Moisture is one of the most important actors affecting building performance and durability, especially in countries with cold climates. Understanding and predicting moisture movement within and through the building enclosure is crucial to the control and the avoidance of moisture-related problems such as corrosion, freeze-thaw, and biological growth.

This thesis comprehensively investigated the control of moisture in above-grade enclosure walls. Emphasis was given to driving rain deposition, rain penetration control, ventilation drying, and pressure moderation. A major review of liquid and vapour moisture storage and transport in porous building materials was undertaken, and the results summarised.

The experimental program involved the temperature, humidity, and moisture monitoring of 26 full-scale test panels exposed to the environment of South-western Ontario for 30 months. Driving rain was measured in the free wind and a t14 locations on a test building. High-speed pressure measurements, of interest to ventilation and pressure moderation, where simultaneously collected at many points. The water permeance of brick veneers under air pressure differences and the moisture absorption of brick were studied in the laboratory.

A method of predicting driving rain was developed and validated with field measurements. The distributions of driving rain event duration, intensity, and direction were investigated. An approximate means of estimating rain deposition on buildings was also developed, supported by measurements and other researchers' results. A rational rain control theory was conceived which led to a useful enclosure classification system. A probabilistic model of rain-building -enclosure interaction was produced which incorporates all of the important variables.

Extensive pressure measurements showed that instantaneous pressure equalisation does not occur. It was also shown that realistic air pressure differences have little effect on the permeance of brick veneers. It was concluded that pressure moderation is not an effective rain control strategy for most walls, especially brick veneers.

The physics of ventilation flow and ventilation drying of walls were formulated. Field measurements of wind pressures and air space moisture content and temperatures behind brick veneers demonstrated the importance of ventilation as a drying mechanism and as a means of resisting inward vapour-drive wetting. It was found that the sun and wind have a large and beneficial influence on ventilation drying. Summer condensation wetting due to inward vapour drives from solar-heated rain-wetted cladding was shown to be a potentially serious performance problem.