Experimental determination of drying capacity of wood-frame envelope systems for comparative studies and limit state verification.

摘要

The building envelope protects the occupants against outdoor weather and contains the indoor environment to provide comfort for the occupants. As demonstrated through field observations and large-scale experimental tests, wind-driving rain can penetrate the building envelope through design defects or through defects which may develop during its lifetime operation. The rate at which the penetrated water can be evacuated, which is a function of the drying capacity of the envelope, can affect significantly the durability of building envelope systems. However, adequate methods for quantifying the relative drying capacity of building envelope systems do not exist. The objective of this research is to develop a methodology to evaluate the relative drying capacity of building envelope systems of different compositions and thereby to assist the performance evaluation and design of envelope systems.;An innovative experimental procedure has been introduced to apply uniform in-cavity moisture loading by placing a water tray at the bottom of the stud cavity to represent the penetrated water. After a preliminary test for verification and improvement, an experimental program was carried out to monitor the processes of water evaporation from the tray, moisture absorption by envelope materials and moisture evacuation from the envelope. Thirty-one full-size wall specimens of various configurations formed the enclosures of a two-story test hut, located in a large environmental chamber. Tests were carried out over five test periods in 283 days under steady-state "outdoor" conditions that were selected from 10% worst-drying months of Montreal based on 31-year weather data. Over 1,000 electronic sensors and 750 gravimetric samples were installed. By implementing the water tray, a quantitative relationship between in-cavity moisture loading and the moisture responses in the envelope systems was established experimentally for the first time for each wall specimen.;A drying capacity indicator has been developed to quantitatively characterize and compare the relative drying capacity of wood-framed building envelope systems. First, load-response profiles are developed by monitoring the evaporation from the water trays as the moisture source in the stud cavity and by monitoring the moisture absorbed in the gravimetric samples in the sheathings. Second, an allowable moisture limit of wood-framed envelopes is set at 20% MC by weight. Third the region from this 20% MC to the fiber saturation point (FSP), about 28% MC depending on wood species, is deemed as a safety margin against fungal decay. Fourth, the loading at which the 20% MC limit is reached in the moisture response of sheathing was determined from the load-response profiles and was defined as the In-Cavity Evaporation Allowance (ICEA). By comparing the ICEA values obtained from the experimental data for the 31 specimens, the drying performances of these wall configurations were characterized and compared; and ICEA has shown to be a good indicator for evaluating drying capacity of envelopes.;To demonstrate the potential of the newly proposed experimental method and the ICEA concept, a procedure is presented to verify quantitatively the acceptability of a wall configuration by matching potential moisture penetration of the wall against its drying capacity. This verification procedure adopts the concept of the LSD (limit state design) principle used in structural engineering. A case study applying this procedure to 12 testing wall assemblies is presented.;This thesis research on experimental and analytical investigation on the drying performance of building envelope systems has explored innovative concepts, validated them with quantitative testing procedures, advanced the current understanding and design of building envelope systems, and posed new challenges for future research.