If you dip your toe into the world of building science, it won’t be long before you encounter the concept of the rainscreen. The term refers to siding (or “cladding”) systems that include a space for air circulation and water drainage behind the siding, between the siding and the weather-resistive barrier (WRB). While rainscreens may still be in the minority, they’re increasingly the go-to system for custom builders who are building high-performance structures, especially in colder climates.
The purpose of the rainscreen is to promote drying of the cladding assembly, while allowing for good management of vapor and rainwater. A rainscreen is an upgrade over the simple application of siding directly to the WRB. That’s true for any building, but it’s especially true for buildings with more highly insulated wall assemblies, and especially in cold climates. In the old days, heat driven through wall assemblies would dry out the walls and siding materials. But in modern high-performance structures, the walls don’t conduct enough heat outward to reliably dry the wall exteriors. By allowing for the circulation of air behind the siding, rainscreen systems supply the requisite drying capacity to keep walls that get wet from staying wet.
The system provides dual benefits. On the one hand, it allows the wall behind the siding to stay dry and avoid water damage, mold, and rot. And at the same time, it increases the durability of the siding itself. Because it can dry toward the house as well as to the exterior, siding that is part of a well-designed and well-implemented rainscreen will be more stable, and paint jobs will last longer.
Creating the Air Gap
The classic, low-tech way to create an air gap behind the siding is with the use of strapping. In the typical application, vertical 1x3 strapping is applied to the wall over the housewrap or felt paper, and horizontal siding such as wood clapboards or fiber cement is applied over the strapping. An air gap at the bottom of the wall assembly allows air to enter, and in many (but not all) applications, there’s also an air gap at the top of the wall, creating the capacity for convective airflow. Screening and/or ridge-vent material is applied to the wall at the top and bottom openings to keep bugs out of the cavity.
An effective alternative to strapping is a synthetic woven air gap material. Available from numerous manufacturers, these fabrics are fastened directly over the WRB, and the siding material is fastened through the fabric into the sheathing. This method simplifies the process, particularly if the siding material is wood shingles.
A less effective alternative is simply to apply a drainable housewrap before installing siding. Several vendors supply housewrap that has wrinkles or bumps sufficient to create a small capillary break between the siding and the wall (on the order of 1/16 inch). This small gap is enough to allow water to drain out from behind the siding. But it’s not enough to allow effective air circulation, so it can’t be termed a true rainscreen system.
Focusing on the WRB
In a rainscreen siding system, bulk water management is the job of both layers in the system. The siding handles most of the rain that falls on the wall. But any water that penetrates behind the siding is managed by the weather-resistive barrier—the housewrap, tapes, and flashing. That WRB has to be carefully detailed to direct any incidental water down, out, and away from the wall. The WRB should be constructed so that the flashing is integrated into the WRB.
This practice is most challenging at penetrations in the wall. Windows and other penetrations such as lights, electrical outlets, vents, or hose bibbs have to be flashed in a way that captures any incidental water and directs it into the drainage space, or else directs it out onto the surface of the siding. Ideally, the penetrations should be detailed in a way that promotes the flow of air into and out of the drainage space, so that effective drying can occur.
Naturally, the cost of installing a rainscreen is greater than the cost of simply applying siding directly over the WRB. But advocates argue that the upcharge is worth it because of the reduced risk of a costly repair down the road, when water may damage the siding. In fact, simply the reduction in routine maintenance cost may go a long way toward recovering the cost of a rainscreen: Routine paint jobs are required less frequently when the siding is more able to dry out and stay dry.
