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Morrison Hershfield Limited undertook to study and investigate the performance of the Rainscreen Principle applied to residential claddings on wood frame construction for the Canada Mortgage and Housing Corporation. The study included a full scale simulation, in the laboratory, of the rain penetration control performance of three cladding types with each having a sealed and leaky air barrier system. The three cladding types include vinyl siding, stucco, and a brick veneer. Ail cladding systems were mounted on a conventional wood frame wall. The wood frame walls were equipped either with a flexible polyethylene air barrier system or a gypsum interior finish air barrier system. The walls were subjected to simulated wind driven rain from a water spray rack and a simulated wind pressure load. In addition, the test walls were subjected to steady state wind load conditions as well as variations in wind gusts. In the latter tests, the distribution of air pressure loads on the cladding elements and other components within the walls were recorded and analyzed. The study also undertook an examination of the pressure equalization performance of one of the cladding systems on a construction model placed in a boundary layer wind tunnel, specifically the BLWT2 wind tunnel of Western University in London, Ontario. A fully compartmentalized cavity, a continuous cavity and other permutations of compartmentalizations were examined. This allowed for a detailed examination of the pressure equalization performance of the cladding system under real wind conditions and the compartmentalization requirements of the cavity behind the cladding system. In addition, the study undertook to develop a simple mathematical model that simulates the pressure equalization performance of a rainscreen wall. The model was developed using the basic gas laws, for pressure, temperature and volume. It was designed to predict the cavity pressure and time response of the cavity for various gust load rates and magnitudes. The laboratory investigations of the test walls have shown that the pressure equalization phenomenon reduces the amount of water that penetrates the cladding system. This phenomenon is particularly evident with the brick veneer cladding system. It was found
that the decrease in rain penetration can be as much as ten times depending on the cladding air pressure load difference. The stucco and vinyl cladding tests have shown little water penetration. Both cladding systems were substantially pressure equalized by the action of the sheathing and paper even though the air barrier systems were made leaky. The compartmentalization study revealed that the pressure equalization of the cavity was dependent on the compartment seals. From the wind tunnel tests, the cavity pressures behind the cladding approached the exterior cladding surface pressures when the cavity was fully compartmented at the corners. It was noted from the study that the air pressure load on the air barrier varied between 10 and 100 percent. The compartment seals, on the other hand, were subjected to Ioads of two and three times the wind load pressure. The mathematical model was developed, tested and computerized. .It was found that the output of the model provided a satisfactory correlation between predicted results and the laboratory measurements for pressure equalization performance. This study has revealed that the rainscreen principle applied to the design and construction of cladding systems on wood frame walls limits the penetration of rain more effectively than a non pressure equalized wall. To achieve rain penetration control, the rainscreen wall is dependent on certain design and construction features. These include an effective air barrier system within the wall, sufficiently rigid components, the air barrier system in particular, adequate venting and drainage of the cladding system, and effective compartment seals located at corners. To further understand the requirements of the rainscreen principle and to quantify more fully the benefits, it is recommended that the study be extended to:
- investigate the water penetration properties of a broader selection of wall systems to include systems constructed with insulated sheathings and other cladding types,
- develop and test compartment seals to include the use of fiberglass, foams, tapes, plastic and metal compartment seals,
- determine the effects of drips, flashings and capillary breaks on the reduction of water penetration in a rainscreen wall,
- undertake a detailed study of various window systems designed on the rainscreen principle,
- undertake a parametric study using the mathematical model to determine the optimum venting to volume ratios, venting area to leakage area ratios, volume to stiffness ratios and other permutations,
- develop the math (computer model) to incorporate water penetration rates under simulated 15 minute, one hour and four hour rain storms.
Morrison Hershfield was pleased to have undertaken this important assignment on behalf of Canada Mortgage and Housing Corporation. The knowledge gained from this study will undoubtedly lead to innovations in the design and construction of exterior walls and cladding systems. These innovations should enhance rain penetration control while providing durable and cost effective cladding systems for the Canadian climate.