What they are, why you need them, some basic science behind them and how we are the best to help.
Raincreens were first developed in the mid part of the 20th century in Scandinavia. Until fairly recently, they’ve been in predominant use in extreme weather areas in Europe and Canada with all regions of the United States starting to fully embrace the system in the last few decades. And rightly so. There are significant benefits from this clever solution.
What is a rainscreen? In architectural speak, a screen is a barrier. And a rain screen is the first defense and first layer on the building envelope.
A rainscreen is defined as the entire system of the siding, drainage plane and a moisture/air barrier. A rainscreen is a veneer of some variety that stands off from the wall sheathing to create a space between the wall and the screen.
The difference between a wall cladding and a rainscreen is this space. To what extent the veneer extends from the building, and how it is attached are among a number of factors determining efficacy.
Our metal systems and custom products we engineer may be fastened directly to a wall for a traditional cladding with permanence and water-tightness, but in many cases, rainscreens make even more sense for some of the following reasons:
-Air is one of the best insulators. A correctly engineered system will trap air and keep it appropriately stagnant so the building stays warm in the winter and cool in the summer.
-In many cases rainscreens can be a significant contributor to acoustic performance and noise abatement.
-Design aspects. A rainscreen sets the tone of the building’s design. Indeed, it is the predominant feature. We can create virtually anything you can devise.
-A properly devised system will keep moisture from accumulating. As this is arguably the most consequential topic, let’s discuss the science of it in greater depth:
As all building professionals know, water is the most significant factor in the premature deterioration of buildings. It can cause corrosion of metals, rotting and mold in organic substances, dissolution of materials, reduction in effectiveness of insulation and exacerbates movement and breakage due to freeze/thaw cycling.
Three conditions are required to move water through the building envelope:
– A source of water;
– an opening or path for the water to follow and
– a force to drive the water through the opening.
If one of these conditions is absent, moisture penetration cannot occur. The rain screen wall addresses the latter two, but aspects of building design also contribute.
The first condition, the presence of water, cannot be eliminated. Water management strategies can reduce the frequency and intensity of moisture at critical points in the building envelope by diverting water away from these areas.
Leakage paths exist at any opening in the wall surface, whether intended or unintended. Joints between materials and around windows and doors, vents, cracks, and porous surfaces are all potential entry points for water. Approaches to controlling rain penetration that rely on sealing openings without also dealing with the forces driving rain into buildings are unreliable. Engineering is key.
The forces that drive rain into buildings can be summarized as kinetic, gravity, capillary action and surface tension, and pressure gradients. In some circumstances only one or two of these forces may be present, but in a windy rainstorm they will probably all be acting to move water through any available leakage path. Each of these forces must be taken into consideration in designing buildings to prevent rain penetration. Of these forces, the most significant are gravity, capillary action and wind pressure differences. Materials, construction and orientation determine which force is dominant in a given situation.
Kinetic force refers to the momentum of wind-driven raindrops. This force will carry raindrops directly through openings of sufficient size. Components can be used to protect intentional openings, such as drains and vents, from direct entry of rain. The design of these elements must recognize that rain does not simply fall straight down. Wind driven rain can have a significant horizontal velocity, and near the top of a building this force may even have an upward component.
Dealing with water movement due to gravity may seem elementary, but leakage due to gravity action still occurs far too frequently in modern buildings. With near-horizontal or moderately sloped building
elements, gravity is usually the main concern. These problems can usually be traced to errors in the design or construction of elements such as flashings, or to the restriction of drainage paths by dirt or
ice, causing water to build up and follow an unintended path. Care is required in detailing and construction to avoid creating inward sloping leakage paths or areas where water can pond or overflow
drainage paths. Gravity can be used to advantage in controlling rain penetration of vertical building elements. Flashings can intercept water coming from above and direct it to the outside and away from
building surfaces. This also reduces the source of water that could be driven into the wall by other forces.
Surface tension allows cohesion of water droplets, even against gravity and across small openings. Water movement due to this attraction of water droplets to one another is referred to as capillary action. These forces allow water to cling to and flow along horizontal surfaces, such as soffits, and to move against gravity through cracks and pores in building materials. The force with which capillary action can work against gravity is inversely proportional to the size of the openings (small cracks allow more capillary suction), and also depends on the attraction of the materials to water. In certain circumstances and depending on the materials, gravity can overwhelm capillary forces and the water will drain.
In porous materials, such as masonry, capillarity is usually the dominant force in water penetration and will tend to hold water in, drawing it through, even against gravity and air pressure, until the material is saturated. Other forces, such as wind pressure, gravity or kinetic energy may then drive this retained water further through the building envelope. While capillary forces can act all through brick, testing in brick walls shows most water penetration occurs at the mortar joints, primarily through cracks at the mortar/brick interface. With impervious claddings, capillarity is still a major concern at cracks and joints. Sealing exterior joints is unreliable as seals fail under stresses and due to weathering, creating ideal capillary paths. An engineered and planned system is needed to prevent capillarity.
Air pressure differences across the building envelope can create suction drawing water through available leakage paths, while air movement due to pressure differences can carry water droplets directly. Pressure differences across the building envelope can result from wind and other factors.
Of primary concern in controlling water infiltration is the pressure difference due to wind, as it is generally much higher and more variable. Even a steady wind does not create uniform pressures across a building, as airflow patterns around building edges create varying wind velocities and forces. Air pressures due to wind will be positive on the windward faces of a building, and negative (for example,
suction or uplift) on the leeward side and, often, the roof.
Cyclic pressures due to gusting winds, meanwhile, can create significant variations over very short time periods.
Some areas of buildings are more vulnerable than others to rain penetration. As mentioned, all openings in the wall surface are potential entry paths for water. At a larger scale, wind flow patterns also affect how much rain hits various areas of a building. Wind direction is a factor, as windward-facing walls will be subject to more driving rain, while leeward walls will be protected. The aerodynamics of wind flow around buildings also mean that different areas of a single wall are subject to different wind forces, especially in larger buildings. Wind paths are a complex subject. There are ways to use the flow of air against itself to create safe spots that protect your building. We can address these details and more when assisting on your project.
Typically for a multi story building, wind accelerates around the side and top edges driving rain more forcefully against these parts of the wall. Studies have shown that these edges can receive more than 20 and as much as 50 times the amount of rain at the center of the wall. This discrepancy in wetting intensity is greater with taller and narrower buildings.
As you can see, there is much to consider when designing and selecting an appropriate rain screen for your building. We understand the complexities of each project and have the correct solutions you seek…Innovative solutions, that in many cases can be found no where else.
From spacing to fastening to environment and hosts of additional issues, planning a correct system that won’t cause more problems that it resolves can come down to the most miniscule of tolerances. We help you with all aspects of your rainscreen engineering. Partner with us with confidence!
For your next rainscreen project, rely on our specific expertise to ensure an effective, permanent and beautiful outcome.