This calculation assesses the amount of energy or heat transferred to the outside via the building envelope, air infiltration and ventilation.
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It is normally used to size the heating system that is required to avoid frost and keep a comfortable internal temperature, taking into account, the building envelope performance, air infiltration, ventilation and heat gains, such as people, sunlight, lights and appliances.
Ideally it is required to have a floor plan with dimensions including windows and doors.