The On The Face Of It blog series has been written to give property professionals an overview of the curtain wall façade, differences in system types and defects which are commonly observed during inspections.
Blog #3 in this series will look at two commonly used drainage systems – Water Managed and Pressure Equalised.
Why is drainage so important?
It is generally accepted that it is not possible to totally prevent water from entering a façade. I have inspected face-sealed facades (designed to prevent rainwater ingress) during my career and all have exhibited various failures as water always finds a way in… especially with help from its best friends, wind pressure and gravity.
“Water can enter the exterior wall system by means of five different forces: gravity, kinetic energy, air pressure difference, surface tension, and capillary action. To mitigate water infiltration, all of these forces must be accounted for in the system design.” ¹ Curtain Walls – Whole Building Design Guide
Considered the most reliable is the pressure equalised system. The water managed system is also considered reliable but it does not have the same belts and braces features we see in the pressure equalised system. Both are discussed in more detail below:
Pressure Equalised Systems
The forces described in the Whole Building Design Guide quote above, are what a pressure equalised system is designed to resist:
- The exterior face of the glazing, external gaskets and the exterior face of the framing form a rainscreen. The rainscreen sheds the water from the surface of the facade.
- The junction of the interior face of the glazing and the glazing pocket are sealed with gaskets or wet seals to create the air tight barrier.
- Between the rainscreen and the air tight barrier is the pressure equalisation chamber (within the glazing pocket). This chamber removes the pressure difference between the two faces which reduces water penetration
Water Managed Systems
Air barriers are not present in water managed systems which means it is possible for the pressure difference to allow leaks to occur under certain conditions. However, the system does have the ability to drain water away.
Where water enters the glazing pockets in which the glazing unit sits, this water can drain from the façade through the weep holes.
Identification
The best way to tell the difference between these two systems is to remove the transom and mullion cover flashings to see whether each glazing section is isolated from the next and air barrier detailing is continuous around each glazing unit. If this is the case, then you are looking at a pressure equalised system.
By Victoria Richardson
References
¹ Wbdg.org. (2016). Curtain Walls | WBDG Whole Building Design Guide. [online] Available at: https://www.wbdg.org/guides-specifications/building-envelope-design-guide/fenestration-systems/curtain-walls [Accessed 3 Apr. 2018].
On The Face Of It Blog Series:
The next blog in this series will be published later this year.
Blog #4 will look at common gasket defects and gasket replacement.
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