Q: Many buildings use steel wide flange beams as lintels in larger masonry openings that have a plate welded to the underside to carry the exterior wythe of brick. Since the steel plate is exposed on the outside and on the inside, this would appear to create thermal short. Is this a problem in cold weather?
A: This type of steel lintel does create a thermal bridge from the exterior to the interior, as shown in Figure 1. This affects not only the temperature of the steel plate but also the temperature of the head of window below the lintel. Any thermal break in the window system, if aluminum windows are used, would be bridged. As a result, this can lead to condensation problems at the head of the windows.
This is a relatively common detail, especially in older buildings and warehouses. In many cases, these lintels do not have condensation problems for several reasons. First, since most of the steel is located on the interior portion of the wall, the majority of the steel will be warm. This is especially the case with insulated cavity walls where the steel beam is located on the interior side of the insulation.
Therefore, depending on the amount of steel that is exposed on the exterior, the temperature of the steel will likely be closer to the interior temperature than the exterior temperature.
Second, most of the older buildings and warehouses where these lintels are used are not humidified during cold weather. As a result, there would not likely be sufficient moisture within the air to result in condensation.
Condensation problems can occur in older building where the use of the building changes. The interior may be humidified during cold weather. In these cases, the moisture content of the air may be high enough to raise the dew point above the temperature of the steel plate and condensation will occur.
A significantly worse situation occurs when renovation projects of older buildings include adding insulation on the interior of the masonry. When this is done, the entire steel lintel will be located on the exterior side of the insulation. Therefore, instead of being closer to the interior temperature, the steel lintel will be closer to the exterior temperature.
One approach to control condensation is to reduce exposing the steel lintel by covering the exposed portion of the steel plate on the exterior using a thin layer of insulation and sheet metal trim (Figure 2).
This may be effective where insulated cavity walls are used. But it will have limited benefit when insulation is added to the interior of the wall.
Another approach is to virtually eliminate the thermal short altogether. You can accomplish this by welding a clip angle onto a stiffener attached to the lintel beam (Figure 3). The steel angle that supports the brick veneer can then be bolted to this clip angle.
In this case, plastic shims can be used as an isolator between the interior angle and the angle welded to the flange of the beam. Since these clip angles are not continuous, the cavity rigid insulation would be placed between them. This virtually eliminates the thermal bridge.
Another advantage of this third approach is that anchors for the window will not penetrate the flashing. When the steel plate is attached to the underside of the steel lintel beam, anchors of the head of the windows can penetrate the flashing. Separating the veneer lintel from the lintel for the backup simplifies the anchor installation.
A Principal at Wiss, Janney, Elstner Associates Inc., Northbrook, Ill.
