measure a Junctions Heat Loss

  • Creates Energy Efficient Homes

  • Meets SAP Calculations & Building Regulations

What are the Benefits of Psi Values?

Super low psi values could mean very low energy loss and provide continuous insulation in both Winter and Summer Months.

In addition to its Occupiers being more Comfortable all year round, the Building has a better chance of achieving compliance / SAP Assessment and the need for more thermal modelling.

As the Fabric of our Buildings becomes more critical and new Building Regulations are introduced, Innovative Manufacturers are developing more Building Elements to assist with thermal Bridging, including Lintels and New Insulation around the junction Products.


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