An analysis of the subfloor cavity climate in a residential building

As roof and wall insulation have become standard practice in residential building design and construction, the conditions in the subfloor cavity have gained relative importance to a building's thermal performance. However, the modelling of the subfloor cavity is considered a weak point in many building thermal performance software programs. Recent research suggests that improvements to the subfloor model are crucial to improving the accuracy of Australia's benchmark building thermal performance program, AccuRate. Another recent study finds fault with the criteria that established the current standard for subfloor ventilation design, questioning the subfloor's ability to maintain the subfloor humidity below the design limit. These findings suggest the need for a review of the subfloor thermal model. As very little measured data on Australian subfloor conditions exist, this research seeks to explore the subfloor conditions experimentally. This research investigates the subfloor cavity climate of a small residential scale test cell for a period of over one year. Energy and mass transfer relationships linking subfloor ventilation, ground evaporation and the subfloor and outdoor climate conditions are explored theoretically and using observed data. The observed data are compared to design limits, previous research findings and AccuRate's predictions. The relative humidity in the subfloor is found to exceed the ventilation design limit. The conditions that lead to mould or decay are complex and when compared to these limits the data are on the threshold of conditions thought to be conducive to deterioration. The subfloor climate conditions are found to vary based on time elapsed since construction. Between one year and five years after construction, the subfloor air temperature, specific humidity and ground moisture evaporation rate are observed to drop considerably. Though the relative humidity remains constant over this time, both the energy and moisture in the subfloor are reduced, changing the role of the subfloor vents. Whilst initially the net effect of the vents is to nearly always decrease both the moisture and energy of the subfloor air, six years after construction the vents are shown to increase moisture 24% of the time, and energy 35% of the time. This shift in subfloor climate over time is found to affect the apparent accuracy of the AccuRate subfloor model. Previous research, based on data collected one year after construction, had shown that the observed subfloor temperature was several degrees above AccuRate's predicted subfloor temperature. However, when considering data six years after construction, this study finds that the observed and AccuRate temperatures are more closely aligned. These results emphasize the importance of building thermal performance research to consider the time elapsed since building construction, as ground temperature and moisture stabilization have a noticeable effect on the subfloor climate.