Numerical evaluation of the impact of hemp lime concrete moisture-buffering capacity on the behaviour of relative humidity sensitive ventilation system

Hemp Lime concrete (HLC) is a bio-based material, which knows currently a growing development. HLC is a low embodied energy material. It has an excellent moisture buffer performance and is considered as good indoor climate regulators. Recent field study has confirmed the ability of HLC to maintain hygrothermal conditions at winter and summer comfort levels.  
On the other side, relative humidity sensitive ventilation systems help to save building heat energy by reducing the amount of the exhaust airflow during unoccupied periods or low activities depending on the level on the indoor relative humidity. Previous studies showed the ability of such systems to save energy without compromising IAQ throughout the whole year. They are widely used in new French residences. However, the moisture-buffering capacity of bio-based material such as HLC can modify the behaviour of humidity sensitive ventilation as it lessens the variations of indoor relative humidity. 
The objective of this paper is to assess the impact of the moisture buffering capacity of hemp lime concrete on the behaviour of humidity sensitive ventilation system in single detached dwellings. A numerical approach based on the energy simulation tool TrnSys coupled to the multi-zone air-flow and contaminant transport model COMIS was used. A real case of HLC dwelling was studied. First, measured indoor relative humidity is compared to the simulated results in order to assess the moisture buffer model of Trnsys. Second, humidity sensitive ventilation system is added in order to analyse the dynamic interaction between the moisture buffering capacity of HLC and the performance of humidity sensitive ventilation system in terms of energy efficiency and indoor air quality. Results show that in the case of humidity sensitive ventilation, the moisture buffer capacity of hemp lime concrete helps to maintain the indoor relative humidity within the range of comfort zone between 40% and 60% through the whole year. They confirm that the use of moisture-buffering materials is a very efficient way to reduce the amplitude of daily moisture variation. However, the yearly average exhaust airflow is slightly higher, yet the heat load increase is less than 5%.