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The Two Types of Rainscreens:

Tuesday, October 16, 2007

Many are not aware that rainscreen systems come in two distinct types: Drained/Back Ventilated, and Pressure Equalized. Both systems have applications in commercial and residential construction provided they are properly specified and constructed for the application in which they will be used.


Drained/Back-ventilated Rainscreens:

The current system employed by most system manufacturers is drained and back ventilated.  This is a worthy system provided you have reasonable workmanship on the air/water barrier.  It involves the assumption that water enters the cavity in limited amounts and is prevented from entering the building by the waterproofing. Additionally, the moisture can easily dry since the cavity is vented, causing the water to simply evaporate. 

The system in force in most panel installations does not pressure equalize because of the upward or downward or sideways flow of air in a large cavity. Specifically,  these large volumes of cavity space behind the panel system do not allow rapid equalization of pressures (if equalization occurs at all).

For example, if the pressure is 20 psf at the top of the wall and 15 psf at the bottom of the wall, there will probably never be 20 psf pushing back out of the cavity joints because the pressure simply migrates to a lower pressure point in the large cavity. 

Therefore the planned scenario is that at least small amounts of water will enter… but then dry out due to the ventilation that is free flowing and available.


Pressure Equalized Rainscreens:

In contrast, the pressure equalized system differs in that the cavity will be broken up horizontally as well as vertically so that individual (smaller) panel areas can pressure equalize.  The amount of venting is then carefully calculated based on the cavity volume. 

The benefit is that pressure can build up rapidly in the cavity and push back at the same pressure that it is pushing in the vents/seams and gaps, resulting in less water (or no water) entering the system. 

Another advantage is that the pressure presses equally on both sides of the panel resulting in a theoretical Net 0 psf on the panel system itself. Consequently, the building design pressure can be applied to the wall behind the panel system.  Although this is potentially true, building codes and safety factors require that you design fastening systems able to withstand the design pressures.


Other Notes:

It is also important to realize that regional concerns come in to play with both types of rainscreens. For instance, it is recommended by the ABAA (Air Barrier Association Of America) that the building Vapor Barrier should always be installed on the warm side of the insulation. In primarily air conditioned climates, the warm side may occur on the exterior of the building envelope, whereas in Seattle, the warm side typically occurs on the interior of the building envelope.

Another common concern is the use of insulation board behind a panel system (on top of the sheathing). The joints between these boards may hold water if not properly sealed, and without placing an air-impermeable barrier on top of the insulation it may be difficult to obtain an airtight cavity as is required for PER systems.

Also note that the vapor permeable barrier applied to the exterior of the building in PER systems will be required to withstand negative pressures associated with the design of the rainscreen. For this reason, it is recommended that the barrier (Tyvek, for instance) be attached with 1" washers at all locations where the barrier is subject to cavity pressures.

These, among other considerations, are extremely important to your rainscreen design.


Other Reading:

Metal Construction Association (2006). "Understanding 'The Rainscreen Principle'"

IRC (1999). "Designing Exterior Walls According to the Rainscreen Principle"

IRC (1998). "Pressure Equalization in Rainscreen Wall Systems"

IRC (1997). "Evolution of Wall Design for Controlling Rain Penetration"

Air Barrier Association of America (2005). "Difference between Air Barrier, Vapor Barrier, and Water Resistive Barrier"

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