Building in Use - Environmental aspects

Heating energy consumption of the building

The following picture shows the average contribution to energy losses made by the various elements of a properly insulated detached house:

The heating and cooling energy consumed by a building will depend on various factors:

Location of the building/climate:
The colder the climate, the greater the energy needed for heating although this can be reduced by the intensity of solar heat gains during the year.

Geometry of the building (size, shape, volume/surface ratio):
The smaller the building, the higher the specific heating energy consumption. But a simple shape (ideally a cube) and a high volume/surface relation (a big volume with a small surface) means lower heating energy consumption.

Thermal performance of the building envelope (U-values of walls, windows, roof, cellar)
U-values will depend on the type of wall construction. Building regulations vary from country to country and required values will depend on the local climate. The lower the U-values of external building elements, the lower the energy required for heating. It has recently been shown that solid clay brick walls can reach U-values as low as 0.20 W/m²K. Clay brick cavity walls and clay brick walls with additional insulation can in principle reach any required U-value by varying the insulation thickness. In many countries the trend is toward low energy houses (LEH, heating energy requirement approx. 40-60 kWh/m²a) or even passive houses (PH, heating energy requirement < 15kWh/m²a).

For these energy standards, the following U-values are necessary:

Especially in houses that have very low energy consumption it is important to take into account thermal (cold) bridges.

Thermal mass to use energy gains
When considering the internal environment of a building, it is important – especially in summer – to have sufficient thermal mass to store the solar energy absorbed by the construction (see also Living comfort and Internal environment). Thermal mass has a direct effect on the energy required for heating. Massive clay brick walls can store solar heat gains and radiate the energy out when it is needed, whereas lightweight constructions cannot exploit this energy or only small parts of it.

Ventilation of the building
The lower the energy consumed to heat a building, the higher will be the effect of heat losses through ventilation. For low energy or passive energy houses, this is a significant part of the total heat loss (i.e. over 50 %). In several countries, state of the art mechanical ventilation systems incorporating heat recovery are now common. These ventilation systems reduce the heating energy requirement by an average 20 kWh/m²a and when combined with monolithic external clay brick walls can help attain passive house standards.

Efficiency of the heating system
The total energy consumption of a building also depends on the efficiency of the heating system. Normally electric heating systems have the lowest efficiency; modern gas boilers or heat pumps have a high efficiency.

Lifestyles of householders
People’s lifestyles have a significant effect on overall thermal efficiency. Research has shown that careless actions and routines can triple the energy required to heat a building. Excessive ventilation, such as windows open all day even in winter, can negate the benefits otherwise associated with energy efficiency measures featured in the construction. It is therefore important that householders have good awareness of energy efficiency.

The following table gives the heating energy consumption for different clay brick wall types on a typical 18-apartment residential block (see picture above). The wall constructions were chosen on the basis of their high level of thermal insulation. The consumption was calculated with different sets of U-values for the other building elements (roof, windows, doors, basement, etc.) – the first 2 lines show the results of the calculation with good practice U-values (first line without, second line with mechanical ventilation system), the last line (“minimum”) shows results with the lowest available U-values on the market.

Products	50 cm mono- lithic block	38 cm mono- lithic block	30 cm block +10 cm insul.	25 cm block +12 cm insul.	20 cm block +16 cm insul.	cavity wall 10 cm insul.	cavity wall 15 cm insul.
U-value wall	0,27	0,37	0,24	0,25	0,20	0,34	0,20
Thickness (cm)	54	42	42	39	38	39	56
wall construction
Heating Energy Consumption (kWh/m²a)
without vent. system	43	46	40	41	38	45	40
with vent. system	24	28	22	23	20	26	21

minimum	17	20	14	16	13	19	13

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