Our Systems and Products:Sustainable Building Glossary
Psi-value (Ψ-value): The Ψ-value, which quantifies additional energy, is a correction value that is caused by linear shaped thermal bridges, and is not taken into account in one dimensional energy calculations with U-values. The magnitude of the Ψ-value depends on several values: The quality of the construction, the quality of the measurements, as well as the U-value which calculates the loss of heat of the undisturbed component. Hence, it is possible that a poorly insulating flat roof protection has a better Ψ-value than a well insulating window-wall junction, because in the case of the former, the calculation of the Ψ-value is done with exterior measurements through which part of the energy is already covered by thermal bridge energy losses by flat roof and wall measurements. Ψ-values can also be negative.