Heat and moisture transfer in a bed of gypsum boards

Abstract

Several recent projects in building science have examined the hygric performance of building materials. Most building materials adsorb from and desorb water vapour to their environments. This phenomenon could be used to help control relative humidity fluctuations in buildings, experienced during periods of moisture production such as cooking, washing or bathing. They could also be used to reduce the need for mechanical ventilation and air conditioning to remove excess moisture. To understand how a building material responds to transient changes in relative humidity, testing is required. This thesis outlines the testing performed on gypsum board, a common wall and ceiling finishing material used inside buildings. The effect of paint coatings on the gypsum boards and heat and mass transfer coefficients of the air passing over the gypsum bed was tested. The data produced from these experiments was used to validate several numerical models through an International Energy Agency/Energy Conservation in Buildings and Community Systems (IEA/ECBCS), Annex 41: Whole Building Heat, Air and Moisture Response. The validated models are important for simulating the process of adsorption and desorption in building materials to predict failure in the building envelope and expected indoor air conditions. A sensitivity analysis is also presented which examines the effects of the sorption isotherm and vapour permeability of the gypsum and paints as well as the heat and mass transfer coefficients the boards are exposed to. The sensitivity range used was determined from the tests performed on the gypsum boards and paints which were also performed during the work of Annex 41. The results of this thesis produced a high quality data which can also be used to validate future numerical models. All information required for validation of future models is available such as dimensions of test section, test conditions, material properties and the experimental data. The results show that when designing for passive humidity control in buildings using gypsum boards, the most influential factor is the type of coating or paint applied to the surface. The sensitivity analysis showed that material properties such as vapour permeability and the sorption isotherms, for the expected temperature range, should be well known for increased accuracy of the simulation. The material properties were determined from inter-laboratory testing at 14 different institutions to achieve confident values. The effect of increasing the heat and mass transfer coefficients, over the range of coefficients studied in this thesis, showed negligible differences in the results. The simulated results had very good agreement between the models and were mostly within experimental uncertainty of the measurements.