Infiltration through leaks? A careful analysis of minimum air change according to DIN 1946

Purpose of the work

Can the minimum air change in naturally window-ventilated units be covered only through leakages?

Content of the presentation

In general, the infiltration airflow rate is dependent on meteorological conditions, especially wind pressure on the building / unit, and with regard to thermal effects, the temperature differential between the interior and the exterior, i.e. the infiltration airflow varies strongly.

According to the German Industrial Standard DIN 1946-6, units with window ventilation for reasons of moisture protection require a part of the minimum airflow to be user-independent. During the heating season, the airflow rate must infiltrate through leakages. Should the calculated infiltration airflow rate not suffice, because of the “general” leakages in the building envelope, additional ventilation measures, as for example window rebate ventilators, must be designed and installed.

Modern buildings today are usually made airtight to the extent that the user-independent air change rate is smaller than 0.1 h^-1. The building stock up to the early 1970s, however, featured a user independent air change rate of 3.0 h^-1 to 7.0 ^h-1. Installing windows with lip sealing has resulted in a reduction of the user-independent air change rate to the level of 0.1 h^-1 of new buildings. According to the Energy Savings Regulation, a minimum air change of 0.5 h^-1 has to be ensured for  health reasons. It can easily be seen that we are facing a deficit of approx. 0.4 h^-1, which has to be offset through manual window-ventilation.

The calculation conditions for the German Industrial Standard DIN 1946-6 with regard to all four ventilation methods (ventilation for moisture protection, reduced ventilation, nominal ventilation, and intensive ventilation), refer exclusively, contrary to other standards, to the area of the unit. The clear room height is not taken into consideration, resulting in a variation of the air change rate depending on the room height. Considering a room height of 2.5 m, the air change rate at nominal ventilation varies from approx. 0.7 h^-1 for smaller units (30 m² of floor space) to 0.4 h^-1 for larger units (210 m²). This means that even for larger units, the minimum air-change rate is lower than required by the German Energy Savings Regulation. Respectively, at greater room heights, the air change rate is smaller.

Results and their significance

The air change required by building physics is defined by “ventilation for moisture protection”. In buildings with thermal insulation exceeding the requirements of the 1995 Thermal Insulation Regulation it is 0.3 times the nominal ventilation, and in less insulated buildings 0.4 times the nominal ventilation. At a room height of 2.5 m, this results in a user-independent air change for moisture protection between 0.12 and 0.28 h^-1. These values often exceed the air change through leakages.

Conclusions

It should be reconsidered whether we have to continue to build with such a high degree of airtightness. After all, because of the possible airtightness, additional openings have to be installed as a substitute measure. However, such considerations should not cause any problems with regard to building physics later.

Note

For more information, please contact the reference author at: info@pbs-planung.de