Laboratory investigation and field monitoring of pressure equalized rainscreen walls.

The rainscreen principle is not new. It was proposed as early as the mid sixties by researchers of the Division of Building Research of the National Research Council of Canada and the basic principles were developed. It has been applied to certain exterior wall types but it remains largely unknown because of the absence of factual data to support the claims of performance. It is only recently that interest has grown in the application of the rainscreen principle because face sealing and the drained cavity approach do not allow for the satisfactory control of moisture in cavities from rain or from condensation. The rainscreen principle is well developed qualitatively but not quantitatively. There are no technical or engineering criteria to assist designers and few established prescriptions for the builder. The actual field performance of the rainscreen with respect to rain control is unknown and the relation to pressure equalization is also unknown. Canada Mortgage and Housing Corporation (CMHC) recognized the need to undertake further research into the engineering and technology of the rainscreen principle. This project was commissioned by CMHC and Public Works Government Services Canada (PWGSC) to further advance the application of the rainscreen principle to exterior wall design and construction of both residential and commercial buildings. This project included three distinct areas of interest. First, the development of a method to monitor the performance of existing rainscreen wall systems and to gain insight into the actual or field pressure equalization performance. This work was also coupled to a laboratory investigation of the wetting and drying of a rainscreen cavity in a metal and glass curtain wall. Secondly, the development of a field performance and design compliance testing procedure. The procedure is termed the Cavity Excitation Method or CEM. It is a field test that does not require elaborate preparations and substantial mockup facilities. Third, the development of performance criteria for the design of rainscreen systems and the development of commissioning guidelines for rainscreen wall system. This report is the first of three reports on rainscreen performance research. It examines the first area of interest, field performance monitoring and wetting and drying of rainscreen cavities. The field monitoring of existing rainscreen walls was undertaken in 2 buildings. These included a metal and glass curtain wall at the University of Quebec in Hull, Quebec and a Limestone cladding and architectural precast rainscreen wall on the new Canada Life Building in Toronto, Ontario. A laboratory investigation at Queens University examined the wetting and drying of a metal and glass curtain wall rainscreen system. This project revealed considerable information. While there were only 2 rainscreen systems monitored in the field, those two and similar experiences elsewhere by the author confirmed that pressure equalization in current rainscreen wall and window systems is virtually nonexistent. Several reasons were found but for the cases reviewed; the weep and vent openings were too small for the volumes served and the compartment seals were deficient or nonexistent. The rain penetration monitoring method using humidity and temperature of the cavity is quite promising but complex to interpret and will require further investigation. While pressure equalization was found to be poor in the cases studied, there were no observed water penetration problems. Further work in this area should include a more detailed examination of the wetting and drying of rainscreen wall types to distinguish between the effects of water storage in masonry claddings and the cavity moisture balance and the effects of other non-water absorbing rainscreen wall systems.