Photo courtesy Owens Corning
Drainage and ventilation
The best way to let both the masonry and the substrate dry rapidly between wetting events is to install a mesh drainage plane at least 7 mm (¼ in.) thick between the WRB and scratch coat. Canadian codes require a minimum 10-mm (3/8-in.) thick drainage plane. (For more information, see Lstiburek’s “BSI-029: Stucco
Woes–The Perfect Storm.”)
It may help to think of the drainage plane as serving the same function as the cavity in a masonry cavity wall—it allows water and air to move freely behind the veneer for both drainage and ventilation, and acts as a bond break to prevent mortar bridging so water cannot travel from the masonry to the substrate. It also helps eliminate localized water stresses on the sheathing and other components, because it allows moisture to migrate rapidly away from areas like wall openings and control joints. Further, a drainage plane allows water vapour to rapidly migrate from behind the veneer so hydrostatic pressure will not force it through small holes into the substrate, and allows masonry to dry from the back, so even if wetting events are frequent, the masonry can dry more quickly and avoid saturation.
A polyester drainage mesh with a factory-adhered water- and vapour- permeable fabric on one side is currently available in rolls of various nominal thicknesses. A new system that has a 90 per cent open-weave mesh factory-assembled to a galvanized, G-60, 1-kg (2½-lb) expanded metal lath is also available, with variable thickness and weight.
Conclusion
A drainage plane between the veneer and substrate is not yet required by most North American building codes for adhered masonry. However, changes to building materials and construction methods are needed to meet more stringent energy codes and increase building sustainability, meaning the old ways simply do not work anymore. Further, modern building science has proven drainage planes can significantly reduce or eliminate moisture problems in adhered masonry veneer walls.
Twenty-five years ago, mortar-dropping collectors in masonry cavity walls were not part of code either, but as their value was demonstrated over and over, codes caught up, and now no mason or masonry designer would consider building a cavity wall without them. Drainage planes for masonry veneers are currently at the same stage as mortar-dropping collectors were then—quality builders are using them because they reduce problems and callbacks, and because they make the buildings that implement them a source of pride for both the builder and the designer. When codes catch up, everyone will benefit.
| FURTHER READING |
For more information on the topics broached in this article, professionals can consult:
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Art Fox has been the head of marketing and communications at Mortar Net Solutions since 2012. He was also the chief operating officer of the company when it was initially formed 25 years ago. Fox has been involved in the building trades since he was a contractor specializing in new home and light commercial construction in New Mexico in the 1970s.
He can be reached via e-mail at afox@mortarnet.com.
