Inwards vapor diffusion due to high temperature gradient in wall assemblies

摘要

Moisture is the main source of deterioration of building envelope systems. One source of moisture that has not been studied to a great extent is inwards vapor flows, due to high temperature gradients. Such flows occur, for example, when exterior claddings like brick are wetted by rain and then exposed to solar radiation. This thesis investigates this phenomenon, which can bring large amount of undue moisture within in the envelope assembly. This thesis presents in detail the experimental work performed on large-scale wall assembly specimens to document their hygrothermal performance under conditions leading to inwards moisture flow. A large-scale experimental facility was utilized for the purpose of this work which consisted of weighing apparatuses to monitor the change of mass of the cladding and back-wall parts of wall specimens, a spraying array, a radiation array, and a test hut to provide controlled interior conditions. Five insulated wood-framed walls were monitored, four of which were brick cladded and one stucco cladded. Constructions of the backwalls varied by the material utilized for the exterior sheathing and the interior finish and also by presence or lack of an air cavity. The walls and the environment were equipped for electronic monitoring, complemented with manual weighing of gravimetric samples. The results of the experiments demonstrate that the presence of vapor tight interior finishes result in the accumulation of moisture in the interior gypsum board. Furthermore, it is shown that even a vapor tight sheathing does not reduce sufficiently vapor flow to prevent moisture accumulation in the wood studs and the interior gypsum board. The highly positive effect of air space, and its ventilation, in reducing the magnitude of the inward vapor flow is shown. During the process of studying the vapor diffusion through various wall assemblies, a vertical temperature and moisture content gradient was observed along the gypsum board-insulation interfaces. This phenomenon was also further investigated and studied both experimentally and numerically using CFD. The work indicated that stratification of humidity occurs within the insulation cavity of the wall. This thesis demonstrates the mechanism of inward diffusion: under high thermal gradients, large amounts of vapor are transported, inducing high relative humidity against low-permeable layers in assemblies. By sorption, materials exposed to high relative humidity display high moisture content.