Switching to metal stud backups: changing from a brick veneer and concrete block backup exterior wall to a brick veneer and metal stud requires a system redesign

Masonry Construction, Jan, 2003 by Michael Gurevich

In many cases, an original architectural design will call for a brick veneer and concrete block backup exterior wall system. Later, due to an economic or construction management decision, the original design is changed to a brick veneer and metal stud backup (BVMSB). This change requires a significant architectural/engineering redesign of the entire exterior wall system.

One of the weak points of the BVMSB system is the masonry ties that anchor the brick veneer to the metal studs. The Brick Industry Association (BIA)'s Technical Notes 28B, "Brick Veneer/Steel Stud Walls," revised November 1999, recommends the use of four types of masonry wire ties for the BVMSB system.

These ties should be connected to the steel studs with screws. About two threads hold this screw in the 16-gauge outside flange of the metal stud. Thermal bridging at the stud could cause condensation to form at the inside face of this outside flange in the cold weather months.

A rust or a galvanic cell reaction could develop at the juncture between the screw and the stud. The connection between the two could eventually deteriorate and become loose, which means that the masonry tie and the brick veneer also could become loose, causing cracks in the horizontal (bed) joints of the brick veneer.

Cracks lead to water penetration through a brick veneer and provide an avenue for moisture to get inside the building, All associated problems could then be blamed on the masonry contractor who installed the brick veneer and masonry ties. However, it is actually a design problem.

Eliminating thermal bridging

The International Building Code (IBC)--specifically the Energy Conservation section--includes provisions to eliminate thermal bridging at the metal studs. For example, Massachusetts, which adopted the IBC provisions in 2001, has a minimum design requirement to provide an exterior sheathing on the outside face of the metal studs with a rigid insulation (R = 3 minimum).

Therefore, a 3/4-inch extruded polystyrene board with R = 3.75 should be installed on the outside face of the metal studs to eliminate thermal bridging at this location. This board also could eliminate the possibility of screw to metal stud connection deterioration. Check the conductivity through the screw surrounded by the extruded polystyrene insulation because the temperature at the head on the outside face of the stud could be different from the temperature at the tip of the screw inside the stud space.

Exterior walls in the last century were designed with a fiberglass batt insulation located between the metal studs with no insulation over the exterior face of the studs. The batt insulation has a kraft paper or foil facing the interior of the building intended to act as a vapor barrier in the cold winter regions.

Flanges of the kraft paper or foil are stapled to wood studs and should overlap the interior of the studs to create a continuous vapor barrier. Flanges can't be stapled to steel studs; therefore, they should be attached to the steel stud flange with a spray adhesive or a tape.

In many cases, the open vertical joints left between the studs and the insulation's kraft paper or foil create gaps in the vapor barrier at each side of the exterior wall stud. Moisture could accumulate within the stud space causing rust and a loss of R-value for the wet batt insulation.

It is my strong opinion that thermal bridging through metal studs could cause condensation within the metal stud wall backup system, which might contribute to mildew development. Good thermal performance of a wall should have the entire insulation located at the outside face of the studs. No batt insulation would be required in this case. For example, the Massachusetts building code would require R = 7 (1 1/2-inch-thick extruded polystyrene board, R = 7.5) on the outside face of the metal studs.

In cold climates, a wall is better insulated with a combination of 1- to 2-inch extruded polystyrene insulation board at the outside face of studs and a batt insulation between the studs. Condensation analysis helps determine the location of the vapor barrier in the wall system.

For fire-rated walls, a glass fiber or mineral wool batt or blanket insulation must be installed between the studs. Wall acoustical ratings require batt insulation between the studs as well.

The horizontal joints between the exterior gypsum board or extruded polystyrene boards at the exterior sheathing should be sealed. If not, air moves through the entire wall system from outside into the conditioned inside space (infiltration). Air can also move from the conditioned inside space out (exfiltration).

If moist air enters the wall system, it often follows a relatively long path until it finds a crack to enter--perhaps at a gypsum board sheathing joint. The gypsum board or extruded polystyrene board sheathing joints must be sealed to create an air barrier. The IBC requires permeability of the air barrier to be 10 times more than the permeability of the vapor barrier.

Thermal bridging through metal studs could substantially reduce the R-value of the wall. It could also increase the energy usage for the heating/cooling system of the building.