I haven’t spent any time writing about rain screens — a building technique that has tended to be associated with wetter climates. The point of rain screens is to let the water that gets behind the facade drain out so that the building stays dry. As our buildings are designed to be tighter and tighter, any penetration can lead to water getting into a wall assembly but having nowhere to get out causing all kinds of havoc from wood rot to mould build-up. Eventually, these two things can lead to structural failure and health issues for building occupants.
I contacted Dave Petersen from Outside In Design Build to discuss rain screens, a technique for constructing a wall assembly that has gained traction over the years due to its ability to keep water away from infiltrating walls. According to Dave, it is a requirement of most local building codes. While it is used on the (usually) rainy West Coast (note they’ve instituted drought restrictions in Vancouver), it is also a good method for building here in the east, even in colder climates, and, in fact, many cladding materials require its use with their products.
How it works (from correspondence with Dave Petersen):
The rain screen assembly allows for water getting past the outer (face) barrier to weep down and outward (gravity assisted and pressure equalized) once the wind abates through a series of engineered flashings and weep-assemblies. These often include bug screens, through wall flashings in metal and ice and water shield materials. The key with this system is to allow for pressure equalization behind the face materials which will allow the water to drain away instead of continuing its way through the wall assembly. Most wall systems (brick and stone veneer, siding, EIFS*, cement board, etc.) are designed to work as part of a rain screen wall system – there are few barrier walls left, other than precast concrete panels, which have a rain screen caulking system that helps drain these assemblies. Hot/dry climates can even benefit from a rain screen cladding as it may act as a radiant barrier and slows heat transfer through the façade into the building.
Traditional Face-sealed facades (Diagram from: http://www.buildingscience.com/documents/digests/bsd-013-rain-control-in-buildings)
Rain screen assembly (Diagram from: http://www.buildingscience.com/documents/digests/bsd-013-rain-control-in-buildings)
The pros and cons of rain screens are listed below (again, thanks to Dave Petersen).
- Enhanced water management in all climate zones (USA and Canada)
- Improved material durability
- Better IEQ
- Effective at blocking radiant heat gains
- Possibly higher costs
- More detailing at the site level
- May be prone to detailing errors that limit its effectiveness (mortar dams, etc)
The cons can be minimized by using an integrated system approach and most cladding products are readily detailed for these types of walls. Education of trades and proper site management will minimize most of the other issues.
(*EIFS – “Exterior Insulation and Finish Systems”).
For a detailed explanation of how rain screens work to keep water from infiltrating building envelopes, visit Building Science Corporation‘s thorough explanation of how they work: http://www.buildingscience.com/documents/digests/bsd030-rain-control-theory/?searchterm=rain%20barriers
and this one: http://www.buildingscience.com/documents/digests/bsd-013-rain-control-in-buildings
Thanks again to Dave Petersen for his time and knowledge on rain screens!
Visit: Outside In Design Build for more information on sustainable building.