The best way to protect the structure, siding, and paint or stain from moisture damage is to design the outer layer of the house as a decorative “rain screen”. A rain screen should be sturdy enough to block most of the wind and rain, but porous enough to dry to the exterior when wet.
This is accomplished by separating the outer cladding from the building’s water-resistive barrier with an air space. This approach accepts the fact that no siding system is entirely waterproof and relies, instead, on the drainage layer for waterproofing and moisture control (see illustration).
The interest in rain-screen systems has grown in response to problems that modern builders were experiencing with premature failure of sidings and exterior finishes.
Modern construction is a lot more challenging to exterior finishes for a number of reasons. Insulated walls keep siding materials colder and wetter. And, in some cases, material quality has suffered as well.
Older, uninsulated walls were warmed by heat loss from the house and and kept dry by wind blowing through the plank sheathing. Also premium, quarter-sawn siding was dimensionally stable and held paint well. The result was durable exteriors, but high energy bills. Rain screens bring some of the same resilience and durability to modern energy-efficient wall systems covered with the types of siding and finishes commonly used today.
Rain screens are the optimal approach to exteriors and will increase the siding performance with any wall system. They are especially important with foam-core panels (SIPs), walls filled closed-cell foam, or other wall systems that need to dry to exterior. This includes the installation of wood siding over foam sheathing, a detail which has had a long history of problems.
The rain-screen system has four components: an exterior cladding, an air space, a drainage plane, and weep holes.
1. Cladding. In a rain-screen wall, the exterior finish material — whether wood, brick, vinyl, or stucco – is mainly aesthetic. In addition, the cladding must sheds most of the water that strikes the side of the building and also protect the sheathing wrap from wind and ultraviolet (UV) radiation. While some exterior claddings are more porous to water than others—for example, brick, vinyl, and vertical-wood sidings are particularly leak prone— all can function well with a proper drainage plane. Some siding materials, such as vinyl and aluminum, and to some extent, wood shakes and shingles, are self-ventilating, so less in need of an added rain screen.
There are many choices for exterior cladding. In choosing one, consider both its first costs and its durability and maintenance costs. A cheap siding that need frequent repainting may not be so cheap in the long run. If you need to replace the siding in 10 years, you’ll definitely wish you had chosen a more durable product the first time.
2. Air space. The air space behind the cladding serves several important functions. First, it allows water that has penetrated the cladding to drain safely away. Second, it provides a gap (called a capillary break) between the cladding and the building paper. Wet wood siding or stucco has been shown to degrade both building paper and plastic housewrap if it is directly touching the wet cladding. Cedar and redwood sidings can leach out tannins that are particularly corrosive to building papers. Third, the air space helps promote drying from the back of wood siding or from the framing and sheathing in the event of a leak. With painted or stained wood sidings, the air space will add years to the life of the finish.
How big should the air space be? Most experts agree that a small gap of 3/16 to ¼ -inch space is adequate for a capillary break, drainage, and some ventilation to assist drying. A few manufacturers and researchers claim that a space as small as 1/16 inch is sufficient, but long-term performance data on real job sites is lacking. And since many of the “draining housewraps” are well under 1/16 inch thick, I would not consider these suitable for a rain screen. In general more space is better, so with wood, stucco, or other siding types in a wet climate, I would not use less than a 3/16-inch air space. The two main options for creating the air space are: furring strips and plastic drainage mats.
Furring Strips. For horizontal wood siding, an air space can be created by nailing the siding to vertical strips of 1×3 furring. Although furring out the siding provides excellent protection for the siding and structure beneath, it also adds significant cost and complexity to the job, so it is used mainly in high-end custom building. To simplify the detailing around doors and windows, some builders use thinner material for furring, such as ¼-in.-thick wood lath or strips of ¼-in. or 3/8-inch plywood or plastic.
Plastic drainage mats. Where you’re looking for the protection of a rain screen, but don’t want to cost and hassle of furring out the siding, consider one of the new plastic drainage mats available for sidewalls. Two well established products are Benjamin Obdyke’s Home Slicker 6 and MTI’s Sure Cavity SC 50. These products create a 6mm (1/4 in.) gap between the siding and housewrap (water-resistive barrier in Code-Speak).
Home Slicker is also available laminated to the housewrap Typar for a one-step installation. Both drainage mats are strong and rigid enough to resist compression by the siding but thin enough that windows, doors, and trim can be installed without shimming or furring. A thicker version (10mm) of each product is available that complies with Canadian code. These products simplify rainscreen installations, but can cost $0.50 to $1.00 per square foot.
3. Drainage plane. The drainage plane typically consists of asphalt-impregnated building felt or a plastic housewrap that is fully integrated with all door, window, and wall flashings. The system must provide a clear drainage path out the bottom of the building. In general, the housewrap must be cut to lap over window and door cap flashings and under window and door sill flashings. In addition, the house-wrap should lap over step flashings, the upper leg of abutting roof flashings, and deck ledger flashings. Upper courses of sheathing wrap should lap lower courses by at least 6 inches and vertical seams should lap 6 to 12 inches.
4. Weep holes. Any water trapped in the air space must safely drain to the outdoors at the bottom of the wall and above doors and windows. For this purpose, brick veneer has small “weep holes,” and stucco has a perforated flashing called a weep screed. If furring strips are used with wood or composite sidings, the openings at the bottom should need be screened against insects.
To simplify the installation, most builders now use pieces of corrugated plastic ridge vent material across the bottom of each drainage channel to provide drainage and solid backing for the siding. If you are using a plastic drainage mat, closely follow their installation instructions for these terminations. Some of the drainage mats, such as MTI’s Sure Cavity, have accessory products for these terminations.
Optional Top Vents. With a 1/4-inch or larger gap, you can generate some vertical airflow in the gap by creating vents at the top (protected from insects). Researchers have shown that adding a top vent speeds up drying of the cavity, making this a good detail in wet climates or on walls with a lot of rain exposure and limited protection from overhangs.
Stucco & Brick Veneer
Traditional stucco, as well as brick veneer, have always been applied over a type of rain screen to allow for adequate drying of the wall system. Because both stucco and brick veneer are very porous to water, drainage is an essential part of the system.
Both systems absorb water and dry to both the exterior and interior. As with all types of siding, some water also penetrates at openings, cracks, and penetrations in the exterior finish.
In a stucco wall, the metal lath provides the air space, and the building paper is the drainage plane. Any water that accumulates in the wall flows down the building paper and drains out the weep screed at the bottom. When the system works as planned, the sheathing and wall cavity remain dry.
It’s interesting that this traditional approach, used by tradespeople for decades, follows the same principles that building experts are just beginning to apply to other types of siding.
Rain Screen Over Foam Sheathing
A rain-screen design will improve the longevity of any siding and finish, but it is critical when installing wood siding over foam sheathing. Wood sidings installed directly over foam sheathings are more prone to problems such as cupping, cracking, and paint failures than wood siding installed over wood sheathings. Because it absorbs excess water, wood sheathing acts as a temporary reservoir for moisture that penetrates the siding, releasing it back to the air in warmer weather.
With foam, on the other hand, the moisture tends to build up on the back of the siding and cause problems such as cupping and paint peeling. An air space between the foam and siding, even a shallow space of ¼ to ½ in., will reduce or eliminate these problems. However, to provide a solid nail base for wood siding, most builders use 1×3 or 1×4 vertical strapping over the foam.