It occurred to me that we haven’t really written a good post dedicated to vented rain screen assemblies. A rain screen is basically a gap created in between the sheathing of a house and the exterior cladding. It is considered by most building experts to be the most durable method of building an exterior wall and almost all commercial cladding details implement some type of a rainscreen. We have decided to use a vented rain screen on all of our Postgreen Homes and will talk about what we have learned to date in more detail below.
Benefits of a Vented Rain Screen
There are a couple of key benefits to using a vented rain screen assembly on any building that improve the performance and durability over the life of that building. Let’s put them in list form.
- Equalized pressure means less air infiltration inside your building. Since there is an air gap between the exterior sheathing and cladding, the pressure is equalized with the exterior atmosphere. I’m no scientist, but basically this means that air is not going to be fighting hard to enter your building when it hits the cladding because the rain screen has eliminated the large pressure difference that usually exists between the exterior and interior of the home and various layers in between. This helps when you are trying to build homes as air tight as we are in order to achieve maximum energy efficiency.
- Water the makes it past the cladding will drain and dry out, keeping the guts of your walls dry. This is where the durability claims come from. Water will eventually make it through any and all cladding assemblies on a building. The vented rain screen makes sure that when water does get through it is able to drain down the face of the Water Resistive Barrier and out the bottom or simply dry out on its own due to the air gap behind the cladding. This not only helps to keep moisture from getting into your walls assembly (which is bad), but it prolongs the life of whatever cladding is used by allowing it to dry out when wet from both sides.
- An extra layer of air keeps your building cooler. In most homes, the sun hits your cladding which is in direct contact with the sheathing. The heat from the sun is transferred directly from the cladding into your walls. This makes your building warmer than necessary. The air gap created by a rain screen keeps the face of your sheathing much cooler. This is more important in the South where cooling loads dominate heating loads, but it’s still a nice feature in the north.
How to Build a Vented Rain Screen
There are many other people on the interwebs that I have linked to below that have described the how-to’s of rain screen building, so I’ll keep this brief. Here are the basic elements of any vented rain screen assembly:
- Furring strips or a Drainage mat to create the air gap in between your sheathing/WRB and your exterior cladding. We list options for this material below, including our new favorite, The Furring Master!
- An insect barrier at the bottom of the rain screen assembly gap with proper mechanical flashing. You want to keep the bugs from nesting in your rain screen from the bottom and water from draining into your basement.
- A path for air to flow at the top of your cladding assembly. If you seal the top of your cladding to your roof cap or top trim piece, the rain screen is compromised by insufficient air flow from top to bottom.
- A bit of extra care and planning at all window and door openings in terms of proper flashing. A good designer or flashing expert can help out here, but there are many details online for this as well that are simple to follow if planned for.
Material options for implementing Vented Rain Screens
There are many simple material options for creating the vital gap needed to build a vented rain screen system in residential buildings. There are much fancier systems implemented in commercial construction that we are no going to get into here. Some of the choices and their pros and cons are listed below:
- Pressure treated lumber – This option is one of the most popular as it is a familiar material, readily available and not too expensive. Rips of pressure treated plywood can be used or 1x material. This is what we used on the Passive Project. The main drawbacks are that the wood is not consistently flat and straight for precise cladding applications and water can be trapped between the wooden strips and the WRB with no room for venting where all strips are located.
- Mesh ventilation product – There are many mesh products available that are thin and easily applied in roll form over your WRB with a slap stapler. These products work great and eliminate the problem of trapped moisture completely that occurs with wooden furring strips. The main cons are the price and the uneven compression that can result in wavy cladding when fasteners are attached with different pressures. This is the product we used on the 100K House.
- Metal furring strips (The Furring Master) – Metal furring strips are our current favorite option due to a number of factors. The price is much better than the mesh products and not that higher than wood, depending on what type of wood strip you are using. They are perfectly straight and rigid which lends itself to the best finished installation of siding. One of our favorite aspects of it is that it has a hollow channel behind it that allows water and air to flow behind the surface, unlike wood. Lastly, it’s made of galvanized 22 gauge steel that is recyclable and can be left exposed in certain joints if desired without concern for UV degradation, decay or rusting.
- Miscellaneous other products – Although less common, we’ve seen others use creative products like corrugated plastic, rigid insulation and fiber cement strips.
Above is a picture of our installed Furring Master strips. We have studs at 24″ on center on the Skinny Project, but we installed the furring strips at 12″ centers. The strips in the middle that are not connected to studs are basically just maintaining the spacing from the wall and keeping the somewhat fragile panels rigid and secure.
Better links to Vented Rain Screen Instructions
Best post on Rainscreens ever by the BUILDblog – Fantastic diagrams and pictures from guys who really know what they are doing.
Green Home Building’s take on Rain Screens (PDF) – Good info, details and lots of hose wrap bashing. What more could you ask for?
Rain Control in Buildings by Building Science Corp – All sorts of good info about how rain and wind are trying to destroy your building and how to prevent them from succeeding.
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