The U-value (also called thermal transmittance) is a measure of how well a building element (like a wall, roof, floor, or window) conducts heat.
It tells you how much heat (in Watts) passes through 1 m² of the material for every 1 °C temperature difference between inside and outside.
Units: W/m²·K
In simple terms: the lower the U-value, the better the insulation.
Why U-Values Matter in Heat Loss Calculations
Foundation of heat loss calculation: U-values are used to calculate fabric heat loss through each part of the building envelope.
Accurate sizing: Correct U-values ensure your heat pump is neither undersized (risking comfort) nor oversized (higher costs and inefficiency).
MCS compliance: From June 2025, heat loss calculations must follow BS EN 12831-1:2017, which uses U-values as the basis for transmission losses.
Subsidy eligibility: Getting U-values right helps maintain audit-ready records for MCS certification and government schemes like the Boiler Upgrade Scheme.
How to Use U-Values in autarc
Use defaults or custom values:
Our software includes default U-values based on building age bands (helpful when exact data isn’t available).
If you have measured or specified U-values (from EPCs, SAP data, or manufacturer specifications), enter those directly for higher accuracy.
Consistency check: Make sure the U-values match the construction type and insulation levels. Outliers should be double-checked.