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
Featured articles and news
A case study and a warning to would-be developers
Creating four dwellings for people to come home to... after half a century of doing this job, why, oh why, is it so difficult?
Reform of the fire engineering profession
Fire Engineers Advisory Panel: Authoritative Statement, reactions and next steps.
Restoration and renewal of the Palace of Westminster
A complex project of cultural significance from full decant to EMI, opportunities and a potential a way forward.
Apprenticeships and the responsibility we share
Perspectives from the CIOB President as National Apprentice Week comes to a close.
The first line of defence against rain, wind and snow.
Building Safety recap January, 2026
What we missed at the end of last year, and at the start of this...
National Apprenticeship Week 2026, 9-15 Feb
Shining a light on the positive impacts for businesses, their apprentices and the wider economy alike.
Applications and benefits of acoustic flooring
From commercial to retail.
From solid to sprung and ribbed to raised.
Strengthening industry collaboration in Hong Kong
Hong Kong Institute of Construction and The Chartered Institute of Building sign Memorandum of Understanding.
A detailed description from the experts at Cornish Lime.
IHBC planning for growth with corporate plan development
Grow with the Institute by volunteering and CP25 consultation.
Connecting ambition and action for designers and specifiers.
Electrical skills gap deepens as apprenticeship starts fall despite surging demand says ECA.
Built environment bodies deepen joint action on EDI
B.E.Inclusive initiative agree next phase of joint equity, diversity and inclusion (EDI) action plan.
Recognising culture as key to sustainable economic growth
Creative UK Provocation paper: Culture as Growth Infrastructure.
Futurebuild and UK Construction Week London Unite
Creating the UK’s Built Environment Super Event and over 25 other key partnerships.
Welsh and Scottish 2026 elections
Manifestos for the built environment for upcoming same May day elections.
Advancing BIM education with a competency framework
“We don’t need people who can just draw in 3D. We need people who can think in data.”