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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 rainscreens. 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 rainscreen.
We now know that in addition, a pressure-moderated rainscreen
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 rainscreens
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 troweled-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.
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