Rain screens, while common in Europe and North America, have begun to emerge to a greater degree in discussions within the Australian architecture and design community. A rain screen system offers numerous benefits in moisture-management and energy-efficiency but when I dig deep into conversations over the past couple years with local architects, façade consultants, suppliers and installers there is a huge variety of perceptions on what “rain screen” actually means and what the key design components are.
I recently came across an excellent document jointly published by the Ontario Institute of Architects and available for free for download from the Canadian Mortgage and Housing Corporation.
Page 26 includes an excellent checklist of 10 critical points in rain screen details.
- Provision for water shedding at the outer cladding, away from joints, with drips under any projections to prevent water collecting at the building face.
- A cavity of appropriate width to allow pressure equalization across the cladding system and prevent capillary movement (allow for construction tolerances!).
- A continuous and effective (for example, airtight to a maximum air leakage of 0.1 L/s/m²) air barrier within the backup wall.
- Drainage of the cavity through continuous flashings and weep openings, and proper management of drained water.
- Adequate venting of the cavity provided through properly located openings in the cladding (appropriate ratio achieved between vent area and leakage of air barrier and seals).
- Additional provision for drainage at the backup wall (located on the “warm” side of the insulation to avoid condensation problems).
- Effective compartmentalization of the cavity at each building face with airtight seals, and additionally across the width of the façade as required (refer to calculations).
- Sufficient rigidity and/or structural support of the air barrier to resist wind loads and limit deflection.
- Sufficient rigidity of the cladding to limit deflection and resist wind loads as required.
- Special attention paid to water-resistance and drainage at building edges and parapets (areas subject to heaviest rain-wetting and wind pressure differences, where pressure equalization may not be achievable)
The National Association of Home Builders (NAHB) in the US outline the advantages of rain screen
- It neutralizes physical forces inducing water intrusion.
- It is a simple, forgiving system with built-in, multi-layered redundancy, and
- It has integrated drainage and ventilation that accelerates cavity moisture removal.
Rain screens prevent or reduce moisture problems in exterior walls, including siding rot and peeling paint. Thermal shock, solar driven moisture effects, and pressure forces are diminished. In high moisture environments, the additional cost and complexity of Pressure Equalized Rain Screen construction may be cost effective over the long term.
Rain screen designs do not increase the cost of drainable EIFS and brick veneer wall systems. To install the air space behind the cladding in most assemblies, however, costs for furring or other spacer materials cause costs to increase. The few cents per square foot spent on a rain screen offer exceptional value to design professionals seeking liability protection, builders wanting to avoid call backs, and homeowners looking for comfort.
The Proctor range of vapour permeable membranes ProctorWrap High Tensile, Air Tight and Black Label can play an important part in meeting these requirements and provide a continuous air barrier. In particular the Black Label has been designed with high UV resistance for use in open joint rain screen applications.
One area of debate is if a flexible membrane can be relied upon as the air barrier in pressure equalized rain screen design without a rigid substrate. Point 8 above advises that a “sufficient rigidity and/or structural support of the air barrier to resist wind loads and limit deflection,” is required.
Studies by the Institute for Research and Construction in Canada report that the performance of the air barrier system affects the ability of the wall assembly to achieve pressure equalization across the rain screen, by reducing the flow of air through the wall assembly, and greatly contributing toward reducing the air pressure differential across the rain screen.
“Under dynamic-pressure conditions, recent IRC studies indicate that excessive flexibility of the air barrier system will result in fluctuations in the volume of the air chamber compartment. These fluctuations adversely affect the potential for rapid pressure equalization across the rainscreen.”
Chown, G.A., Poirier, G.F. and W.C. Brown, “Designing Exterior Walls According to the Rainscreen Principle”; Technology Update No. 17, Institute for Research in Construction, National Research Council of Canada, 1998. http://www.nrc-cnrc.gc.ca/obj/irc/doc/ctu-n34_eng.pdf
It is clear that in order to attain the full benefits of rain screen then we need at the outset a clearer understanding of the function of the rains screen and what is required for the building envelop to function as such.
Download the full CMHC document here: http://www.cmhc-schl.gc.ca/en/inpr/bude/himu/coedar/upload/The-Rain-Screen-Wall-System.pdf
Asia Pacific Manager