In addition, the exterior CI can prevent issues related to condensation and moisture. Many dew point calculations in rigorous building science studies have proven that walls with no insulation between studs and using only exterior ci are fully protected from air leakage condensation during cold weather, if the ci is installed on exterior side of the vapor barrier. The reverse—heavy interior insulation and little or none outside—lead to condensation occurring on the inside, and the potential for mold and rot with the attendant structural deterioration and occupant health risks. For overcladding scenarios, rainscreen systems provide the best opportunity for including ci.
Important air barrier considerations
The purpose of an air barrier is to stop airflow or resist air pressure within the enclosure (or between conditioned and unconditioned spaces) to control the flow of heat and moisture. As such, any air barrier must be continuous and durable, with all transitions and penetrations carefully sealed. Unchecked air movement creates both moisture management problems and thermal performance issues, since air carries moisture and water vapor as well as heat, or lack of it.
With respect to overcladding masonry, the act of repairing and smoothing the existing wall by parging, as well as by tending to and sealing cracks in masonry and mortar, results in an enhanced air barrier function. This is further improved with the installation of a fluid-applied vapor barrier within the overclad assembly, creating an additional impediment to the movement of moisture through the system. To state plainly, the air barrier is not a single component of the enclosure assembly. When thinking of the air barrier, one should think about the flow of air through the assembly in total, as an integrated system.
How much air should pass through the enclosure? Typically, the answer is none. To the definition that an air barrier should be continuous, structural, and uninterrupted, according to Wagdy Anis, FAIA, writing for the Whole Building Design Guide. As he stated, the enclosure and the overcladding layer should address three types of air leaks through an architectural enclosure:
- Orifice flow, e.g. in a slit unintentionally left between a window rough opening and its frame.
- Diffuse flow, such as through some brick or concrete block.
- Channel flow, the common type of air leaks where the air entry point and exit point are distant from each other, giving the air enough time to cool below its dew point and deposit moisture in the building enclosure.2
Taken together, these principles help to effectively design and specify overcladding for one of the most common challenges faced by long-term building owners: poorly performing and unattractive masonry walls. Failure modes in masonry facades where the masonry wall is supported from steel lintels begin with age alone and can be exacerbated by older practices, such as missing or ineffective masonry expansion joints. Some cavity wall construction may lack thermal insulation as well.
Thermally displaced masonry will result from the lack of expansion joints, which may be visible in brick facades. In extreme cases, these may impact glazing systems. Some exterior walls may not even comply with modern codes for resisting lateral imposed loads.
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