What is the heat loss area?
The heat loss area (Als) is the total area of all the separating constructions that enclose the heated part of a building: facades, windows, external doors, roofs and the ground floor. In Dutch it is called the verliesoppervlakte. “Loss” refers to the energy lost through that envelope: in winter, heat leaks out, and in summer, heat gets in. The term comes from the former Dutch EPC calculation (NEN 7120) and is still used in the current Dutch calculation method, NTA 8800.
What counts and what does not?
The parts of the envelope that border on the following count:
- the outside air (facades, roofs, windows and doors);
- the ground;
- a crawl space;
- an adjoining unheated space, such as an unheated garage or conservatory.
Separations from other heated spaces do not count, such as the party wall of a mid-terrace house. The area is measured on the outside, so including the thickness of the construction.
Link with BENG
In the Netherlands, the BENG 1 requirement (maximum energy demand in kWh per m² of usable floor area per year) depends on the ratio Als/Ag: the heat loss area divided by the usable floor area. The larger that ratio, the less compact the building and the more lenient the limit. Under the Dutch Bbl, the following applies to a ground-level dwelling:
- Als/Ag up to and including 1.5: maximum 55 kWh/m² per year;
- Als/Ag between 1.5 and 3.0: maximum 55 + 30 x (Als/Ag – 1.5).
Different limits apply to apartment buildings. A mid-terrace house has a low ratio because of its shared walls; a detached house or bungalow has a higher one.
Application and tips
- Compact design pays off: every extension, bay window or dormer increases the heat loss area more than the usable floor area. That is not prohibited, but the envelope must then perform better to achieve the same energy demand.
- Insulation and frames: because windows are part of the heat loss area, the choice of glazing and frames has a major influence.
- Have the calculation done by an energy consultant: for new build in the Netherlands, you must demonstrate with a BENG calculation according to NTA 8800 that you meet the requirements. Have it done early in the design, so you can still adjust the shape.
Related terms
- BENG (nearly zero-energy building)
- EPC (energy performance coefficient)
- Energy label
- Thermal bridge
- Usable floor area (GBO)
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