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Stucco Rainscreen
Introduction
Over the last number of years stucco finishes, both cementitious and
acrylic, are under scrutiny and have been the topic of many technical
and non-technical discussions. The thickness of cementitious
stucco has been an issue in Alberta over the last 5 years and
acrylic stucco has been the main topic of discussion among designers,
building owners and building officials.
The problem with “leaky condos” in British Columbia has
been the topic of many newscasts and it has affected many individuals.
Many papers have been written on the topic most of them by the
proponents of the system (especially the acrylic stucco), which
provided little or no explanation as to why so many buildings
experience so many problems. The problems experienced by these
systems are blamed on applicators, contractors, designers, etc.
This paper deals with the issues and factors affecting the performance
of these systems and introduces the principles of vented rain
screen to stucco finishes. The principles of rain screen wall
assemblies have been known to designers and building officials
for many years and because of the problems encountered with other
wall assemblies it is now the preferred system, and one recommended
by designers and building officials.
Current Issues
The main factor contributing to the problems encountered with the
acrylic stucco when applied on top of the insulation known as
Externally Insulating Finishing System (EIFS), is the inability
of the assembly to dry through the “EIFS making the performance
of the system somewhat climate dependent. In a document prepared
by the National Research of Canada (NRC) in September 2000 a
number of deficiencies have been identified, the most important
one being failure of the system to drain and dry when water enters
through the exterior causing softening of the drywall, delaminating
of drywall sheathing, expansion and warping of OSB or plywood
sheathing, corrosion of metal fasteners were some of the concerns
raised by NRC in the above mentioned publication.
The problems encountered with this system in its current state are
extensive and have resulted in the cancellation of liability
insurance for professionals. Associations have notified their
members about potential problems and issues warning in the form
of identifying face sealed wall assemblies as systems with increased
liability claims. The OAA (Ontario Association of Architects)
is quoted as saying the following: “The association will
not cover you, pay damages, provide you with a defence or make
supplementary payments for claims made against you, directly
or indirectly arising out of, contributed by, caused by ingress
of precipitation, resulting from, or in connection with your
design or selection of an exterior above-grade wall which has
no provision for drainage of precipitation that penetrates the
wall system, or otherwise provided in the Practice Bulletins
issued by the OAA.” The NRC quoted document above is further
stating that reduction of moisture is required for these systems
to perform. For a system which can reduce the moisture load the
NRC publication further states that no testing will be required. “Such
a system could be achieved with screen principles for a mechanically
attached EIFS that includes a vented, wide air cavity.” This
system will also satisfy the requirement of the OAA by providing
for drainage of precipitation that penetrates the wall system.
The system which is described in the following section provides for
all these requirements as it satisfies all existing code requirements
as they relate to a vented rain screen wall.
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