Internal thermal mass for passive cooling and ventilation: adaptive comfort limits, ideal quantities, embodied carbon

Abstract

How effective is naturally ventilated internal thermal mass for obviating air-conditioning, mitigating heatwaves and storing carbon in buildings? This study combines detailed climate model outputs with scaling rules for optimizing internal thermal mass coupled with buoyancy ventilation. It identifies regions where this passive design strategy is most effective during future heatwaves and determines how much internal thermal mass each person needs to stay comfortable in these regions, with a special focus on Canada. Results suggest that naturally ventilated internal thermal mass is likely to become less effective due to future global heating. Regions where internal thermal mass will no longer be sufficient to obviate air-conditioning and where it can still play a significant role in hybrid cooling are identified. By comparing the ideal per capita thermal mass quantities in different regions, it is found that biomass-based materials require approximately 3.5 times the volume of cementitious materials to perform equivalently, if thermal proportions for surface area and thickness are respected. Finally, an analysis of the per capita embodied carbon of these ideal internal thermal mass quantities is undertaken, suggesting a fair functional unit to compare thermal mass materials. reasonable and similar to dimensions used in construction (≈ 6.3 cm-thick thermal mass with an interior surface area of 26.6 m 2 for concrete, 13.6 cm thick and 47.0 m 2 for mass timber, and 8.8 cm thick and 41.1 m 2 for oriented straw structural boards—OSSB). Based on these findings, concrete substitution factors are developed for biomass-based materials. Finally, an analysis of the embodied carbon emissions of per capita optimal thermal mass quantities is performed, suggesting that the latter could be used as a fair functional unit for inter-material comparative lifecycle assessments (LCAs) in low-carbon building design to allow a comparison of the environmental impact of equivalently performing, rather than equivalent volumes of materials.