Investigating the role of moisture in vernacular and conventional building typologies for occupant comfort and health

Abstract

Moisture is omnipresent and critical to sustain life on the planet. In buildings, it is observed in the form of indoor moisture, dampness on walls, seepages, etc. As building occupants, humans perceive moisture in the air during respiration and through their skin (and eyes) for comfort. The COVID-19 pandemic has emphasized the appropriate regulation of indoor moisture for occupant wellness, bringing to forefront the importance of ventilation rates and air changes. As on date, comfort studies have majorly dealt with indoor temperature, with humidity perceived in conjunction with temperature as a sensational response. Though humidity is acknowledged as a critical determinant of comfort, it has not been explicitly examined, especially in the case of naturally ventilated dwellings. Naturally ventilated buildings carry the largest building stock in India. The building envelope and its characteristics (materials, surface finishes, texture, openings, etc.) regulate the indoor environment for comfort in response to external climatic conditions. Indoor air thus connects the building and the occupant. While it is well acknowledged that the building envelope regulates indoor temperatures in response to external climatic conditions, its role in regulating indoor moisture is less understood and examined. The current study specifically examines the role of earth-based building materials in regulating indoor moisture and its impact on occupants' comfort and wellness. Indoor moisture regulation is typically dependent on the hygroscopicity of the building envelope, indoor surface finishes, and furnishings, which vary with building typology. Moisture buffering is the process by virtue of which hygroscopic materials dynamically interact with and moderate indoor (air) moisture. A cluster of buildings in Jamgoria village (Jharkhand) comprising vernacular (earth-based) and conventional (RCC/Fired Clay Bricks) building typologies were examined for indoor temperature and humidity conditions indoors for a year. It was observed that in vernacular buildings, the humidity levels varied to a noticeably lesser extent than conventional dwellings. The high variation in indoor RH observed in the conventional dwelling potentially causes variable heat stress on occupants, leading to discomfort and poor health outcomes. This signified the role of varying moisture buffering characteristics attributed to the indoor surfaces. Also, the average daily humidity ratio observed in both the building typologies was higher than the 12g-wv/kg-da upper limit prescribed by ASHRAE for comfort. Further, to examine in depth the contribution of the building envelope on indoor humidity ratio, surface level measurements were conducted. A novel, low-cost sensor assembly was fabricated and adopted to study water-vapor transfer occurring between the surface air film and the bulk indoor air. It was found that hygroscopic earth-based surfaces in vernacular dwellings permitted spontaneous mixing of water vapor from surface to bulk (air), resulting in dampened indoor peaks in humidity levels. Earth-based materials from 3 clusters were subjected to lab-scale tests to examine their hygrothermal characteristics. The water vapor adsorption/desorption characteristics for the earth-based samples were investigated in both dynamic and static conditions. It was found that the earth-based plaster adsorbed moisture close to 5% of its dry weight in comparison to 1% for cement plaster. Thermal conductivity, density, and specific heat capacity of the samples were also measured. The experimentally derived hygrothermal properties were relied on to develop whole building simulation models, adopting a modified BESTEST geometry, to examine the benefit of adopting earth plasters. Window-to-wall ratio and Air change rates critically impacted moisture-buffering performance in passively regulating indoor moisture. To examine whether replacing conventional cement-based plasters with earth-based plaster provided comfortable indoor conditions, it was deemed essential to identify the optimum humidity-ratio range for comfort. Occupant comfort (respiratory, thermal and skin perception) was examined based on current empirical relations using year-round building data and comparing the results with occupant responses obtained using a novel, aggregated comfort survey method. Discrepancy observed between the computed and survey results challenges the applicability of existing models in the Indian context, justifying the need for region-specific stipulations. The current study represents a maiden effort to identify indoor humidity-ratio ranges conducive to naturally ventilated dwellings in a composite Indian climatic zone. A new computational approach for describing and evaluating humidity-related thermal, skin (IAQ), and respiratory comfort in buildings has been proposed in the present study. To investigate into thermal and skin (IAQ) comfort, a 2-core thermo-physiological computational model of a human being was adopted and validated. The model accounted for occupants' physiology (height, weight, heart rate, and blood pressure), and clothing characteristics. Skin temperature, evaporative loss, and skin wettedness were found to be critical parameters determining thermal and skin (IAQ) comfort. An energy balance approach describing heat and mass balance with a human respiratory tract was adopted to examine respiratory comfort. The psychrometry of the inspired air as it traverses the respiratory tract was examined. Indoor humidity was found to be a critical indicator of respiratory comfort. Indoor environments with regulated humidity, like in hygroscopic-earth-based dwellings, were found better for respiratory conditioning and comfort. Further, the acceptable comfortable humidity ratio was found to be in the range of 17.4 to 22.6 g-wv/kg-da for the participants of the study. The comfortable humidity ratio range was further examined viz-a-viz building simulation results for indoor temperature and humidity variations in dwellings with indoor earth-based plasters. This intervention was found to be effective in Composite, Warm and humid, and Hot and Dry climate zones of India for comfortable indoor humidity. Recommendations for using this intervention with varying geometry and climate zones have also been presented. Including humidity-related comfort parameters in building simulation tools would aid in selecting building materials for improved indoor comfort. Earth is a natural material with low embodied carbon and energy. Its use can offset the otherwise used cement-based plasters to aid in energy efficiency and promote occupants' wellness hand-in-hand. Traditional approaches for comfort evaluation in buildings need to expand, incorporating critical parameters impacting the overall wellness of occupants.