To lower the risk of wall rot, it’s increasingly seen as sensible to provide a ventilated air gap between the siding material (also known as cladding) and the structural sheathing. As rainscreens become more common, mainstream builders are more often being compelled to ask, “What is a rainscreen? How do I know if I need one?”. This article will discuss the most common questions about rainscreen gaps between siding and sheathing.
What is a rainscreen?
A rainscreen siding installation is one that includes an air gap between the siding and the water-resistive barrier (the asphalt felt or housewrap). Every well-designed rainscreen wall needs:
- a water-resistive barrier (WRB).
- an air gap between the WRB and the back of the siding.
- flashings at all penetrations and vulnerable areas.
- weep holes at the bottom of the wall.
- ventilation openings at the top of the wall on a case-by case basis.
What is the benefit of a rainscreen?
Rainscreens manage moisture by drying the sheathing, which may accumulate moisture due to condensation in cold weather. It also helps to dry rain-soaked siding. These benefits are due to four functions of the rainscreen gap:
- The gap provides a capillary break between the back of the siding and the WRB to limit wicking.
- The gap allows moisture held in the siding and sheathing to be redistributed to adjacent materials by evaporation and diffusion, thereby limiting damage due to water that concentrates at leak points.
- The gap provides a path for liquid water to drain down to the weep holes at the bottom of the wall.
- If there are ventilation openings at the top of the gap, the rainscreen provides a path for moving ventilation air. This is due to the stack effect, which is strongest when sun shines on the wall. Research shows that this type of ventilation is a powerful drying mechanism.
Does every building need a rainscreen gap?
The decision about whether to include a rainscreen gap is a judgment call. Among the factors to consider:
- In some places rainscreens are required by local building code. Since April, 2010, the residential building code in Oregon has required a minimum 1/8-inch-deep gap between siding and the WRB for all new homes.
- A rainscreen installation is more important in wet climates than in dry climates. Some experts advise rainscreen installations for all houses where annual rainfall exceeds 60 inches. In areas with rainfall amounts in the range of 20 to 60 inches per year a rainscreen may not be mandatory, but is still a good idea.
- A tall wall with a short overhang is more vulnerable to wind-driven rain — and therefore more likely to need a rainscreen — than a short wall protected by a wide roof overhang. If the wall faces a wide porch, it probably doesn’t need a rainscreen.
- A wall sheathed with OSB, which is more vulnerable to rot than plywood sheathing, should almost always include a rainscreen gap. If the walls are made of SIPs, or if the OSB wall sheathing has closed-cell spray foam on the interior, it’s absolutely essential to include a rainscreen — because the OSB on these walls can’t dry to the interior.
- If the walls are sheathed with rigid foam, most types of siding — especially wood sidings — require vertical furring strips.
- Rainscreen gaps are always beneficial: a few minor leaks in a rainscreen wall are less likely result in major rot than the same leaks in a wall without a rainscreen. The only drawback to rainscreen walls is the added cost associated with these details.
How big a gap is appropriate?
A rainscreen gap doesn’t have to be very big- even a 1/16-inch gap provides a capillary break, allows drainage of liquid water, and permits “diffusion redistribution.” However, job-site realities and variations in material thickness usually require a the gap to be at least 1/4 inch deep. Many builders use 1/4-inch wooden lath or rips of 1/4-inch plywood for their furring strips while others use 1/4-inch-thick plastic drainage mats.
Admitting of installation errors, a deeper gap — 3/8 inch to 3/4 inch —allows for possible field-installation glitches. While 3/8-inch gap is more than sufficient for drainage and ventilation drying in most climates, homes in wet climates will benefit from deeper, 3/4-inch gaps to allow faster ventilation drying.
When choosing rainscreen gap’s depth, factors other than water management must be considered. 1/4-inch or 3/8-inch rainscreen gaps simplify exterior trim details: it’s simply easier to trim out and flash a wall with 1/4-inch furring strips than one with 3/4-inch furring strips.
Finally, if the furring strips are being installed over thick rigid foam, 3/4-inch thick 1x3s or 1x4s are needed in order to have something to attach the siding to.
What materials are best for furring strips?
Furring strips are best installed vertically, directly over the studs. Because a rainscreen is designed to stay dry they need not be pressure-treated.
For thin gaps 1/4-inch lath board or rips of 1/4-inch plywood can be used. For a deeper gap, thicker plywood can be used. Although some builders use 1x3s for furring strips, others are frustrated by the fact that 1x3s tend to split easily. If you want a 3/4-inch-deep gap, and are worried about splitting, use 1x4s.
Plastic furring strips
At least six plastic furring strips are currently on the market. These are: 1). BattenUp furring strips by BattensPlus, 2). Drain-Plane sells polyethylene furring strips, 3). 3/8” plastic battens by El Dorado, 4). VaproBatten by VaproShield, 5). Sturdi-Strips by Cor-a-Vent, and, 6). CedarVent strips by DCI Products.
Coroplast battens, BattenUp battens, El Dorado battens, Sturdi-Strip battens, and CedarVent strips all have channels that allow cross-ventilation between the cavities. None of these plastic battens are good at holding nails and screws, so longer fasteners are mandatory to reach the studs.
What about bug screening?
In the early days of rainscreen gaps, builders stapled a length of horizontal insect screening to the bottom of their walls, with the screen hanging below the bottom of the sheathing, before installing the furring strips. Once the furring strips were installed, they folded the extra width of the insect screening up on top of the furring strips and stapled it up in place.
These days, most builders use a section of ridge-vent material at this location, or a commercial product like the Cor-A-Vent SV-3 Siding Vent.
How much does rainscreen detailing cost?
Every project is different, but reports indicate that rainscreen details add about 30% to the cost of a siding project.