Value of ventilation
Today’s building enclosures have lower moisture storage capacity and drying potential than in old times.
Not long ago, framed buildings had better drying capacity. Since there was little vapor resistance, less insulation, and more air leakage, walls dried just from the energy used to heat them, even though they had minimal moisture storage capacity. As energy efficiency improved over time, enclosures began to retain more moisture. When insulation was added for greater comfort, it led to moisture problems in colder climates, most noticeably in the 1930s, because the heat energy was no longer acting as drying energy.
In order to improve enclosure design, building professionals learned the importance of:
- incorporating drainage spaces to rid the wall assembly of bulk water;
- including air spaces to provide ventilation for enhanced drying potential of the wall assembly;
- separating air gaps with an impermeable plane;
- adding capillary breaks; and
- controlling solar-driven moisture when using absorptive claddings.
The newer technical construction of rainscreens allows the surface of the cladding to be separated from the wall, enabling the continual flow of air, leading to significant benefits in drainage and ventilation and a decrease in thermal load. The air exchange in rainscreen walls is expected to provide ventilation drying if excess moisture is absorbed in the wall construction. Using heat and air movement helps reduce the mass of water more quickly than no ventilation, just air movement, or just heat (e.g. warming from the sun).
Extensive research has been done on evaluating the airflow rate in the air cavity and the moisture removal by cavity ventilation through laboratory testing, field measurements, and simulations. Findings indicate rainscreen ventilation helps drying (read Bassett and Mcneil 2005a, 2005b; Davidovic 2004; Hansen et al. 2002; Hazleden 2001; Straube and Burnett 1995; VanStraaten and Straube 2004; Onysko 2003). Panels with both bottom and top vents dried faster than the comparable panels with only bottom vents (‘drained screen’), and the walls with a 19-mm (3/4-in.) cavity dried faster than panels with a 10-mm (3/8-in.) cavity.
Current practices vary in terms of specifying cavity depth and slot vent heights for panel systems. In 2012, a team at Lund University in Sweden conducted a ventilated rainscreen cladding study that focused on the drying process. They found the cavity design is of major importance for the drying rate if the material adjacent to the cavity is wet over its entire extension. Of significance, a small cavity depth (<10 mm) prolongs the ventilation drying process.
The cavity depth and vent size have a positive impact on the airflow rate in the cavity. However, the amount of moisture removed by the ventilation is governed by the properties of the sheathing membrane once the airflow rate reaches a threshold value. For panel systems, an air cavity depth of 19 mm provides higher airflow and drying rates compared to a 10-mm cavity. For cavity depths greater than 19 mm, higher ventilation rates can be achieved through larger vent openings, but the drying rates are not significantly influenced.
When designing a rainscreen, it is important to understand why one is putting a space behind the cladding and what performance criteria can be met. A common question is just how big that gap needs to be. If the gap is for drainage only, it may be quite small. Even the space between two layers of a WRB is adequate to provide drainage. However, the gap must be bigger to include ventilation.
To better understand why rainscreen systems include drainage and ventilation cavities, one must understand the different expectations for each. The drainage cavity will remove much of the bulk moisture by gravity. However, moisture can still remain adhered to or absorbed in materials within the wall assembly. The amount of moisture that can be safely absorbed or stored depends on the material properties. Drying can occur by vapor diffusion, evaporation, desorption, or by air convection (i.e. ventilation).