To design energy efficient buildings, it is crucial to understand how heat loss occurs. While the insulating properties of walls, roofs, and floors (U-values) are a major factor, a significant amount of heat is also lost through the junctions where these elements meet. This is known as thermal bridging.
The Psi Value
The Psi-value, or linear thermal transmittance, measures the additional heat flow that occurs at the junction between building elements, over and above the heat loss through the adjoining plane elements. It is a critical factor in building energy assessments like SAP (Standard Assessment Procedure) and SBEM (Simplified Building Energy Model).
In simple terms, the Psi-value quantifies the extra heat loss from a thermal bridge. A lower Psi-value indicates better thermal performance and less heat loss. Accurately calculating Psi-values is essential for achieving compliance with building regulations and for designing buildings that are as energy-efficient in reality as they are on paper. AutoPSI helps you calculate these values precisely, ensuring your designs are robust and your energy calculations are accurate.
The fRsi Value (Temperature Factor)
The temperature factor, or fRsi, is a measure used to assess the risk of surface condensation and mould growth at a specific point on the internal surface of a building. It is a ratio that compares the temperature drop across the building fabric to the total temperature difference between the internal and external environments.
The fRsi value is a number between 0 and 1. A higher fRsi value indicates a warmer internal surface and a lower risk of condensation. Building regulations in many countries set minimum required fRsi values to prevent mould growth and ensure a healthy indoor environment.
AutoPSI calculates the fRsi value in accordance with the conventions set out in BR 497, helping you identify and mitigate potential condensation risks early in the design process.