Psi

Psi, also written with the Greek letter ψ has many different uses including in psychology, quantum mechanics and computing as well as in the design and construction of buildings.

In terms of building design the Psi or Ψ value is the property of a thermal bridge junction between two elements and expresses the rate of heat flow per unit length of at thermal bridge. It might be considered as an adjustment figure for performance and is considered when calculating or allowing for U values and het loss in building fabric.

Psi should not be confused with PSI, which might also be found in the construction field and stands for pounds per square inch (PSI). PSI is a measurement of pressure in the Imperial system of measurement, commonly used to measure the pneumatic pressure of gasses or the hydraulic pressure of liquids. PSI is also used as a measure of tensile strength, as a resistance to pulling forces, and elastic modulus strength, defined as resistance to deformation ie the stiffness of materials.

NB Part L 2021 Where to start, A guide for housebuilders and their advisors, Masonry construction, published by the Future Homes Hub in June 2022, states Psi-value or linear thermal transmittance is: ‘the measure of heat loss along a non-repeating thermal bridge calculated as per guidance in BR 497 (2nd edition 2016) and IP 1/06 (2006); expressed in terms of W/mK.’

See also: Linear thermal transmittance.

[edit] Related articles on Designing Buildings

• Air tightness in buildings.
• g-value.
• k-value.
• Heat transfer.
• Insulation specification.
• Limiting fabric parameters.
• Linear thermal transmittance.
• Thermal bridge.
• U value.