ARCHIVED - The science behind better walls
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April 04, 2010— Ottawa, Ontario
If you're a homeowner who likes to stay dry, NRC's dynamic wall testing facility (DWTF) has you covered.
The DWTF is used to assess the watertightness of wall sections and find ways to improve the sealing around pipes, wiring, vents, windows or anything else that passes through the "building envelope." The results are then shared with architects, builders, construction associations, regulatory bodies and other government agencies, which have produced guidelines based on this work.
"In different climate zones across Canada, indeed in North America, rainfall varies in terms of intensity and duration. In Canada, we often see fairly substantial wind-driven rain conditions," says Dr. Michael Lacasse, head of the team that runs the facility. "The DWTF can mimic the basic climate conditions that bring about wind-driven rain."
The team custom-builds in-house wall sections for performance assessment and also assesses structures submitted by the building industry. It features different sized test rigs that spray water at building components. On the largest unit, a huge 2.44 metre-diameter piston is used to change air pressure on the outside of full-sized test walls.
While labs exist elsewhere that can simulate hurricane conditions, the DWTF is unique because the level and rate of pressure fluctuations together with the water spray can be pre-set for desired test conditions, says Dr. Lacasse.
Evolution of the facility
Initially, the DWTF was used for research on commercial buildings, but since Dr. Lacasse has led the team, the facility has focused on North-American-style, low-rise, wood-framed residences. His team first performed experiments on exterior claddings, such as wood, vinyl, brick and stucco. Then, as questions arose about which construction details were most critical, they began to evaluate different installation methods for components such as windows, pipes, wiring and vents. Lately, their research efforts have centred on the "window-wall interface," in response to growing industry needs for information on how to specify and build these interfaces effectively.
"A window is a building component that needs to fit in a wall assembly, and the details around that are rather important to get correct — otherwise you increase the risk of water entering right into the wall system," he says. Water ruins the effectiveness of insulation very quickly and, in the long term, can promote mould growth or rot the framing.
To prevent problems from water entry into walls, critical construction details include how the flashing around a window is installed, how the sheathing membranes or vapour barriers are cut and positioned, and where the sealant goes — even the order of assembly. "There are many different details to consider — flashing, membranes and sill pans," explains Dr. Lacasse. "There isn't one single component you have to worry about, but rather all the components working together as a system."
The DWTF allows his team to explore a combination of factors — for example, how well-built walls or components work under ideal conditions or, on the other hand, what poorly built walls will tolerate before "failing."
Through a comprehensive experimental program, the team has found that different types of window installation become less watertight in different ways. The researchers have also shown that it is less effective to prevent leaks by sealing a wall against moisture at the outside of a wall than by installing the air barrier further inside, where it's better protected from water. In addition, the team has explored the performance of sloped sill pans under windows, which are designed to collect any water that penetrates the windows and drain it back to outside.
"If there's an opening, water and a pressure difference at that opening, the water will be driven through," says Dr. Lacasse. "But ultimately, what you don't want is the structure to deteriorate. You want to keep the components and framing as dry as possible. Our work demonstrates that if you're not careful with the choice of details and their installation, you run a risk of premature deterioration. That's why the installation guidelines based on our work are so important."
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