Lesosai is software for the certification and thermal balance calculation of buildings containing one or more heated or cooled zones. It was created in 1984 and continuously imporved since then. The software can be used in the following languages: French, German, Italian and English. It can use many calculation methods, which can be selected one after another. (see “Countries and Standards” below). It is available from http://www.lesosai.com/ Lesosai allows the calculation of environmental impacts of the energy consumption, taking into account the energy used in the building, but also the building’s construction materials (Life Cycle Impacts Analysis / Environmental Assessment). This calculation is based on a building life cycle approach, and uses the list of impacts maintained by KBOB (extracted from the EcoInvent database), and the methodology conforming to Swiss Draft Standard SIA2032 (Life Cycle Impacts Analysis complete with ECO+® module). To calculate the heat transfer coefficient (U-value) we have integrated the software USai, which allows the creation of constructions and materials while controlling condensation. It also allows you to synchronise the database with those of the many materials producers who participate in the materialsdb.org project. To simulate buildings anywhere in the world, Lesosai includes the meteorological data generator, Meteonorm®. Lesosai can also calculate required heating power, follwing EN 12831 and SIA 384.201 by zone or by room. Lesosai offers the possibility of calculating the thermal inertia of the buidling by introducing layers of walls (calculation possible under the standard SIA382/1).