What is a Rain Screen?
The rain screen is an exterior defensive coating that sits away from a structure’s outside wall's weather contrary barrier, creating an air crater directly behind the covering that helps to protect the buildings important weather contrary barrier, by providing a moisture management instrument allowing air intrusive through to vaporize any lingering moisture between the structure’s walls.
It is an attempt to understand the architectural and design structure concerning the background check and application of vital principles used in back ventilation and pressure equalized rain screen principles. These systems provide drainable compartmentalization to limit water penetrating during heavy rains and provide rapid pressure. A rain screen is a building platform mechanism created to limit the quantity of water that could penetrate into a buildings’ moisture protective area.
Rain screen provide a way of correcting buildings’ energy efficiency by facilitating exterior covering. It reduces chances of water penetrating a wall assembly against the following forces driving water into the building: gravity force, capillary force, surface tension force, kinetic force and pressure gradient equalization. Pressure equalized minimizes and eliminates water in the rain screen cavity.
A rain screen provides the designers and architectures of buildings with some benefits that include:
- Water vaporizer and insulation by reducing the potential of mould.
- Flexibility in design for simplified detraining and cladding material integration.
- It assists in the process of draining the water and enhances drying through the provision of space of air by between the drainage system and the cladding that is done on the outside.
- Reducing energy loss by reducing hot and cold air movement through the wall.
- Use of lightweight covering options versus classical building materials reducing buildings dead load.
The amount of water that enters volume of is not constant. It alters depending on the design system because both use different mechanisms of eliminating the leakage of water. However, it does not automatically stopping water penetration through the exposed outer wall. The BS 5250:2002 is a practice code for condensation control in buildings was meant to reduce damage caused by entering of rain water into the outside walls.
When built and detailed well, the rain screen wall provides a workable preventive solution to the penetration of moisture from the outside environment. Typically, there exists a myriad of forces allow rain to move into buildings. The rain screen is one feasible way through which this situation can be curbed. In essence, the principle of rain screen refers to a design principle which provides solutions on how the penetration of the rain screen by water from the rain may be prevented.
Ideally, a wall that is devoid of any leaks that has applied the principle of the rain screen should have some elements that are essential to it. There should be a rain screen on the outside or a barrier that is deterrent to the penetration of water. It is also important to have a space of air that is confined or exposed to the outdoors. Insulation should be done properly and there should be a barrier on the inside too. The presence of these elements is to ensure that vapor and air do not pass through the exterior walls. They also aid in preventing the walls against pressures caused by the wind.
Penetration of Rain into Walls
It is well known by professionals in the building and construction sector that water is a factor that contributes significantly to the unprecedented destruction of buildings. Water can result into metal corrosion, efflorescence, movement and breakage as a result of freeze cycling, decrease in the effectiveness of insulators, material dissolution and stresses. Clause 15.1 of the BBA requires that property is supposed to be surveyed before an ‘Envirowall External Insulation System’ can be constructed. This will assist in meeting the various regulations of building. While it is understood that moisture can bring damage to the envelope of the building from the within or outside, rain water also use the vertical elements such as walls to penetrate into the building.
Rain screens are specially designed to reduce the penetration of water into buildings through the outside. The rate of absorption of the exterior material of the wall is a vital factor of consideration. For the case of a masonry wall, not unless a sealer is used to protect the surface, a significant amount of water is taken in by the walls. BS 5628-3 : 2001 outlines the practice code in the use of masonry. It highlights the use of components and materials, workmanship and design.
Generally, the absorption is distributed over the whole surface of the wall. Faults and weaknesses on the surface of the wall present a significant risk of penetration of rain water into the wall.
Rain Water Penetration Approaches
The forces that lead to the penetration of rainwater into the walls of buildings have been counteracted using a number of techniques. Systems of mass wall such as concrete block, stone or brick, and hard logs or timber, depend to a great extent on the surface of the wall to shed a lot of rain. The thickness of the material of the wall permits it to absorb and hold the surface moisture that remains. Indoor or solar heat aids in the process of evaporating the water that had been absorbed. According to BS 5628-3: 2001 and in particular Clause 5.5.2 of the practice code on the penetration of rain, ‘the designer should select a construction appropriate to the local wind-driven rain index, paying due regard to the design detailing, workmanship and materials to be used.’
The Drained Cavity Wall
Although it contains some aspects of the rain screen mechanism and takes account of the forces that result into penetration of rainwater, it is not particularly a rain screen wall. A cavity is used to separate two layers. Inside the cavity wall is installed a free material for draining the water. The kinetic force caused by the rain is received by the layer on the outside. It is important that the water that is penetrating the wall from the outside is retrieved and directed away from the cavity with use of weep holes and flashing.
This system involves draining/removing off most of the rain water at the primary surface of the wall and providing for cavity draining and evaporation of the remaining water. High buildings exteriors must have well assemblies that provide a superior level of control and performance within current building science and practical application.
The main problem with the drained cavity wall design and that which inhibits its performance is its inability to address gradients of air pressure. Suppose there is air tightness on the on the outermost layer of the wall, the surface of the wall will be subject to significant forces of the wind. Suction will be created by the relatively lower pressure of air in the cavity. This results in the infiltration of rain water via tiny opening on the surface of the wall. In line with BS 6399-2: 1997, the design of the system can such that it withstands the rising loads of wind attributed to buildings that are above twelve metres tall and high exposure areas.
These opening may take the form of pores, cracks, gaps, or surfaces that have been bonded poorly. The amount of water from the rain that enters the outside walls may outweigh the amount that can be drained internally by the wall. If the water is unable to be drained sufficiently well from the inside, the materials used to construct the wall may get destroyed with time.
How it Works
A group of clads put together as support channels for the outer exposed barrier, the outer walls are opened but designed to prevent water entering by capillary force, surface tension force and kinetic force, the clads put together are not designed to stop the water penetrating but to reduce. Water leakages from air pressure will occur; hence water might penetrate the outer wall through the clad knots. Water is controlled by cladding design by allowing it to flow vertically through the inside face of the wall on the outside, by vertical channels combined with horizontal knots which direct the flow of water to the vertical elements (BS 5427-1 : 1996). Wetting of the inner surface might occur and is allowed.
The vertical architecture of the cover is made in such a manner that it is able to gather the water that enters the wall direct it by gravity force and pressure gradient moving downwards and to the outer wall. Penetrations of the system must be such that it is in a position to gather water and make sure that it moves to the outer wall exterior or into the vertical drainage.
A minimum allowable width of space between the outer wall and the inner wall is of essence to facilitate back ventilation, promoting rapid evaporation of any water in the inner wall surface. The inner wall has an outside face that requires a water barrier at the stern of the cavity, because designer considered the inner wall to be wet at some point. According to the BBA Clause 8 of the ‘Envirowall External Wall Insulation Systems’, the thermal conductivity declared has to reach 90% of the level of production. The confidence level is expected to be at 90%.
The air barrier controls the flow of air through the wall, decreasing constant air pressure equilibrium along the covering, allowing the rain screen covering and the inner wall to perform more effectively and efficient. The inner wall must be flashed to prevent water from entering the building and also prevent direct water intrusion to the outer wall.
The inner wall is developed in such a way that it encompasses the interior environment and its nature, it can be constructed in many different ways and generally the inner wall is the structural enclosure. Use of insulation maximize usage of building space, it helps prevent condensation of the face on the inside of the interior wall. ‘ If the systems are to be used on the external walls of rooms expected to have continuous high humidities, care must be taken in the design of the rooms to avoid possible problems from the formation of interstitial condensation in the wall’ (BBA Clause 11.2: 2005). This reinforces the BS 5250: 2002 clause that recommended consideration be given to the entire design so as to mitigate the risk associated with condensation. Water resistance insulation can be used for the outward part of the barrier of moisture.
Since this system is not equalized by pressure, the wall on the outside must be constructed to hold 100% of the building’ wind load. BS 6399-2 : 1997 outlines the practice code for loads of wind for building loading. Traditional methods of testing water and air are applied to finalize on the building of the inner wall.
A pictorial of the drained/back ventilation is represented as follows:
Component elements of this system include:
- The outer wall
- Vertical drainage channel
- Penetration flashing
- Cavity ventilation
- Moisture barrier
- Compatible flashing
- Inner wall
- Resistant insulation
- Building wall
Point of noting in drained/back ventilated is that covering are allowed to leak with no deliberate attempts to curtail the impact of wind by the process of equalization pressure. Rather, the cavity behind the covering is drained and back ventilation allows promoting the quick evaporation of water from the rain on the inner wall. The BBA has given out several certificates to ensure proper insulation of walls. The construction of the outside walls should be sound and watertight. This is enforced by the issue of EWI certificates. Same method is used to remove water vapour which passes the inner wall.
The Pressure Equalized rain screen
The pressure-equalized rain screen is an approach of controlling rain penetration that involves employing several lines of approach. In other words, it is an approach that entails multi-defense. It derives its foundation from the principle of open screen, whose objective is to control those forces that can direct water into the assembly of the wall. These forces include; difference in the pressure of air, surface tension, momentum of rain drop, gravity and capillary action. Out of the mentioned forces, difference in the pressure of air is usually a major one with the possibility of pushing a reasonable quantity of rain water into the assembly of the wall.
Pressure Equalization Wall Components
It employs a system of drainable compartmentalization to limit water penetration due to disequilibrium in pressure facilitating rapid pressure equalization. Under certain weather conditions it minimizes and almost eliminates water in the rain screen cavity.
There are three components that are basic in the assembly of a wall meant to minimize the penetration of rain water especially as a result of differentials in the pressure of air. These are; an air barrier system that is effective, air chamber and a rain screen. These components must have features such as vent holes that are specially designed. The air chamber is situated between the air barrier system and the rain screen. The ability of the assembly of the wall to attain equalization of pressure transcending the rain screen is affected by the air barrier system’s performance.
This wall has supplementary characteristics in the cavity design in order to boost performance as compared to the typical design of the rain screen. The strategy of the pressure equalization is sometimes called the ‘pressure modification’. This is due to the fact that there is no system of walls that can attain instant and fixed equalization when subjected to dynamic pressures of the wind.
How it Works
Pressure equalized rain screen are more intensified in their design, there alteration and discrepancies are more sensitive. The entry spaces are designed to allow for counterbalance in pressures of dynamic and static nature that take place in the rain screen. Inequality with the Drain/Back Ventilation is with the use of compartments and the type of design applied within the depression. The reason for this that pressure comparison between the faces are not the same and because of these comparisons they only occur in limited time with known and controlled intervals of the depression behind the rain screen.
The compartmentalization is of essence in that the pressure of wind that blows across a particular phase are consistently changing, there is a known number of vents calculated based on known cavity volume allowing sufficient air to enter and leave the space on the inside in response to the dynamic wind changes. This enables the pressure difference between the plane of the rain screen and the internal compartment pressure reduced, thus, when the pressure of air is equalized on the internal and external sides of the exterior rain screen covering parts.
The vent holes effective holes depend on;
- The tightness of air present in the barrier system in the inner wall.
- Stiffness of the rain screen covering and inner wall.
- Volume of the different compartments that comprise the air space inside the wall.
The compartments are important in that they;
- Determine size of the opening of the vent holes
- Control and infiltrate water drainage at the time of disequilibrium in the pressure of air.
- Control lateral and vertical air flow.
The main point to note without dependence on the use gaskets focus is to minimize leakages into the cavity and it’s important to be cautious on drainage mechanism, components of these system include;
- Outer wall covering system
- Vertical drainage channel
- Penetration flashing
- Ventilation cavity/compartment
- Moisture cavity
- Compatible flashing at all penetration
- face of building structure wall
- Moisture resistant insulation
- Pressure equalization and drainage path
- Horizontal air dam
- Building structure wall.
Clause 14.2 of the BBA on finishing explains that ‘a spardash finish will break up the flow of water on the surface and reduce the risk of discoloration by water runs. The finish may become discoloured with time, the rate depending on the initial colour, the degree of exposure and atmospheric pollution, as well as the design and detailing of the wall. In common with traditional renders, discoloration by algae and lichens may occur in wet areas.’
Conclusion
Too often we are faced with requirement that amalgamate the elements of both rain screen systems and cladding design elements. It’s up to us professional and academies to educate the market on different designs and understanding of the scientific principles revolving around the main approaches to rain screen cladding. Proper understanding of both the design, principles and origin of the technology and development is of paramount importance, otherwise confusion may lead to development of hybrid versions which may not achieve organizational’ goals and objectives.
Various elements including outer wall, insulation, air barrier and inner wall can be supplied by different supplies hence contracts on purchase of these elements must be monitored, it’s important, to prevent buying elements that will not fit in the specific applicants hence contracts must be entered by design and construction professional.
The present invention provides a pressure equalizing compartment for building an inner wall facing both inside and outside surface and having a bottom and top edge. Both the inner and outer part contain uniformly coated with air barrier and a closure member. Present innovation also provides pressure equalizing exterior insulation and building structural layer, uniformly coated, with an air and moisture vent communicating to the outside environment. It’s therefore an objective of the present invention to provide an exterior insulation and well finished system incorporating a pressure equalizing covering and compartments.
Works Cited
Chudley, Roy, and Roger Greeno. Building Construction Handbook. Hoboken: Taylor and Francis, 2012. Print.
Controlling the Elements with Watertight Walls. N.p., 1997. Print.
Gifford, Clive. Technology. New York: Scholastic, 2012. Print.
Pressure Equalization and the Control of Rainwater Penetration Under Dynamic Wind Loading. N.p., 1994. Print.
Roberts, Simon, and Nicolò Guariento. Building Integrated Photovoltaics: A Handbook. Basel: Birkhuser, 2009. Print.
