Impact of airtightness on the heat demand of passive houses in central European climate

Excessive air leakage through the building envelope increases the infiltration heat loss and therefore lowers the energy efficiency. Therefore, very good airtightness is required in case of well insulated buildings equipped with a mechanical ventilation system with heat recovery (e.g. n50 < 0.6 h-1 for passive houses). Although the building industry has progressively adopted strategies to comply with such strict limits, it is still important to study how and how much the airtightness influences the energy efficiency of different types of buildings in different climatic conditions.
This study investigates the impact of building envelope airtightness on the heat demand of a single-family house and a multi-family residential building in the central European climate (Prague). Both model buildings are passive houses, equipped with a balanced mechanical ventilation system with heat recovery. Their heat demand was calculated in function of the envelope airtightness (n50 varying from 0 to 1 h-1). Several combinations of leakage distribution over the building envelope and wind shielding conditions were considered. The single-family house was modelled as a single zone building. In the multi-family building, each flat and the staircase were considered as separate pressure zones. The heat demand was calculated considering the following alternatives of internal air leakage between the zones:
- no internal air leakage between zones
- each flat connected with the staircase (air leakage between flats not allowed)
- each flat connected with the staircase and neighbouring flats (air leakage between flats allowed)
The air leakage distribution over the residential building envelope and the characteristics of the internal leakage paths were estimated from results of airtightness tests of a real building. For the purpose of this study, transient thermal and air infiltration models were developed using Matlab – Simulink. Iterative approaches were adopted for a reliable coupling of the thermal and air infiltration models (differently in the single and multi-zone models).
The heat demand increases noticeably with the building envelope air permeability. The increase is more pronounced in case of the residential building (eg. 3 kWh/(m2∙a) per unit of n50 against 4 kWh/(m2∙a) for the single-family house under the same conditions). The wind shielding and the leakage distribution influence significantly the results. The highest heat demand was identified in cases with the air leakage paths distributed half and half at the bottom and on the top of the building. The internal air leakage does not affect significantly the heat demand of the residential building, which depends mostly on the envelope air leakage and its distribution. However, significant air flow rates were detected between the zones (up to 24 m3/h between flats). The internal leakage may therefore cause an issue for IAQ, ventilation system function and fire safety.