Recommendations for adhered stone veneer installations over exterior insulation
In order to comply with NECB 2011’s high R-value requirements in colder climates, adhered masonry veneer products are being installed over exterior insulation. Similar recommendations
are utilized to ensure sufficient bond for installations of adhered masonry veneers over exterior insulation.
Most installations over exterior insulations use Z-bars to fur out the wall. Although long fasteners can be employed, it is often more difficult to engage a stud backup wall, and the fasteners tend to be more robust and expensive than Z-bars. Focusing on the Z-bar installation, the temptation is to design the Z-bars to be installed horizontally (i.e. perpendicular to the studs) to reduce thermal bridging and, in theory, eliminate cutting the insulation boards (typically 610 mm [24 in.] wide) down to 406 mm (16 in.), as is necessary to fit between Z-bars when they are vertically oriented and fastened to studs typically spaced at 406 mm o.c. However, installing Z-bars horizontally loads them along their weak axes, and if this method is used, an increase in the Z-bar gauge is required to reduce the deflection. In addition to excess deflection, horizontally oriented Z-bars can more readily trap moisture, and gaps must be intentionally left to allow water to flow past them.
The required spacing for lath or cement board is 406 mm (16 in.) o.c. This eliminates the possibility of spacing horizontal Z-bars to be consistent with the insulation board width of 610 mm (25 in.). Therefore, it is recommended to install Z-bars vertically (parallel to the studs). The other recommendation is to use cement board to replace both the lath and scratch coat, as doing so means the designer is guaranteed the thickness specified. Ideally, this size is 16 mm (5/8 in.) to reduce deflection between the Z-bar supports. The use of cement board requires ANSI A118.4-compliant mortars. A 13-mm (½-in.) oversizing of the Z-bar can provide drainage behind the adhered stone veneer without the necessity for a drainage mat. Figure 11 provides a foundation and floor detail of adhered stone over exterior insulation.
Conclusion
Adhered masonry veneers replicate the look and feel of full-bed masonry, but their reliance on mortar bond to anchor them to the structural backup makes it extremely important to use the proper products and pay additional attention to installation practices when exceeding 3 m (10 ft) above-grade. This becomes even more important when a multi-storey building uses different materials, such as a combination of steel stud backup walls and concrete floors, or when cantilevering over rigid insulation.
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Mark D. Hagel, PhD, P.Eng., is the executive director of the Alberta Masonry Council. He holds bachelor’s degrees in actuarial science/applied mathematics and civil engineering, and a doctorate in civil engineering. Hagel was previously employed as a technical services engineer for the Canadian Concrete Masonry Producers Association (CCMPA) and as a structural and building envelope engineer with a Calgary design firm. He can be reached via e-mail by contacting markhagel@albertamasonrycouncil.ca.
Nicholle Miller is the marketing co-ordinator of the Alberta Masonry Council. She holds a degree in business administration with a double major in marketing and human resources. Prior to her employment with the Alberta Masonry Council, Miller worked as a proposal co-ordinator with Aecom, formerly Flint Energy Services. Additionally, she has served as a Calgary Corporate Challenge co-ordinator and a CANstruction co-ordinator for the Calgary Food Bank. Miller can be reached via e-mail at nicholle@albertamasonrycouncil.ca.
