Prior to the energy crisis in the mid-seventies, buildings tended to be leaky, and allowed a lot of air-flow through (i.e. between the outside and inside of) their wall cavities. Later, when buildings were tightened up to save energy, this ventilation, and its ability to dry out any moisture that got behind the siding, was diminished. You may remember the rash of lawsuits in the late 80s and 90s, related to projects whose Dryvit (EIFS) siding failed – these were due to just this issue. EIFS systems now have to incorporate a ventilation layer behind the siding.
In houses, a conventional wall assembly consists of wood studs, usually at 16” on center (in new construction advanced-framing techniques should be used, with studs at 24” o.c.), drywall on the interior side, and structural sheathing on the exterior side of the studs. Over the sheathing (usually plywood or O.S.B.) a weather-resistant barrier (WRB) is applied – this can be asphalt-impregnated building paper, Tyvek or similar house wrap, or newer materials such as Vapro-Shield. The siding is then applied directly over the WRB. Any moisture that does find its way behind the siding in this assembly has virtually no way to get back out.
By contrast, in rain-screen the siding is held away from the wall. There are several reasons rain-screen siding is the preferred method for installing siding:
- It causes any water that does make its way behind the siding to quickly evaporate;
- It equalizes the pressure on either side of the siding. In a conventional wall, where the siding is tight against the drain-plane, blowing winds will create an area of high-pressure on the exterior face of the siding. Wind-driven rain can be forced to travel through (even up and over) the siding to find the area of relatively lower pressure behind. Once that water is behind the siding it’s trapped;
- It improves the vapor-diffusion characteristics of the wall (more about this in a future posting);
- It minimizes heat gain in the wall;
- It discourages the buildup of moisture in the wall cavity (insulation and framing), which can lead to decreased R-value, and deterioration of the structural members. Water that does make it into the wall cavities tends to occur at especially leaky areas – even though buildings are tighter than they used to be, they’re still not tight enough!);
- It discourages the buildup of mold and mildew, and associated indoor air quality (IAQ) problems such as asthma and allergies.
The rain-screen siding assembly is applied over furring, which can be 1 x 2 cedar strips, or proprietary metal hat channels with neoprene gaskets. These assemblies can be either closed-gap or open-gap. Closed-gap rain-screens don’t have to look any different than conventional siding, except they’ll have a vent at the top and bottom of the wall to allow the flow of air behind the siding. Open-gap rain-screens leave a visible space between the pieces of siding, which can provide even better airflow behind. Extra care has to be taken in detailing and spec’ing open gap systems, to properly discourage excessive water intrusion, insects, UV rays, etc. Variables that come into play are the depth of the siding relative to the gaps, the thickness of the furring strips (i.e. how far the siding is held off the wall), detailing the base and top of wall conditions, detailing the vertical gaps properly, etc. Often an open gap system will have exposed fasteners, which can become a strong element in the overall design, and can be very attractive.
Water will get behind siding, no matter what we do. Let’s accept that fact, and work with it rather than against it, by using rain-screen siding.