What`s a rainscreen

Twenty years ago, very few residential builders knew what a rainscreen was. These days, however, it`s no longer unusual to see siding being installed on vertical furring strips or a plastic drainage mat. As rainscreens become more common, mainstream builders are beginning to ask, What`s a rainscreen? How do I know if I need one?

This article will pull together information to answer the most common questions about rainscreen gaps between siding and sheathing.

What is a rainscreen?

You can`t really point to a rainscreen, because it isn`t a thing…  it`s a system. That`s why I prefer to talk about a rainscreen installation, a rainscreen approach, or a rainscreen gap. For most residential builders, a rainscreen siding installation is one that includes an air gap between the siding and the water-resistive barrier (the asphalt felt or housewrap). Some purists insist that you can`t call it a rainscreen gap unless you have ventilation gaps at the top of the wall as well as the bottom of the wall.If it only has drainage gaps at the bottom, but no openings at the top, the purists insist that the system is more accurately called a cavity wall. However, the distinction between a cavity wall and a rainscreen wall isn`t particularly useful in part because the distinction is based on a faulty understanding of pressure equalization principles so I side with those who use the word “rainscreen” for either type of wall. 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; and (usually) weep holes at the bottom of the wall. Ventilation openings at the top of the wall are optional. Why would you want a rainscreen? Rainscreen gaps help walls manage moisture. A rainscreen gap helps to dry the sheathing, which may accumulate moisture during cold weather. It also helps to dry the siding when it is soaked by rain. These benefits are due to four functions of a rainscreen gap: The gap provides a capillary break between the back of the siding and the WRB. This limits 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 ventilation air rises 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?

How do you decide whether your house needs a rainscreen gap? Ultimately, the decision about whether to include a rainscreen gap is a judgment call. Among the factors to consider: In some areas of North America notably coastal British Columbia rainscreen installations are required by the local building code. A rainscreen installation is more important in wet climates than in dry climates.

Some experts advise rainscreen installations for all houses in areas with an average annual rainfall of 60 inches or more. In areas with lower rainfall amounts in the range of 20 to 60 inches per year a rainscreen may not be mandatory, but it is still a good idea. A tall wall protected by a stingy roof 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. Some experts (including Joseph Lstiburek) note that a wall sheathed with OSB, which is more vulnerable to rot than plywood or board sheathing, should almost always include a rainscreen gap. Lstiburek has written, We learned through trial and error (mostly error) that if you use OSB, really good cavity insulation and a housewrap, make sure you have an air gap between the cladding and housewrap/OSB interface. If your walls are made of SIPs, or if your 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.

Brick veneer installations always require a rainscreen gap. Because it already has an air space behind it, vinyl siding does not require a rainscreen installation unless it is installed on top of rigid foam that is more than 2 inches thick, in which case it should be installed over vertical furring strips. If your walls are sheathed with rigid foam, most types of siding especially wood sidings require vertical furring strips. If you`re installing vinyl siding over thin (2 inches or less) rigid foam, you can probably skip the furring strips. (For more information on installing vinyl siding over thick foam sheathing, see Can Vinyl Siding be Applied Over Furring Strips?) Rainscreen gaps are always beneficial.

Rainscreen walls are more robust than ordinary walls because a few minor leaks in a rainscreen wall are less likely to lead to a major rot than the same leaks in a wall without a rainscreen.The only drawbacks to rainscreen walls are the hassle of installing the extra details and the added cost associated with these details.

How big a gap do I need?

A rainscreen gap doesn`t have to be very big. Researchers have learned that even a 1/16 inch gap provides a capillary break, allows drainage of liquid water, and permits diffusion redistribution.  However, you`ll probably find that (unless you are using a wrinkled housewrap) job-site realities and variations in material thicknesses usually require a rainscreen gap to be at least 1/4 inch deep. Having read this information, many builders use 1/4-inch wooden lath or rips of 1/4 inch plywood for their furring strips. Other builders use 1/4-inch-thick plastic drainage mats. These 1/4-inch gaps work well. However, some builders prefer more leeway for installation errors. They may be worried, for example, about puckers in the WRB or bulges in the sheathing that may compromise the 1/4-inch gap. They`re more comfortable with a deeper gap perhaps 3/8 inch, or 1/2 inch, or 3/4 inch because it allows for a few minor on-site problems or installation glitches. A 3/8 inch gap is more than enough for drainage, and is also enough to provide useful ventilation drying.

Logic dictates that homes in wet climates benefit from deeper rainscreen gaps up to about 3/4 inch because deeper gaps allow faster ventilation drying. When choosing the depth of your rainscreen gap, there are issues other than water management that come into play. Builders know that 1/4-inch or 3/8-inch rainscreen gaps simplify exterior trim details. It`s much easier to trim out (and flash) a wall with 1/4-inch furring strips than a wall with 3/4-inch furring strips. Finally, if your furring strips are being installed over thick rigid foam, you don`t really have a choice on furring strip thickness.You`ll need 1x3s or 1x4s that are 3/4-inch thick in order to have something to attach the siding to.

How do I create the gap?

There are three ways to create a rainscreen gap: You can use furring strips.Furring strips are usually installed

An illustration of Guertins method

vertically, directly over the studs (16 inches on center or 24 inches on center). You can use a three-dimensional plastic mesh product. You can use bumpy or wrinkled housewrap. What can I use for furring strips? Furring strips don`t have to be pressure-treated, because a rainscreen is designed to stay dry. For thin furring strips, you can use 1/4-inch lath board or rips of 1/4-inch plywood. If you want a deeper gap, just use thicker plywood. 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 One option for creating a shallow rainscreen gap is to use rips of 3/16-inch-thick Coroplast (a corrugated plastic sold at sign shops). Because this product is corrugated, it allows air movement through the furring strips.Another option is to use 1/4-inch-thick fanfold insulation (thin foam that is often installed under vinyl siding), cut into 2-inch wide strips on a table saw. At least six manufacturers sell plastic furring strips: BattensPlus manufactures BattenUp furring strips. These pieces of polypropylene strapping measure 1 1/2 inch wide by 1/2 inch thick by 48 inches long. Drain-Plane sells polyethylene furring strips. El Dorado makes plastic battens that measure 1 5/8 inch wide by 3/8 inch deep by 8 feet long. VaproShield makes a plastic batten called VaproBatten. Cor-a-Vent makes plastic furring strips called Sturdi-Strips. DCI Products makes plastic furring strips called CedarVent strips. Coroplast battens, BattenUp battens, El Dorado battens, Sturdi-Strip battens, and CedarVent strips all have channels that allow water to drain right through the products, even when installed horizontally. VaproBattens aren`t designed for horizontal installation, and won`t drain that way. Note that none of these plastic battens are good at holding nails and screws, so you`ll need long fasteners that reach to the studs if you go this route. Three-dimensional plastic mesh and plastic mat products It`s possible to create a rainscreen gap with a mesh product that look like the plastic wool pads used for scrubbing pots and pans. These three-dimensional mats are especially useful for siding types that can`t be installed over vertical furring strips for example, cedar shingles or board-and-batten siding. At least seven manufacturers make three-dimensional plastic mesh or plastic mat products designed to create a rainscreen gap: Benjamin Obdyke Home Slicker mesh is 1/4 inch thick. Stuc-O-Flex WaterWay Rainscreen drainage mat can be ordered in several thicknesses, ranging from nominal 1/4 inch to nominal 3/4 inch.

MTI Sure Cavity creates a gap of about 3/16 inch. Colbond EnkaMat is about 3/8 inch thick. DC 14 Drainage Mat is about 1/4 inch thick. Because it is made of polystyrene and has a low permeance (1 perm), it depends on ventilation drying through air channels rather than diffusion drying. Cosella-Drken Delta-Dry is about 1/2-inch thick. Delta-Dry is a membrane made of 22-mil high-density polyethylene that has a three-dimensional egg-carton configuration. Like DC 14 Drainage Mat, Delta-Dry is a vapor barrier. The product depends on ventilation drying through air channels rather than diffusion drying. Ventgrid is about 1/2-inch thick. It is a rigid plastic panel that measures 4 feet by 8 feet. Three-dimensional plastic mesh and mat products are fairly expensive; most cost between 60 and 70 per square foot. What about bumpy housewrap?

There are at least seven brands of bumpy housewrap or wrinkled housewrap:

  • DuPont StuccoWrap
  • Pactiv
  • GreenGuard
  • RainDrop Barricade Drainage Wrap Barricade
  • WeatherTrek
  • Valeron
  • Vortec
  • Fortifiber
  • Hydro Tex Benjamin Obdyke HydroGap.

The jury is still out on whether these products provide a significant benefit. In some applications notably when installed between OSB and rigid foam on walls that have spray foam insulation on the interior side of the OSB they can lower the risk of an otherwise risky wall assembly. However, these products have very tiny air gaps StuccoWraps wrinkles are less than 20 thousandths of an inch high. Skeptics wonder whether such tiny gaps have much effect. Moreover, some builders report that many of these products compress easily under certain types of siding, eliminating the wrinkles that provide drainage. According to building scientist John Straube, however, DuPont StuccoWrap works well; Straube says that it`s impossible to fasten siding tight enough to crush StuccoWrap wrinkles enough to prevent drainage.

Good advice: if you want a rainscreen gap, choose a method that gives you a gap of at least 1/4 inch. Should the gap be vented at the top and bottom? For some types of siding, it`s possible to omit the openings at the bottom and top of a rainscreen gap. For example, it`s possible to install cedar shingle siding over horizontal 1×3 furring strips. Even though the resulting gaps won`t have drainage, the installation works fine because not much liquid water gets behind the shingles, and any water that does can evaporate quickly. (If you`re the type of builder who worries about drainage, you can cut kerfs on the back side of your horizontal battens, use corrugated plastic battens with drainage holes, or buy notched battens at a roofing-supply outlet. But I don`t think that level of obsession is necessary.) Most types of siding including (especially) brick veneer require weep holes or drainage openings at the bottom of the rainscreen gaps.

Many builders install a rainscreen without openings at the top, and these installations work fine. However, researchers have shown that including screened vent openings at the top as well as the bottom of your rainscreen encourages ventilation drying (see Image #3, below). Siding installed over a vented rainscreen that is open at the top and the bottom will dry faster than siding installed over a rainscreen that is closed on the top. Is it permissible to vent the top of the rainscreen gaps into the soffit? Opinions differ. Joe Lstiburek says it`s OK, and so do I. The amount of moisture carried by this air isn`t enough to cause any problems in your attic. However, builders in British Columbia should note that while the building code in that province does not require rainscreen gaps to have openings at the top, the code stipulates that if there are openings at the top, they can`t be connected to a soffit or attic.

Some builders wonder whether they need to come up with a detail to provide openings under each window and at each window head. The short answer: no, you don`t. Just provide a gap at the top and bottom of your furring strips so that air can move sideways around the window. Detailing the top of a gable wall Many builders have trouble visualizing the best way to vent the top of a rainscreen on a gable wall.Mike Guertin, a builder from Rhode Island, does it this way:The frieze board is padded off the wall to let air flow over the top of the siding, down the back side of the frieze, and out. The rake board needs to be padded off the wall about 3/8 in. to 1/2 in. more than the face of the siding. An illustration of Guertins method For a fuller explanation, see Rainscreen-Siding Details for Gable Ends. What about bug screening? In the early days of rainscreen gaps, many 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 they installed their furring strips. Once the furring strips were installed (with the bottom 3 inches of the furring strip installed on top of the insect screening), 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. Another option is perforated J-channel (see Image #6, below).

How do you flash windows in a rainscreen wall?

Window flashing is a big topic and is beyond the focus of this blog.

Flashing windows in a “vent” screen If you ever find yourself scratching your head, looking for a solution to a window flashing problem, remember this mantra: Flash the rough opening, not the window. If the rough opening is watertight and able to direct leaks to the exterior, you`re good. Here are some more flashing principles to keep in mind: You can`t flash properly unless you know which surface is designated as the WRB. All flashings should direct water to the exterior surface of the WRB. Laps and gravity are more dependable than adhesives and tape. How much do rainscreen details cost? Every project is different, but many siding contractors report that rainscreen details add about 30% to the cost of a siding project.