Sizing the space
As previously mentioned, the industry standard for a cavity width is 51 mm (2 in.), but according to the Brick Industry Association’s (BIA’s) Technical Note 27, Brick Masonry Rain Screen Walls, the air space should not be less than 25 mm (1 in.) when outboard rigid insulation is being used. This means even though a 51-mm air space is recommended, the code minimum is half that.
When given the choice, walls are often built to code-minimum dimensions, leaving this smaller air space prone to excess mortar droppings blocking the path for moisture to exit the wall system. By building with an all-wall drainage mat, this cavity width will remain clear of mortar droppings, allowing the reduced airspace to still drain and ventilate effectively. When choosing an engineered rainscreen for masonry applications, one should specify a drainage mat with a filter fabric bonded to it in order to prevent the mortar or scratch coat from seeping through.
From a residential standpoint, wall systems are not designed with any air space at all (Figure 9). These designs leave trapped moisture nowhere to go, which often leads to rot, mould, and IAQ issues. Most homeowner’s insurance policies will only cover a maximum $5000 for damage from moisture-related issues—this is frequently only a fraction of the renovation costs. Using an engineered rainscreen drainage mat in residential applications is like offering the cavity wall concept to homeowners, yet making it cost-effective for the residential sector.
Engineered rainscreen products are typically made from either a corrugated sheet, dimpled mat, or a random entangled net material (Figure 10) made from either polypropylene or nylon. These types of drainage mats are favourable for wall applications for numerous reasons. For example, the polypropylene or nylon makeup allows for mould and mildew resistance. The open entangled net design also allows for multi-directional drainage and ventilation. Some engineered rainscreen drainage and ventilation mats are also manufactured from recycled materials. It is important to ensure the engineered rainscreen drainage mat meets the requirements of UL 723, Test for Surface Burning Characteristics of Building Materials, and can handle freeze-thaw conditions.
It is important the capillary break created by these engineered rainscreens be at least 4.8 mm (3/16 in.) or greater according to ASTM E2925-14, Standard Specification for Manufactured Polymeric Drainage and Ventilation Materials Used to Provide a Rainscreen Function. This proper capillary break greatly reduces the bulk moisture reaching the water-resistant barrier as well as the transmission of surfactants contained within some cladding materials.
Again, when specifying an engineered rainscreen material for masonry applications, it is important the drainage mat have a filter fabric bonded on one side. This filter fabric not only acts as a mortar deflection blocking the mortar, but also allows the moisture to pass through and drain down the unobstructed channel. The filter fabric increases the sheer and tensile strength of the product while helping keep a uniform airspace for proper ventilation and drainage. It is also important the materials specified are resilient, lasting the life of the building.
In commercial applications, drainage mats should not be the same width as the air space. Oftentimes, a 25-mm (1-in.) air space is a nominal 25 mm (Figure 11). There needs to be enough space between the back of the brick and the drainage mat for the mason to put their fingers, making it easier to lay the brick or stone. Most excess mortar in the bed joints will be forced out of the wall, not fall within the cavity. Even if there is a slight mortar buildup, the filter fabric will allow the moisture to drain through and down the wall to its exit point.
In order to prevent mortar bridging due to capillary action, the drainage plane is recommended to be 10 mm (3/8 in.) or greater for commercial applications. In Section 126.96.36.199 of the 2010 National Building Code of Canada (NBC), it clearly states there needs to be:
a drained and vented air space not less than 10 mm deep behind the cladding, over the full height of the wall.