Recommendable supply air rates for residential housing – A simulation study considering CO2 concentration, relative humidity, TVOC emissions and mould risk

In an extensive simulation study using a multi-zone airflow and contaminant transport calculation software (CONTAM) recommendations for the supply air rates for residential housing were derived as input for the revision of the Austrian standard ÖNORM H 6038 (2014). The floor plan, the occupancy and the contaminant and humidity sources are modelled to represent a typical Austrian housing situation. A humidity buffering model is also implemented. Based on common thresholds for CO2, relative humidity (r.h.) and TVOC the so-called relative threshold deviation is determined. It is used as a combined parameter to evaluate indoor air quality in terms bio-effluents, air humidity and pollutants arising from building and interior products. Additionally the potential mould risk due to high air humidity and low surface temperatures is calculated using the isopleth model.
The results suggest a supply air flow into the bedroom of 20 m³/h per person for the chosen reference climate. It represents the best compromise between exceeding the target value of CO2 and avoiding overly dry periods during winter. If low emitting building products are used, TVOC concentrations seem not to play an important role for the definition of the supply air rates. If the floor plan permits, the implementation of the so-called extended cascade ventilation principle is recommendable. It allows a reduction of the relative threshold deviation with an air exchange rate as low as 0.3 h-1. Prerequisite for the implementation of such low air exchange rates is a high thermal quality of the building envelope. It ensures surface temperatures that exclude potential spore germination. The same applies for the use of ventilation systems with humidity recovery. For typical recovery rates of 60%, an air exchange rate as high as 1.0 h-1 might be required for the same reference apartment to avoid mould problems, in case the building envelope has a temperature coefficient (fRsi value) of 0.5 as frequently observed in existing buildings (thermal bridge). For residential housing, humidity recovery should therefore be limited to locations with very cold and dry winters as observed in mountain regions.