Some architects provide a membrane or seal to isolate the masonry cavity from the jambs of windows. What is the purpose of the seal? Is it required by codes?

This product is sometimes referred to as a ¡§cavity seal.¡¨ It is provided to prevent moisture in the masonry cavity from coming in contact with the jambs of the windows and from entering the interior in the space behind the gypsum wallboard. It is not required by building codes; however, the use of cavity seals can prevent some moisture problems.

Air within a masonry cavity can become very moist, especially during the summer months following periods of heavy rain. Heavy rains can wet, and in some cases nearly saturate, the masonry wall. When sunlight hits this wall, the temperature on the surface increases. Our firm has measured temperatures in excess of 160„a Fahrenheit on walls with southern exposures.

Air within the cavity also heats up. As the moisture evaporates, some evaporates to the exterior and some into the cavity behind. The warm, moist air in the cavity can come in contact with the cooler window frames and condense if no cavity seal is provided.

Moisture also can enter behind the interior gypsum wallboard in the space between the backup and the wallboard. If the wallboard is sealed against the window system, air from the cavity can be drawn in between the wallboard and the backup and can flow out of any penetrations, such as electrical outlets. This scenario is especially a problem when the interior air is negatively pressurized relative to the exterior.

Negative air pressure occurs when the volume of exhausted air in a space exceeds the makeup air volume. This negative pressure can create an airflow path from the masonry cavity, through the wall system, and out of the wall at any outlets and other penetrations. Condensation occurs along this airflow path and can lead to mold growth if there are food sources, such as paper on the wallboard or paste for wall coverings.

The cavity seal is typically installed between the exterior surface of the backup and the window system. This placement is feasible when the windows are installed before the masonry, as is often the case with wood frame construction using windows with integral nailing fins/flanges. The nailing fins are integrated with an air barrier on the exterior surface of the backup.

For many buildings, however, windows are installed following the completion of the masonry walls. In this case, it may be possible to install a membrane sealed to the surface of the backup wall, which can be integrated with the window system upon installation to create a cavity seal.

An alternative approach is to provide a seal between the veneer and the backup at the jambs and head of the opening before the windows are installed. In this approach, the system should be designed to handle potential water penetration at perimeter sealant joints. This design can often be accomplished by using a sill flashing that seals against the masonry and backup wall at the jambs. This flashing directs water back to the exterior.