28 October 2009

Pressure Equalized Rainscreen Applications

In Brick Masonry  (See BIA Technical Notes 27 - Brick Masonry Rain Screen Walls.)

In Metal Panel (See AAMA 508-07 Voluntary Test Method and Specification for Pressure Equalized Rain Screen Wall Cladding Systems.)

In Insulated Metal Panel (Refer to Centria product line.)

In Clapboard Siding (This is an empirical design.)

In EIFS (See Dryvit product line.)

In Curtainwall (See AAMA CW-RS-1 Rain Screen Principle and Pressure Equalization.)

12 October 2009

Designing Pressure Equalized Rainscreen Walls

Whether or not you are testing you Pressure Equalized Rainscreen (PER) to a standard such as AAMA 508, there are prescriptive practices to design pressure equalized screen walls for maximum pressure moderation. There are essentially three components that require detailed attention: the rainscreen, the compartments, and the air barrier.


There needs to be a screen that permits the passage of air and manages water infiltration.

What seems to be the most counter-productive aspect of pressure equalized design is that the exterior wall needs to be full of ventilation holes. Each compartment needs to be ventilated to the exterior so that as the pressure changes, air can flow in and out of the compartment to make up the difference and keep the chamber equalized. Since holes are ordinarily a problem, it is necessary to pay careful attention to these vents.

To be effective, the vents need to be sized in proportion to the volume and rigidity of the compartment, and the air leakage rate of the air barrier. For example, the ratio of volume to vent area can be as small as 25 m / 82 ft for 13 mm / 1/2 in cavities in masonry compartments (Vent ≥ Volume / 50 m). A larger ratio of 50 m / 164 ft may be required for 25 mm / 1 in cavities in gypsum wallboard compartments (Vent ≥ Volume / 25 m). The ratio of equivalent air barrier leakage area (ELA) to vent area should be no more than 20 (Vent ≥ 20 × ELA).

The holes also need to be located in one location to prevent cross currents. Cross currents would likely create an area of low pressure inside the compartment in accordance with the Burnulli principle. Unfortunately, if you had to choose a chronic problem of either high or low pressure, low pressure is the worst because it will pull water into the cavity. From one point of view, the holes should be in the center of the face of the compartment to permit the most uniform pressure equalization across the compartment. In practice, however, it is best to distribute them along the bottom to assist with drainage. There is some research that suggests distributing them to the side away from the corner of the building may even help to pressurize the chamber, which would help resist infiltration of water.

Otherwise, the vents need to be designed to sound principles that would apply to any construction. They need to be larger than 10 mm (3/8 inch) so that water can’t span and block the opening, promoting capillary infiltration. They need to incorporate labyrinths by design or location to resist water infiltration by momentum. They need to be protected behind drips to resist and located at the bottom of the compartments so that water is always flashed down and out by gravity.


There needs to be drained compartments that are open only to the exterior.

Because wind pressures vary across the face of the building, with the greatest pressures on the corners and along the edges, with the least pressures in the center field, the compartments are usually sized in proportion to the wind loading to keep the cost down. Small compartments no larger than 1 meter (4 feet) in length are constructed in the zone within 6 m (20 feet) of the perimeter. In the central portion of the wall, the compartments may be as large as 6 m (20 feet) in length. This provides an adequate approximation of the wind loading differentials.

Of course, since the point of having different compartments is that the pressure may be different from one compartment to the next, the compartments delimiters must be designed to prevent lateral flow between the compartments. Horizontal delimiters are often easy to design and often pre-exist at every floor line in brick construction as the shelf angle. Other systems such as metal panels or stone veneer require a whole new way of thinking because they tend to be supported off clip angles which then become an impediment to compartment design.

The size of the compartment volume in relation to the size of the vents in the screen is proportional to its ability to equalize. Therefore, to achieve maximize equalization with minimize vent size, the compartment volume needs to be as small as possible. Unfortunately, in most assemblies, this is also the best location for the insulation. If rigid insulation is used, the volume of the compartment is decreased by the volume of the insulation. If semi-rigid or batt insulation is used, only some of the volume of the compartment is lost mathematically; it’s actual effect on the equalization can be either positive or negative. If the insulation is full thickness in the compartment, it provides resistance to airflow within the compartment and hence resistance to its ability to equalize. If a free and clear compartment is maintained, my estimation is the insulation will provide some moderation to the equalization.

Pressure equalizing compartments continue to be subject to water infiltration, even with all the performance enhancement of the pressure moderation. Porous materials, construction joints, line-of-sight weeps, and design or construction errors will continue to provide paths into the cavity. Therefore, it is critical to drain each compartment just like any other cavity construction.

Air Barrier

There needs to be a waterproof air barrier that is capable of maintaining a pressure differential.

Building exteriors are subject to pressure differentials due to HVAC balancing, stack effects in tall buildings, and wind pressures. Depending on the HVAC design, the balancing component can be uniform over the entire exterior or can vary slightly from room to room. Stack effects tend to be exterior positive at the lower levels and exterior negative at the upper levels, and can easily add up to an inch of water in tall buildings. (See CBD-104: Stack Effect in Buildings.) Wind pressures can change rapidly from positive to negative as the wind changes direction, in strength as the wind gusts, and in relative severity depending on location in the middle of the building or near a corner. (See CBD-34: Wind Pressures on Buildings.) All these forces get added together at each particular moment and location on the exterior facade. The design of the building can be such that the pressure differential is distributed throughout the thickness of the facade, or concentrated on a single material such as an air barrier. The air barrier needs to have the structural integrity to withstand these forces without failure. For PER, the air barrier needs to be the plane at the back of the compartment.

Air barriers come in two varieties. There are flexible sheets such as Tyvek that are effective in providing draft control, and coatings or sheets such as Blueskin/Air-Block or Perm-A-Barrier, that are applied to a structural substrate the provide both draft and pressure control. The latter rigid air barrier is required so that the volume of the pressure equalization chamber does not change in volume, inadvertently transmitting some of the pressure differential across the barrier and nullifying the effectiveness of the compartment.

Because there are forces in addition to pressure which could successfully transmit water through product, design, or construction defects into the compartment, the air barrier must be waterproof so that incidental moisture that reaches it will be blocked from proceeding farther into the building. Many special-purpose air barriers are also water barriers (and frequently vapor barriers as well, but that is a consideration for a different topic), providing a one-component solution for multiple considerations.

That's all there is to the PER: the screen, the compartments, and the air barrier. With these concepts, you can conceptually detail your PER very early in the project. By understanding them, you can communicate better with consultants and manufacturers' reps how to efficiently apply these concepts to your design, and to refine your details for greatest effectiveness.

10 October 2009

Water Infiltration through Exterior Walls - Pressure, Part 4 of a 4 Part Series

There are four ways that water enters a building:
  1. By Gravity.
  2. By Momentum (kinetic energy).
  3. By Capillarity.
  4. By Pressure.
When you buy a beverage at a fast-food restaurant, it comes with a cap and straw. The cap is useful if the beverage gets tipped, because it retains most of the liquid--perhaps even all the liquid if a sufficient vacuum is formed inside the cup. But when you use the straw to drink it, you create an area of low pressure in your mouth which is sufficient to overcome the gravity and vacuum that is trying to keep the beverage in the container. If the beverage is thick like a milkshake, a greater pressure differential has to be induced or a larger straw used to overcome its greater viscosity, but it is still a pressure differential that enables you to successfully drink your beverage.

Building exteriors work the same way. As long as they are capped and closed in, water tends to stay where it is supposed to stay--outside the building. But they tend to be full of straws of all shapes and sizes due to changes of materials, construction joints, and material deterioration, and they tend to be subject to pressure differentials due to HVAC balancing, stack effects in tall buildings, and wind pressures that rapidly change not only in strength but also from positive to negative.

Just as pressure can overcome a variety of forces in the beverage example, the pressure differential in buildings can and will overcome gravity, momentum, and capillarity to draw water into the building. As there are ways to manage gravity (using flashing to continuously direct water down and out), momentum (blocking the line-of-sight holes), and capillarity (installing capillary breaks), there are ways to manage pressure as a water transport force.

The outstanding concept is to maintain the pressure inside identical to the pressure outside. Then, only the gravity, momentum, and capillary forces are available for water transport. However simple this sounds, in reality it is impossible when dealing with wind loads. The windward side will be under positive pressure and the leeward side will be under negative pressure. The volume inside cannot match both simultaneously.

However, if the exterior skin has an internal cavity that is composed of multiple compartments, each compartment can be pressurized according to the wind pressure in effect at its location, which may be different from a neighboring compartment. The pressure differentials are isolated to the interior wythe only, between the compartments and the interior of the building, rather than spread out through the entire thickness of the exterior enclosure. Although a pressure differential is still available to help transport water to the interior of the building, if the exterior screen has done its job, no water will be available to be transported at the point where the pressure differential takes place.

No assembly that we have can instantaneously and completely equalize a compartment to the exterior. Fortunately an instant response is not necessary because the water transport is also not instant. If there is a small enough time lag and great enough response to changes in pressure, the assembly will be successful. In other words, it is really a case of "pressure moderation." How much moderation is required to be successful is the subject of research. According to AAMA 508-07 "Pressure Equalized Rain Screen Wall Cladding", it should be 50% of the pressure within 0.08 seconds. Many manufacturers have been developing systems that have been tested to this standard.

08 October 2009

Water Infiltration through Exterior Walls - Capillarity, Part 3 of a 4 Part Series

There are four ways that water enters a building:
  1. By Gravity.
  2. By Momentum (kinetic energy).
  3. By Capillarity.
  4. By Pressure.
Free drops of water will by nature take on a spherical shape - the geometric shape that encloses the most volume with the least surface area - because of the cohesive properties of water. However, many things can interrupt the cohesive forces, making water both a versatile and destructive material. Those forces that can overcome the cohesion of water include gravity, which tends to flatten the sphere, and attraction and adhesion to other materials, which can completely overcome the water's cohesion and "stretch" it along a surface.

Water tends to have a natural adhesion to many building materials, but various pollutants and cleaning agents may break down the cohesion and viscosity of water more than normal to increase its adhesion. We call those combination of forces the wetting ability.

This would not be a problem except that those same building materials often come with pores and cracks - little tubes composed of the wettable surface. The tubes increase the available surface area for a given volume of water, increasing the relative effectiveness of the adhesive forces. If the tubes are the right size, the adhesion can completely overcome the water's cohesion, turning the pore or crack into an effective transport medium to drag the water from one place to another, even over great distances. If the pores or cracks are even further right sized, these forces can even overcome gravity, allowing the water to be drug uphill.

One thing about capillarity is that it tends to take time to transport the water. If the walls are thick enough, the rainstorm will end before the water has made it the whole way to the interior, and the transport will reverse during the following dry period. Therefore, capillarity could more or less be ignored in traditional thick construction.

With our modern high performance thin construction, building designers need to take capillarity into account, just as they do gravity and momentum, if they will design durable structures. The first line of defense is to acknowledge that capillarity will happen, and to use materials with enough integrity to withstand freeze-thaw cycles with very little deterioration over a long period of time. Even porous materials such as brick come in varieties that meet this requirement.

The second line of defense is to provide capillary breaks so that the water cannot be transported the whole way into the interior. The capillary break can be a 10 mm (3/8") or larger gap, or a layer of water-impermeable material. In many modern wall constructions, there may be multiple capillary breaks intentionally or not. The multiple breaks aid in the durability of the building by providing a back-up in case the first break is bridged or compromised.

The right size for water transport tends be be in the range of 5 mm (3/16") to 0.01mm (the thickness of a human hair). Within this range, pores and cracks less than 0.5 mm (the thickness of a business or credit card) can even transport water uphill.  Pores and cracks larger than this range will tend not to support capillarity, but will allow water intrusion by momentum. Cracks smaller than this range can still take on water, but they do not support capillarity in the sense that the adhesion bonds tend to be so strong they will not release the water. Then, if they are subject to freeze-thaw cycles and the expansion of the water as it turns into ice is sufficient to overcome the cohesive forces of the building material, the entrained water will widen the cracks as it freezes, enabling future capillary transport.

Although there is a right size for capillarity, pores larger and smaller can still be effective and even rapid conduits for water transport, if other forces such as gravity or differential pressure are added to the capillary forces. The next essay will look at how pressure differential impacts water intrusion.

05 October 2009

Water Infiltration through Exterior Walls - Momentum, Part 2 of a 4 Part Series

There are four ways that water enters a building:
  1. By Gravity.
  2. By Momentum (kinetic energy).
  3. By Capillarity.
  4. By Pressure.
Free water is frequently blown at buildings by the wind. Wind is, of course, nothing more than moving air. When a moving mass of air reaches a building, it cannot pass through the enclosure. It also cannot build up in infinite quantities on the face of the building, so it diverts itself around the building. To a certain extent, the molecules that make up the moving mass never actually touch the building because they are buffered by a mass of air adjacent to the building.

Rain may get caught up by and carried along with the wind, enabling the water to approach the building in something other than a vertical direction. However, the rain has a tendency to keep moving in the same direction when it gets to the building, even as the air is diverting itself around the building. The tendency is a force we call momentum. Though the moving air might not reach the building, the moving water does, and it accumulates on the enclosure until it is removed by gravity.

If a particular drop of water reaches the building at a point where there is a hole in the facade, it will continue unabated into or even through the building enclosure, depending on the size and depth of the hole.

If the hole is large, such as a doorway, viewport, or ventilation opening, we are intuitively aware of the problem and we mitigate the problem of water entry by providing closures such as doors, windows, shutters, and louvers.

However, buildings tend to have other holes that more easily escape our attention such as weeps in veneer brick,  reveals in metal panels, joints between dissimilar materials, and even expansion joints. These holes cannot be blocked in the same way that a door or window is blocked, but the momentum of flying water can be blocked on the same principle that makes louvers work: designing labyrinths that interrupt every straight line and convert the primary force acting on the water from momentum to gravity.

Buildings also sometimes have unintentional holes resulting from a construction omission, natural disaster or aging, or vandalism. When these occur, the solution is to engage workmen to close up the hole with like material or a complementary material designed for that purpose, such as sealant.

As a final note, holes that are smaller than 5mm (3/16") do not need to be considered in terms of momentum. The reason is that it is nearly impossible for a water drop to pass through such a small opening without being attracted to or otherwise disrupted by the sides of the hole. However, capillarity tends to take over at the moment that momentum is arrested, introducing another means by which water can enter a building. But that is the topic of another essay.