Q: Recently, I have seen many wall system designs where rigid insulation is placed in the air space between the veneer and backup, instead of being located in the stud space behind the brick veneer. I assume this placement is done to avoid thermal shorts that occur at the steel studs.
Wouldn't it be better to put some insulation in the air space and then fill the steel studs with additional insulation? This placement would not only have a greater R-value, but would result in a thinner total wall thickness.
A: Placing rigid insulation in the air space between the veneer and backup rather than in the stud space has two purposes. The first is to avoid the thermal shorts at the steel studs, as you mentioned. The second is to reduce or eliminate the potential for condensation within the studs or sheathing.
There is a greater potential for condensation problems when all or some of the insulation is placed in the stud space. In this wall design, the recommended location of the vapor retarder during winter and summer conditions will change.
Since water condensing within the stud space can cause problems, vapor retarders are installed to prevent moisture from entering there. During warm weather conditions, the desired location of the vapor retarder is on the exterior face of the insulation. During cold weather, the best location is on the interior face of the insulation.
However, placing vapor retarders on both the interior and exterior face of the insulation often leads to moisture problems because any air movement can cause condensation within the space between the two vapor retarding membranes. The trapped moisture may then accumulate.
Placing all the insulation in the air space between the veneer and backup, and using rigid insulation that is not sensitive to moisture, solves this problem since no matter what direction the vapor drive comes from, the system will perform. In this configuration, the water resistive barrier placed on the surface of the sheathing also functions as an air and vapor retarder, which addresses both summer and winter conditions.
In cold weather, moisture is greater within the interior air. Therefore, moisture moves from the interior towards the veneer. Moisture is stopped at the vapor retarder on the surface of the sheathing, which is on the warm side of the insulation, and therefore, condensation does not occur.
In warm weather, especially in areas where the brick can get wet repeatedly, the humidity in the air space is very high. Moisture from the air between the veneer and backup condenses on the cool vapor retarder. Because the vapor retarder also functions as the water resistive barrier, water condensing on the surface of the membrane flows down the wall and is handled by the flashing system, as shown in the illustration. Because rigid insulation in the air space is not adversely affected by water or moisture, this wetting is not harmful.
Positioning the vapor retarder on the exterior face of the sheathing has other advantages. It is not penetrated by electrical outlets or other openings through the interior gypsum wallboard. The membrane is easily inspected from the exterior prior to installing the insulation and masonry. The vapor retarding membrane on the exterior face of the sheathing is integrated with any flashings within the wall system.