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
ECA, JIB and JTL back Fabian Society call to invest in skills for a stronger built environment workforce.
Women's Contributions to the Built Environment.
Calls for the delayed Circular Economy Strategy
Over 50 leading businesses, trade associations and professional bodies, including CIAT, and UKGBC sign open letter.
The future workforce: culture change and skill
Under the spotlight at UK Construction Week London.
A landmark moment for postmodern heritage.
A safe energy transition – ECA launches a new Charter
Practical policy actions to speed up low carbon adoption while maintaining installation safety and competency.
Frank Duffy: Researcher and Practitioner
Reflections on achievements and relevance to the wider research and practice communities.
The 2026 Compliance Landscape: Fire doors
Why 'Business as Usual' is a Liability.
Cutting construction carbon footprint by caring for soil
Is construction neglecting one of the planet’s most powerful carbon stores and one of our greatest natural climate allies.
ARCHITECTURE: How's it progressing?
Archiblogger posing questions of a historical and contextual nature.
The roofscape of Hampstead Garden Suburb
Residents, architects and roofers need to understand detailing.
Homes, landlords. tenants and the new housing standards
What will it all mean?
The Architectural Technology podcast: Where it's AT
Catch-up on the latest episodes.
Edmundson Apprentice of the Year award 2026
Entries now open for this Electrical Contractors' Association award.
Traditional blue-grey slate from one of the oldest and largest UK slate quarries down in Cornwall.
There are plenty of sources with the potential to be redeveloped.