The rainscreen principle dates all the way back to the first half of the 20th century. The first assemblies built according to the rainscreen principle consisted of a cladding, made of a lightweight, low water‐permeance and low water capacity material, installed on the exterior of a solid load‐bearing brick wall with a drained and vented air space between the cladding and the load‐bearing wall. The drained and vented space was not pressurized.
In 1963, the NRC(National Research Council Canada) published the Canadian Building Digest(CBD) 40, “Rain Penetration and its Control” (http://archive.nrc-cnrc.gc.ca/eng/ibp/irc/cbd/building-digest‐40.html). This Paper popularized the term rainscreen principle in today’s construction industry and is still considered a prime reference on the subject. The author of CBD 40, G.K. Garden, wrote:
“It is not conceivable that a building designer can prevent the exterior surface of a wall from getting wet nor that he can guarantee that no
openings will develop to permit passage of water. It has, however, been shown that through-wall penetration of rain can be prevented by
incorporating an air chamber into the joint or wall where the air pressure is always equal to that on the outside. In essence, the outer layer
is then an “open rainscreen” that prevents wetting of the actual wall or air barrier of the building.”
Often, drained cavity walls are confused with rain-screen walls. Here is an example of the subtle distinction. When asked what the possible causes of rain-leakage problems are on a jobsite, the question eventually arises as to whether the design and construction of the walls applied to the rainscreen principle. Often the response is, “Yes, there is a drained cavity behind the cladding.” The answer given fits the description of a cavity wall, but not a rainscreen wall. A “rainscreen wall” is designed and built according to what Kirby Garden referred to as the “open rain-screen principle”. The basic premise is the control of all forces that can carry rain to the inside, though it does handle rain once it gets inside. Usually a heated debate on terminology starts up. It should be clear that a wall built according to the rainscreen principle means the package of requirements set out in Garden’s article published fifty years ago, was attained. Some argue that any wall that uses the cladding as a screen for rain, such as a cavity equalization or moderation. That is, the pressure difference from the front side of the cladding to the cavity area needs to be equal or nearly so. This helps to minimize the potential for rainwater to be forced behind the cladding by pressure differential.
A typical cavity wall design has a CMU or steel stud backup wall, through-wall flashings, mortar deflections, and drainage vents that sit on top of the flashing to allow moisture that that enters the wall system to drain back to the outside. These wall systems usually have a maximum cavity space of 4.5 inches(115mm) with a recommended air space of 1.5 to 2 inches wide(38 to 50mm). Chapter 6 of the Masonry Standards Joint Committee states that a one inch(25mm) minimum air space shall be specified. Other organizations, such as the International Masonry Institute, Brick Industry Association, and National Concrete Masonry Association, recommended a two-inch(50mm) air space.
A pressure-moderated rainscreen wall system usually consists of a CMU or steel stud backup wall with an air barrier and through-wall flashing system, along with an all-wall drainage and ventilation mat with weep vents placed at the bottom and top of the wall. These weep vents allow air to flow through the wall in a convective fashion causing the air pressure within the wall to be equal, or close to equal, with the air pressure outside of the wall.
The application of the pressure-equalized rainscreen principle requires more attention, detailing, and care from the designer and builder because of the intricacies of installing air barrier systems over conventional cavity wall designs. With the proper attention given to detailing, pressure-moderated wall systems are likely to require less maintenance and care from the owner down the road. Compared to traditional practices, the proper application of the rainscreen principle can result in a reduction in strength required for the cladding and its anchorage system. This must go hand in hand with an increase in strength in the air-barrier system and its anchorage system. Why bother switching the loads from one component to the other? Only a systematic and consistent approach to air, moisture vapor, thermal and rain flows will produce durable exterior walls that can withstand all criteria of performance set by designers for the benefits of building owners and users. This systematic approach calls for a thermal flow, noise, pollutants, etc. in a durable fashion. It is practically a prerequisite for rain penetration control.
The recommended approach for rain penetration control is to design multiple-element protection into a wall. Many assemblies incorporate more than one element to control to control rain penetration. Historically, such elements include an air space or drainage plane and a water-resistant layer as well as joint and junction details that also incorporate multiple elements of protection. These features have been observed in both masonry and wood frame construction.
Back in the 1970’s, conventional rainscreens were made up of cladding, a second line of defense which was comprised of a flashed, drained and vented air space, and material installed on the exterior of the back-up wall to protect against moisture.
The original rainscreen wall consisted of a lightweight protective cladding that had low water-permeance and low water capacity materials that were installed on the exterior of the solid load bearing brick wall that were then installed on the outside of a drained and vented air space on the exterior of a structural wall. In many of today’s assemblies, the cladding might not be lightweight. They could be permeable or open-jointed as well. Often, the drained or vented airspace consisted of just two layers of building paper.
In the 1990’s, rainscreen walls were modified to meet new building codes and incorporate new building materials. In order to save costs, thinner wall systems started to be introduced, which meant thinner claddings. New energy codes that require more outboard insulation and new fire codes have all played a role in rainscreen modifications as well.
Around this time drainage/ventilation mats were introduced. the outboard insulation now being required by building codes began to reduce the air space previously available in cavity wall applications. For the new code-compliant wall system to function properly, a need to assure a fixed and clear air space arose. According to Building Science Consultants, to have proper drainage and ventilation, there needs to be a capillary break of anywhere from 3/16″ to 3/8″(4.8mm to 9.5mm). It is also important that these drainage mats do not compress when installed so that an acceptable air space is maintained. Maintaining a clear airspace is also crucial when building with masonry-related items. Mortar droppings or other debris can inadvertently clog the space, inhibiting the desired ventilation and drainage. The use of specifically designed drainage membranes can prevent clogs due to mortar and other debris, ensuring the clear passage of moisture for drainage and air flow for ventilation.
It was soon realized that drainage mats should be used on thin clad adhered veneer systems, bringing the cavity wall concept to thin veneer systems without the expense of construction a typical cavity wall. This new system added a tremendous value to drainage/ventilation mats in the residential sector, too. Under section 1403.2 of chapter 14 of the International Building Code(2012) it states that
“The exterior wall envelope shall be designed and constructed in such a manner as to prevent the accumulation of water within the wall assembly by
providing a water-resistive barrier behind the exterior veneer and a means for draining water that enters the assembly to the exterior.”
The 2005 addition of the National Building Code of Canada Section 18.104.22.168 states
“A cladding assembly is deemed to have a capillary break between the cladding and the backing assembly where…there is a drained and vented air space not less than 10mm deep behind the cladding, over the full height of the wall.”
Remember, to effectively drain water there needs to be a capillary break. Two layers of building paper do not qualify as a sufficient capillary break. Used behind wood, fiber cement and other claddings, rainscreen drainage mats would greatly reduce the risk of mold and structural decay.
There are a few main factors to consider when designing with a drainage/ventilation mat. The first is rainfall totals and frequency. How often does it rain? What are the wetting and drying cycles? What is the wind and storm environment like? For example, the Gulf Coast deals with wind pressure that is far more severe than other parts in the United States. Freeze-thaw conditions play an important role in determining the use of rainscreen drainage mats and which type of drainage mat to use. The other three factors are solar index, temperature and humidity.
There have been numerous advancements in the development of drainage and ventilation mats over the last decade. Some drainage mats come in the form of an entangled matrix with or without a bonded filter fabric. Others are designed to offer dual ventilation and drainage. All of these are high quality products. However, when an assembly varies from the conventional rainscreen, assess the situation carefully to make sure the drainage mat will provide the performance needed for the conditions to which it will be exposed.