Heat losses can be divided into two parts: Transmission losses via parts of the building (roof, walls, etc.) and ventilation losses.

Transmission losses
The size of transmission losses varies according to the areas of the building parts and insulation. The losses are proportional to the temperature differences between indoor and outdoor air.

Ventilation losses
The ventilation in a building is either mechanical or of the natural type. Mechanical ventilation most often consists of a supply and exhaust air unit that makes heat recovery possible. Natural and involuntary ventilation consists of thermal currents causing warm air to rise and leak through openings and unsealed areas.

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