Hygrothermal performance of straw bales split-insulation wall assembly in cold and humid climates

Abstract

The abundance of straw in Quebec makes this material an interesting insulation option to reduce the energy consumption of buildings. At the same time, studies indicate that straw bales’ outstanding hygrothermal performance can provide a comfortable living experience for the occupants. However, due to the slightly higher thermal conductivity of straw bales insulation compared to common insulation materials, the walls are found to be 30–90% thicker, which can lead to moisture-related problems especially under a humid and cold outdoor climate. Therefore, a split-insulation wall structure was used in this study to reduce the wall thickness by combining straw bales with expanded polystyrene (EPS) exterior insulation. Results show that straw bale walls generally have higher thermal inertia and better moisture buffer capacity compared to the reference wall using glass fiber insulation. The hygrothermal profiles show that the sheathing-insulation interface is the position having the highest relative humidity in the walls. The biohygrothermal model also suggests that under Quebec’s climate, the mould grow risk in a straw bales wall with two inches EPS exterior insulation is within acceptable limits.