Numerical Investigation of the Effects of Insulated Envelopes on Hygrothermal Comfort within Habitats of Southern Benin: Test of a Local Material

Aims: The objective of this work was to investigate the effect of the thermal insulation with polystyrene or local straw on the indoor comfort (time evolution of temperature and hygrometry, variation of thermal load). The study aimed also to determine the optimal thickness of the insulators, and to check the contribution of a controlled mechanical ventilation (CMV) to reach a better indoor comfort in humid tropical climates, along with ecological and low cost criteria.

Methodology: The studies were conducted on a typical residential building in Benin located in Cotonou with the following geographic coordinates: Latitude 6°38 'North, Longitude 2°34' East. Dynamic thermal simulation series were conducted to analyse the thermal behaviour of the sample building. Mainly two insulators were examined: a manufactured one (polystyrene) and a local material (straw). The TRNSYS software was used to model the building and to realize the different simulations.

Results: The study showed that the thermal insulation by the inside of the walls and ceiling with polystyrene reduced on average the cooling requirement of 4°C. But an average increase of 25% was observed on the relative humidity. In the same way the complete insulation with straw keeps the average indoor temperature at 27°C which is higher than that of polystyrene (24°C), but humidity with straw was lower for the same average insulation thickness. Moreover, the ecological footprints of manufactured insulation such as polystyrene are higher than those of natural products as straws.

The integration of the CMV has ensured good indoor air and correct humidity control between 67% and 76%. These values ​​are in line with the regulatory values ​​in Benin.

Conclusion: the study outlined the need to insulate building even in tropical areas in order to reduce the energy consumption linked with air conditioning. Moreover it promotes the use of natural insulators as straw to improve carbon footprint of buildings in Benin. With a good building envelope insulation coupled with an appropriate ventilation, the need of artificial air conditioning becomes weak, and may even be unnecessary in some cases.