In low-energy buildings in climates with a heating demand the entire building envelope has to be well insulated. The building envelope consists of all the building elements which separate the inside from the outside. Its main purpose is to provide for a comfortable indoor climate – irrespective of the outdoor climate which is determined by the weather.

During cold periods (typically from the middle of October to the end of April in the winter cold climates) the temperature inside the building envelope is usually higher than it is outside. As a result, heat is lost through the envelope and, unless this heat is replaced, the inside of the building cools down adjusting to the outdoor temperature. The inverse applies for hot climates (or during hot periods) with excessive heat entering the building through its envelope. Therefore, it makes sense to restrict the heat flow in any building irrespective of the climate – and this is where thermal protection comes in.

→ Good thermal protection can be achieved for all construction methods and has already been successfully implemented in solid construction, timber construction, prefabricated building elements, formwork element technology, steel construction and all types of mixed constructions.

→ An appropriate level of insulation can also be applied to existing buildings at any given point of time.

Detailed information on Low Energy Building insulation


The heat losses though a standard building component, i.e. external wall, floor, top floor ceiling or roof, are defined by the U-value or overall heat transfer coefficient (formerly k-value). This value indicates the rate of heat transfer through a specific component over a given area if the temperature difference is one degree (1 Kelvin). The measurement unit of the U-value is therefore “W/(m²K)”. The smaller the U-value the better the level of insulation.

To calculate the heat loss through a wall, one must multiply the U-value with the area and temperature difference. In Iran, the average temperatures measured during severe winter periods are –۱۵ °C outside and 21 °C inside.
To calculate the annual heating losses, one must multiply the U-value with the average temperature difference in the heating period with the duration of the heating period, or in other terms, multiply the U-value by the heating degree hours.

A typical Lower Energy Building compact heating system can provide a heating power of about 1,000 W (that’s the typical output of a hair dryer). The U-value of a Lower Energy Building wall needs to be quite low; otherwise a considerable portion of this power would be used up by the external wall: For typical Central European buildings, U-values of Lower Energy Building walls should range between 0.10 and 0.15 W/(m²K); depending on the climate, these figures may be somewhat higher or lower.

Insulating materials

Such low U-values can only be achieved with very well-insulating materials. The following table shows how thick an external building element, consisting only of the material specified, should be in order to achieve a typical Passive House U-value of 0.13 W/(m²K).