Understanding Rain and Rain Control
Rain control is a critical function of the building enclosure. At our February LAB Event, John Straube discussed why that is, and how rain can be managed by selecting appropriate strategies and materials and developing the right details to implement them.
As John explained, there are three main strategies for managing rainwater: drained, mass/storage, and perfect barrier. Drained (sometimes called rainscreen) walls assume that some rain will get past the cladding, and include a drainage space behind the cladding and a drainage plane to prevent rain from moving further into the wall. Mass/storage walls can absorb and tolerate enough rain, and can dry enough between rain events, to avoid any water reaching inner surfaces. Perfect barrier walls attempt to completely stop rain from penetrating; in practice, this strategy requires a combination of perfect barrier components (e.g. insulated metal panels or structural glazing) and drained joints.
None of these approaches are foolproof, and the choice between them should be made in part based on an understanding of rain loads. As John noted, many walls actually don’t get very wet. The rain load for vertical walls depends on:
- Climate (are you in Halifax or Calgary? Denver or Las Vegas?)
- Topography (are you on a hill or in a valley?)
- Exposure (are you a building surrounded by trees and other buildings? Or are you the thing that sticks out?)
- Height (tall things get wetter than short things)
- Shape (pyramid shapes get wetter than inverted pyramids)
Underlying all of these factors is the presence or absence of wind. Rain falling vertically won’t be much of a problem for a vertical wall – but wind changes the direction of the rain’s trajectory, causing more of it to land on the wall’s surface.
The science behind driving rain is pretty fascinating, as John illustrates in this clip:
An hour and a half is a short time to cover all the nuances of rain control, and we will no doubt hear more about this important topic at future events. In the meantime, if you are curious you can check out some of the resources here.
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