Understanding moisture buffering effects in the indoor environment

Abstract

High or low indoor RH levels may have negative effects on people's health and well-being. To regulate the humidity, air conditioning systems can be used, requiring energy and increasing the environmental emissions. However, some materials, like clay and gypsum, which are described as hygroscopic, can passively regulate the indoor climate, reducing peaks of internal relative humidity, when applied on exposed surfaces to the room air. Their capacity to moderate indoor humidity fluctuations is due to their ability to adsorb and desorb moisture, a process referred to as moisture buffering. This property is evaluated through the MBV, which allows for a simplistic calculation of the potential of materials by considering the material properties and humidity regulation. Due to the simplified interpretation of moisture buffering, the testing methods are not representative of the material behaviour in a real building. Furthermore, moisture buffering can be measured, following various standards that are not directly comparable. Alternative experimental studies have attempted to investigate the actual performance of materials in real buildings, but there is no standard methodology yet and no established relationship between moisture buffering and building performances.

This PhD aimed to understand the moisture buffering effects in the indoor environment, by establishing a method to measure this property in full-scale experimentation and laboratory testing. The research was initially developed, by considering three independent approaches: laboratory testing, field work and simulations.
In the laboratory testing, clay, gypsum, lime and plasterboard's hygrothermal properties were tested, to observe and compare their moisture buffering behaviour and investigate the correlation between material properties and moisture buffering potential.
Successively, the testing protocol boundary conditions and test protocol were investigated. The effect of temperature, RH fluctuation and air velocity on moisture buffering capacity of plasters was investigated.
Field work aimed to study the response of real size rooms to humidity fluctuations, to evaluate the impact of moisture buffering, when buildings are exposed to external climate variations, ventilation and indoor temperature variations. Two hygroscopic rooms were compared to a reference room (non-hygroscopic). The testing methodology and equipment were designed to observe the moisture exchange through ventilation, building infiltration and wall moisture buffering capacity. The investigation showed the important impact of hygroscopic materials on the regulation of the indoor moisture content. When the humidity increases, the walls store moisture from the indoor reducing the amount of moisture removed through ventilation. When the absolute humidity is low, the cold air that moves into the building through ventilation constantly replaces the indoor moist air. Therefore, the outdoor air over-dries the indoor environment. In this case walls release moisture in the room to counterbalance the moisture removed by ventilation.

Based on the rooms tested in field work, simulations were used to analyse the contribution of sub-layers and wall design on the moisture buffering performance of plasters. Materials in direct contact with the environment are responsible for the regulation of the indoor moisture. Materials exposed to the indoor stored and released most of the moisture and depending on the humidity level and moisture load, those materials regulate the amount of moisture that moves into the sub-layers.
The culmination of this investigation converged the three research approaches in order to compare and investigate the behaviour of indoor materials in laboratory and in a real building. By merging the three approaches, significant differences between simulations and experimental in-situ testing were found. In simulations, walls buffer more moisture than in the experimental cells. On the other hand, simulations showed a good agreement with the experimental laboratory testing that demonstrates numerical models are based on laboratory measured properties, which are not always representative of the real moisture buffering behaviour of a material when applied to a building.

The ability to test the moisture buffering performance of buildings is the key for material performance assessment. This thesis provides guidelines that reduce uncertainty to assess moisture buffering. It investigated and introduced different approaches to evaluate the materials performances from the material development to their application on buildings.
The impact of this research is to push the development of new moisture control materials at a laboratory scale, with new confidence in their larger scale performance. This will result in an indoor environment that is healthier and more comfortable, by maintaining of the optimal indoor RH level, whilst reducing the risk of condensation and decay of construction materials.

Date of Award	24 Mar 2021
Original language	English
Awarding Institution	University of Bath
Supervisor	Dan Maskell (Supervisor), Pete Walker (Supervisor) & Andrew Shea (Supervisor)