Is a vapor retarding membrane required in an insulated masonry cavity wall?
Determining the need for a vapor retarder is a decision that should be reviewed with a knowledgeable mechanical engineer. Depending on the exterior wall construction, vapor-retarding membranes can be omitted in an insulated cavity wall in most parts of the United States. Typically, either foil-faced polyisocyanurate or extruded polystyrene insulation can be used in cavity wall systems. Both of these rigid insulation materials will also function as vapor retarders. A wall system consisting of a masonry veneer, a cavity with rigid insulation, and an interior concrete masonry wall will be effective in most climates. In the northern parts of the United States, the predominant vapor drive occurs during cold outdoor temperatures from the interior to the exterior. In these conditions, the dew point will occur within the insulation. Because the insulation serves as its own vapor retarder, there should not be a problem. The same is true for the southeastern United States, where the primary vapor drive is from the exterior to the interior during the hot humid summer months.
In regions with a northern climate, such as northern Minnesota or Canada, hospitals and other buildings that must be pressurized may require a vapor-retarding membrane that also functions as an effective air barrier. In these cases, it is often best to parge the exterior surface of the concrete masonry with approximately 1/2 inch of parging. This parge coat can be covered with a vapor-retarding membrane that can be installed and inspected on the exterior prior to the installation of the rigid insulation and brick outer wythe. In hospitals humidified and pressurized relative to the exterior, this will help prevent moisture-laden interior air from moving through and condensing within the wall system, causing moisture problems.
It is also possible to create an effective vapor retarder/air barrier by using a combination of taping and sealing the joints in the rigid insulation, after the insulation and before the brick veneer wythe is installed. The tape must be applied carefully around the eye-and-pintle wall anchors that project through joints in the insulation and at the top and bottom of the insulation. Sealant may be used at these critical areas to ensure a continuous air and vapor-retarding membrane.
Condensation problems in these systems often occur when gypsum wallboard or additional insulation is added on the interior. If enough insulation is added to move the dew point out of the masonry cavity, then a vapor-retarding membrane will usually be required. Most codes do not allow the use of polyisocyanurate or extruded polystyrene insulation on the interior side of concrete masonry. To satisfy building code requirements, fiberglass batt insulation typically is used. This insulation generally needs a vapor-retarding membrane. In northern climates, this vapor-retarding membrane will be located over the interior surface of the insulation behind the interior gypsum wallboard. Unfortunately, it is often very difficult to install a vapor-retarding membrane in this location. Openings for electrical or telephone wires will cause breaches in the insulation and the air barrier. Such breaches will not occur when the insulation is placed within the cavity wall itself.
Sometimes problems will occur in systems in which the vapor retarder is properly placed. In these cases, the primary problem is air movement. When possible, the best way to control air movement is to control building pressurization. In northern climates, problems will typically be minimized if the buildings are maintained at a slightly negative pressurization. Provided that the masonry walls are reasonably airtight, this will draw the relatively cool dry exterior air through the wall system. In the southeastern United States, positive pressurization of the interior air relative to the exterior will allow the cool dry air from the interior to move through the wall system.