Thermal transmittance U-value (or global K factor) is defined (ISO7345) as the thermal flow crossing a unit area in presence of a temperature difference (1°K) between indoor and outdoor environments (in stable conditions).

The thermal transmittance is the global amount of heat moving from indoor to outdoor through a surface (wall). This value expresses the wall insulation capacity and can be used to define the energetic efficiency of buildings. The value is also required by the Euro-pean directive 2002/91/CE concerning the energy performance of buildings.

LSI LASTEM designes and supplies a compact and easy to use system for the measurement of the as-built thermal transmittances (U-values) in compliance with ISO 9869-1:2014 “Thermal insulation — Building elements — In-situ measurement of thermal resistance and thermal transmittance — Part 1: Heat flow meter method”.

Each measurement spot includes one data logger, two outside face temperature sensors, one inside face temperature sensor and one inside face heat flow sensor. Measurements are collected on the data logger memory and downloaded on PC where, using InfoFLUX program, thermal conductance can be obtained using two methods: “average method” (ISO9869:1994 standards) and “black box method”.

A temperature difference of at least 10°C between the two faces of the wall is needed throughout the measurement. When this difference remains stable during the time it is possible to obtain a stable thermal flow between the wall. In good conditions the thermal transmittance result can be reached in 5-10 days of measurements. If data downloading is carry out during the measurements, using the InfoFLUX program it is possible to understand when the result is completed because the chart shows when conductance value is near to the horizontal asymptote, in this situation the thermal transmittance can be fixed.