Rain Screen Exterior Walls
You have probably seen them before--seeing rain fly over a tent is a simple example of rain screen. Rain screens shed most of the rain and manage the rest, preventing moisture intrusion and the resulting premature decay in homes.
Rather than attacking the symptoms of moisture intrusion, rain screens tackle the source-the forces that drive water into the building shell. By neutralizing these forces, rain screens can withstand extreme environments. They appear to be effective in any climate and handle any weather condition short of a disaster.
All rain screens include the following elements:
- Vented or porous exterior cladding
- Air cavity (a few inches of depth is sufficient)
- Drainage layer on support wall
- Rigid, water-resistant, airtight, support wall
There are four basic approaches to water penetration control in buildings:
- Mass: traditional, solid assemblies that shed most surface water, effectively absorb the remainder, and subsequently release absorbed moisture as a vapor. Examples include solid concrete, masonry, and timber structures. This approach has variable effectiveness in most climates.
- Typical face seal, or barrier: surface designed to completely shed surface water with no moisture penetration. Examples include barrier-type exterior insulation finish systems (EIFS) and stucco or clapboard walls built without a drainage plane (e.g., housewrap, building paper). These are effective in climates with less than 30 inches of precipitation annually.
- Internal drainage plane: a drainage plane or moisture barrier located between the exterior cladding and the supporting wall that provides redundancy of moisture resistance. Examples include typical stucco and clapboard walls built with a drainage plane. These are effective in climates with an annual precipitation of less than 50 inches.
- Rain screen: a moisture-management system incorporating cladding, air cavity, drainage plane, and airtight support wall to offer multiple moisture-shedding pathways. Rain screens diminish the forces attempting to drive moisture into the wall. There are two types of rain screens: simple rain screens and pressure-equalized rain screens (PER). Examples include brick veneer cavity walls, furred-out clapboard walls, and drainable EIFS. Simple rain screens are effective in climates with an annual precipitation of less than 60 inches; PERs are effective in climates with an annual precipitation of 60 inches or more.
Rain screens effectively "drain the rain." They control powerful building wetting forces-gravity, capillary action, and wind pressure differences.
The exterior cladding deters surface raindrop momentum. It is typically porous with many air bypasses. An airspace separates the cladding from the support wall. The airspace decouples most of the cladding from the support wall, thereby reducing splash and capillary moisture transfer. Large, protected openings (e.g., vents, or weep holes) positioned at the top and bottom of the wall promote convective airflow, allowing moisture to quickly drain or evaporate from the air cavity. The exterior face of the support wall is protected with a drainage layer to further protect against any moisture that bypasses both cladding and air cavity. The wall airtightness (i.e., sealed assembly) buffers the remaining differential air pressure force.
Simple rain screens rely on the airspace next to the drainage plane to quickly and freely remove water from the wall. In a brick veneer wall, for example, rain water passing through cracks can trickle down the back face of the veneer to either leak out of weep holes, evaporate, or be reabsorbed into the masonry.
Windy conditions can flex drainage plane materials or thin plastic insulation panels that can draw moisture into the air cavity through a bellows-like action. It is therefore important that these materials be fairly rigid.
Pressure-equalized rain screens (PER), an advanced version of the simple rain screen, carefully integrate porous exterior claddings, compartmentalized air spaces, generous ventilation, and watertight, airtight support walls. PERs terminate the pressure differential across cladding systems that is magnified by winds. This effectively eliminates the remaining moisture force affecting rain screens. PER systems employ barriers to compartmentalize the air cavity, thereby allowing rapid air pressure equalization and minimal moisture intrusion. This limits the opportunity for rain penetration beyond the cladding.
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Walls constructed with brick veneer and furred-out clapboard are two classic simple rain screen assemblies. To ensure against oozing mortar joints, mortar droppings, and other poor practices that allow water to bridge across the air cavity, the air cavity of small, simple rain screen walls should never have less than a continuous, 3/8-inch clear airspace. A properly installed drainage plane (e.g., building paper, housewrap) is needed at the back of the rain screen cavity. Full-head weep holes (e.g., the vertical joint between laid bricks) with minimum 24-inch on center spacing provide opportunity to equalize pressures and expel moisture.
Clapboard rain screens have been recommended for decades to mitigate moisture problems in wood walls. In this approach, siding is typically installed on vertical furring strips (commonly wooden 1x2s) attached to studs. Wood claddings are then back-sealed with a breathable, moisture-resistant, penetrating sealer. Porous plastic matting materials (see Contacts) may be used economically to create the airspace instead of furring strips, but the resulting airspace is narrower than recommended.
PER systems go beyond the simple rain screen design. In brick veneer systems, for example, alternating open head joints in the bottom course of brick improves its quick response in equalizing pressure. The top of the wall is also ventilated with a protected wall vent, gap, or air pathway to a ridge vent. Tall brick veneer walls may need to be fitted with weep holes and flashing between stories. However, capping will reduce convective airflow and slows the drying process.
At a minimum, compartment seals should be installed at building corners and parapets-the more structural and airtight the compartment seal and air barrier assembly, the better. Oil-impregnated, adhesive-backed, expanding foam seals are readily available options. Also check to ensure the support wall materials offer sufficient rigidity against depressurizing flexure during wind loading. Canada Mortgage and Housing Corporation (CMHC) documentation provides technical guidelines for PER design and construction.
The rain screen technique offers significant advantages over other systems:
- It neutralizes physical forces inducing water intrusion,
- It is a simple, forgiving system with built-in, multilayered 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 PER 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 callbacks, and homeowners looking for comfort.
Moisture within a simple rain screen can be drawn into the inner wall if the forces acting on it remain high due to storm or climate. Small weep holes typical of brick veneer walls may be incapable of balancing pressures quickly enough, and vermin and insects may nest in the cavity.
Applying PER technology to a wall or joint demands additional detailing care. Short-lived sealants and foam gaskets that disintegrate will decrease the effectiveness and may incur future maintenance costs. Mechanical seals (e.g., metal flashing, gasketed furring strips) offer a more permanent approach , but increase cost and complication.
PER air barriers of comparatively flexible rigid foam sheathing are inadequate unless supported by and secured to a rigid substrate (e.g., ½-inch thick oriented strand board [OSB]) capable of withstanding dynamic wind pressure loading.
Current building codes do not mandate rain screen designs. They are, however, increasingly under consideration for adoption into building codes. Code criteria is typically based on structural issues. Advocacy and manufacturer recommended practices (e.g., furring strips between cedar siding and foil-face rigid insulation) have developed through trial and error.
Simple rain screen walls are built daily from readily available materials. PERs are not common in residential construction. Compartment seals for PER systems are neither well understood nor in widespread use in the residential market. Commercial PER systems are becoming more popular, which may lead to faster, more cost-efficient, residential integration.
Do you have a specific question about this technology and/or its 'real life' applications? Try the contacts listed below:
American Architectural Manufacturers Association (AAMA)
1827 Walden Office Square, Suite 104, Schaumburg, IL 60173-4628
Phone: (847) 303-5664
"The Rain Screen Principle and Pressure Equalized Wall Design (1996)," Item No. CW-RS-1
Canada Mortgage and Housing Corporation (CMHC/SCHL)
CMHC Information Products
P.O. Box 35005, Station BRM B
Toronto, Ontario M7Y 6E5 Canada
Canadian Home Information Centre
700 Montreal Road
Ottawa, Canada K1A 0P7
Phone (Intl.): (613) 748-2003
Phone (Canada): (800) 668-2642
RainScreen 2.0 Computer Program
Best Practice Guides
Rain Penetration Control
Exterior Insulation Manufacturer's Association (EIMA)
3000 Corporate Center Drive, Suite 270
Morrow, GA 30260
Phone: (800) 294-3462, (770) 968-7945
Akzo Nobel Geosynthetics Company
P.O. Box 7249
Asheville, NC 28802
Phone: (800) 365-7391, (828) 665-5050
Colbond Geosynthetics Company
P.O. Box 1057
Enka, NC 28728
Phone: (828) 665-5050
Enkasonic-0.4" thick three-dimensional geomatrix extruded nylon filament composite
Benjamin Obkyke Inc.
John Fitch Industrial Park
65 Steamboat Drive
Warminster, PA 18974
Phone: (800) 346-7655, (800) 523-5261, (215) 672-7200
Cedar Breather® Wood Roofing and Siding Underlayment -woven wire mesh provides ¼-inch air space
Dow Chemical Co. (FHB #56, 1989)
Styrofoam Brand Products
202 Willard H. Dow
Midland, MI 48674
Phone: (800) 441-4369
Thermadry™-hybrid insulation/drain board (grooved XPS covered with layer of filter fabric)
Jay-K Independent Lumber Corp. (EDU, July 94)
P.O. Box 378
New Hartford, NY 13413
Phone: (315) 735-4475
EndurSide™-modified wood clapboard (Kevin Kelly)
P.O. Box 818
Montrose, CO 81402
Phone: (800) 501-7657
Kil-N-Dri™ Ventilation System-two-sided-waffle-pattern EPS insulation panel (inventor)
Owens Corning Fiberglass
One Owens Corning Parkway
Toledo, OH 43659
Phone: (800) 438-7465, "Facts x Fax:" (800) 354-7465
Warm-N-Dri® mineral fiber insulation board-draining insulation board applied with Tuff-N-Dri foundation waterproofing
Koch Materials Company
PO Box 2155
Heath, OH 43056
Phone: (800) 379-2768
Tuff-N-Dri-foundation waterproofing membrane applied with Warm-N-Dri®
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