This International Standard provides methods of calculation of heat transfer coefficients and heat flow rates for building elements in thermal contact with the ground, including slab-on-ground floors, suspended floors and basements. It applies to building elements, or parts of them, below a horizontal plane in the bounding walls of the building situated — for slab-on-ground floors, suspended floors and unheated basements, at the level of the inside floor surface; NOTE In some cases, external dimension systems define the boundary at the lower surface of the floor slab. — for heated basements, at the level of the external ground surface. This International Standard includes calculation of the steady-state part of the heat transfer (the annual average rate of heat flow) and the part due to annual periodic variations in temperature (the seasonal variations of the heat flow rate about the annual average). These seasonal variations are obtained on a monthly basis and, except for the application to dynamic simulation programmes in Annex D, this International Standard does not apply to shorter periods of time.