Opening a Gap
It is possible to incorporate a drained and vented cavity near the exterior face, outward from the thermal insulation, in an EIFS system. Korax Technologies make a “stucco rain screen” polystyrene beadboard panel that has horizontal and vertical grooves in the back to create a drained and vented cavity. The panel can be used with conventional stucco as well as with EIFS. The panel is attached over the primary thermal insulation, using mechanical fasteners. Vertical channels in the panel create cavities that lead moisture to flashings and vents at the bottom of the wall section, and allow for ventilation.
First Generation of EIFS Rainscreens
At about the same time as the survey, the EIFS industry was investigating the possibility of modifying their systems to create pressure moderated rain screens. This challenge was met only partially. With other cladding materials, the weaknesses of a face-sealed design can be mitigated by providing a drained exterior screen, or a pressure moderated space behind the screen to intercept most exterior moisture, and deal with moisture that the screen fails to exclude. The features required to do this are:
- an exterior screen (the visible surface) to shed and/or store and evaporate most of the incident water
- a cavity behind the screen, flashed and drained to the exterior to drain water that leaks through the screen back outside.
- an air barrier on the dry side of the cavity, protected from water in the cavity, so that air pressure differences don’t push water through imperfections. This requires that the cavity be wide enough, and that bridges and ties be designed so that surface tension and gravity cannot cause water to bridge the cavity.
- ventilation to provide air circulation for drying, to remove water that cannot drain, since it would otherwise have to exit as vapour through solid materials of the screen or the interior wall.
In the past, we might have been satisfied to call this a rain screen. We now know that in addition, a pressure-moderated rain screen requires these additional features
- baffles to separate regions of different time-averaged exterior pressure from each other, because cavity pressure will otherwise be the average exterior pressure, not the exterior pressure at a particular point.
- a cavity volume small enough in relation to the leakage and flexibility of the air barrier, and the vent area and flexibility of the screen, to allow air pressure in the cavity to track rapid changes in exterior pressure (on the order of 1 kHz in frequency).
To date, the EIFS systems promoted as pressure-equalized rain screens have neglected some of these requirements. To their credit, they have demonstrated the ability to equalize transient pressures across the lamina.2 So, they do reduce the air pressure that would otherwise force water on the exterior surface into the insulation to near zero. They allow for drainage through porous insulation or grooves in the back of the insulation to the exterior. However, they do not have cavities that are capable of keeping water away from the air barrier, and they do not provide for drying by ventilation, nor can they. The cavity has to be as small as possible, because it is on the warm side of the insulation, and because of adhesive attachment to the backup. If the cavity were large enough to provide ventilation or, worse yet, cross-ventilation, it would negate the insulating value of the system. In addition, reduced contact area would make adhesive attachment less reliable.
Sto, Dryvit, Durock, and Preswitt all offer CCMC-evaluated systems of this general type. Some have grooves in the back of the insulation, others use thick beads or ribbons of adhesive to hold the insulation far enough away from the air barrier to allow air movement for pressure-moderation. In summary, these systems have the following disadvantages:
- the air-barrier is trowelled-on. This makes it subject to thickness variation, and requires use of emulsified polymer materials that may not withstand long-term exposure to moisture.
- the ventilation channels are small - just enough for pressure moderation but not adequate for drying, particularly since air flow through the system has to be prevented to preserve thermal performance.
- entry of exterior moisture is still possible by gravity flow, and capillary forces. Once inside, water can reach the air barrier.
References
2 Nelson, Peter E. & Kroll, Richard E., “Exterior Insulation Finish Systems (EIFS): Materials, Properties, and Performance”, ASTM STP 1269, Philadelphia, 1996.