by Michael Gurevich
Loadbearing walls with brick veneer and concrete masonry units (CMUs) were built as solid walls in the first half of the 20th century. Brick veneer and the CMU backup wall were bonded to a solid wall to carry the dead and live loads. In the second half of the century, a cavity wall system was introduced with an air space between the brick veneer and the CMU backup wall that was designed to carry the load. The insulation was located at the interior face of the CMU wall, and the brick veneer was completely separated from the loadbearing function.
In the 21st century, practices evolved further. Loadbearing wall systems are often modified by locating all insulation in the air space between the brick veneer and the CMU backup wall. Thus, the CMU backup wall is designed to carry the dead and live loads, and is located behind the insulation in the same environment as the interior space of the building, with a constant temperature of approximately 20 C (68 F). Brick veneer is thermally separated from the CMU backup wall, with its temperature dependent on the weather.
With this system, the CMU backup wall is constructed with the bond beam at the building perimeter located at every floor window head level. Continuous steel shelf angles are attached to the bond beam with through-bolts and back plate. Shelf angles are designed to carry the brick veneer, which should have a minimum bearing of 63.5 mm (2 ½ in.).
Masonry ties should project from the CMU horizontal joints and through the insulation board into the air space to receive the hook part of the masonry ties, which should be embedded into the horizontal joints of the brick veneer. The International Building Code (IBC) limits the vertical deviation between two parts of the tie to 31.75 mm (1 ¼ in.). Thus, the coursing of the CMU backup wall should match the coursing of the brick veneer. The vertical spacing of the masonry ties should not exceed 406 mm (16 in.).
Rigid, extruded polystyrene (XPS) insulation board should be installed continuously over the outside face of the CMU wall. This should be 406 mm wide (to be installed between the masonry ties) and 2438 mm (96 in.) long. All horizontal and vertical joints between the insulation boards should be sealed.
A vapor barrier should be installed over the outside face of the CMU backup wall (behind the insulation board). A liquid-applied, spray-on vapor barrier should seal around the masonry ties projecting from the CMU horizontal joints. A peel-and-stick membrane vapor barrier could be used, but it is labor-intensive to seal around the masonry ties with this barrier type.
Brick veneer should be installed with a running bond pattern, and with masonry ties anchoring it to the CMU backup wall. Brick Industry Association (BIA) recommends in Technical Note 28B, Brick Veneer/Steel Stud Walls, providing a 51-mm (2-in.) clear air space behind the brick veneer, with one exception. If the XPS insulation board with sealed horizontal and vertical joints is installed over the vapor barrier and the CMU backup wall, the air space could be reduced down to 25 mm (1 in.). The reason is the XPS insulation board is a closed-cell product and not sensitive to moisture. If moisture reaches the XPS insulation board, it will migrate over the board surface down to the weep holes and drain out from the wall system. IBC limits total size of the cavity to 114 mm (4 ½ in.) from the outside face of the CMU backup wall to the inside face of the brick veneer, to limit the unbraced length of the brick veneer ties. If the total size of the cavity exceeds the 114-mm limit, structural analysis of the brick veneer ties is required.