What are Psi values? – Can I Just Ask? | Ep.29

Psi values — written as the Greek letter ψ — are a measure of linear heat loss at junctions: the additional heat that escapes specifically at the edges and corners of a building, around window apertures, at floor-wall connections, and wherever two elements meet. They are distinct from U-values, which measure heat loss through the centre of an element, and from the correction factors applied for repeating thermal bridges like fixings or timber studs. Psi values capture what those other calculations miss — the additional conductive pathway that exists precisely because two elements join together at a junction, creating a geometry that is harder to insulate effectively than a flat plane.

In practice, Psi values are fed into SAP alongside U-values to give a more complete picture of a building’s overall heat loss. They are highly design-specific — move a stud, adjust the position of a sole plate, or change the depth of an insulation return at a window reveal, and the Psi value changes. This makes generic published figures of limited use; a Psi value is really only accurate for the precise junction detail it was calculated for. One practical benefit of wood fibre systems is that the way junctions are typically detailed — with insulation returned fully into reveals and corners — results in very low Psi values, which can allow a slightly higher U-value target in SAP while still achieving equivalent or better overall performance compared to a system with worse junction detailing.

In new build, Psi values have become the new frontier of energy efficiency. U-value performance has largely been optimised as far as it’s practical to go, and the meaningful gains now lie in reducing junction heat loss — particularly in Passivhaus and other super-low-energy standards where every watt matters. In retrofit the emphasis shifts somewhat: rather than chasing minimal heat loss at junctions, the priority is moisture safety. The question becomes whether the insulation has been returned sufficiently around window reveals and floor junctions to keep surface temperatures above the threshold where condensation and mould become a risk. In that context, Psi values are less about energy and more about protecting the building fabric.