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.

Rainscreen

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.

Compartments

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.

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