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Q: Are there special considerations for selecting
finishes for autoclaved aerated concrete (AAC) wall assemblies?
A: Two critical aspects of selecting
AAC finishes are attachments and drainage. The proper handling of
all forms of moisture and careful selection of the means of finish
attachment should be examined when detailing AAC wall finishes.
Liquid Moisture:
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| Thin, lightweight portland cement plasters
are polymer modified, so they adhere well, resist cracking,
repel water, and breathe. The finish coat can be painted with
vapor permeable masonry paint. |
AAC has a closed cell structure that inhibits the passage of water
(by capillary action) deep into the material. Water does not harm
the structural integrity of AAC, but can temporarily increase its
thermal conductivity. For this reason, the (overall) exterior finish
must be designed to minimize water penetration. Some finishes require
an unobstructed air space (like cavity wall construction) and secondary
means of allowing water that migrates behind the primary finish
layer to drain down and out of the assembly.
Common best practices for finish attachment typically result in
an exterior wall that performs well. Systems that benefit from a
ventilated and weeped air space on the back side of the finish,
like wood siding, should be installed over furring that is mechanically
fastened to the surface of the AAC. A suitable drainage plane like
building paper along with appropriate flashings should be placed
outboard (on the exterior face) of the concrete to prevent free
water from remaining within the assembly, potentially reducing thermal
insulating performance or damaging the exterior finish.
Water Vapor:
Finishes that repel water yet allow the wall to breathe through
both the exterior and interior faces are ideal choices. Lightweight
portland cement stuccos are polymer modified to enhance bonding
and workability. They can be applied directly to AAC. They prevent
free moisture penetration, but have high vapor permeability, and
therefore allow small amounts of moisture that might penetrate the
stucco to dry to the exterior.
As a result of high-temperature steam curing, AAC materials have
an initial moisture content of 18%. Over time and with drying to
one or both wall faces, this internal moisture dissipates. Non-breathing
finishes are not recommended. A synthetic stucco system with low
permeability on the exterior combined with vinyl wall covering on
the inside would be an example of a combination of finishes that
would trap internal moisture within the AAC.
Curing moisture within newly manufactured AAC poses no long term
implications for air handling systems. Given time, the moisture
simply diffuses (as long as finishes are breathable). The heating,
ventilating, and air conditioning system should be designed for
the anticipated in-service loads of the occupied building. The moisture
level of AAC materials will decrease during construction, but a
fast tracked schedule with early occupancy may make the use of temporary
dehumidifiers desirable during the first year of occupancy to supplement
the ability of the mechanical systems to maintain optimized indoor
conditions.
Mechanical Attachment:
Many finishes can be anchored or adhered directly to the face of
AAC. In either case, it is essential to understand the anticipated
loading of the finish to be applied, and the capacity of the anchors
that will hold the finish materials to the block. For anchored systems,
a cavity space may be created by furring strips. For adhered finishes,
a typical application would involve installing a weather barrier,
attaching metal lath to the structural system, and applying a base
coat of plaster. Once that is prepared, finishes such as lightweight
manufactured stone can be applied. The American
Society for Testing and Materials (ASTM) is developing standards
for materials and installation of manufactured stone. And in all
cases, it is recommended to contact the AAC manufacturer for guidance
about appropriate types of anchors and their rated capacities.
 Donn
C. Thompson, AIA, CGP, LEEP AP is Director
of Low Rise Buildings for the Portland Cement Association (PCA).
A licensed architect, Donn has more than 20 years experience in
commercial and residential construction.
More
on Donn Thompson.
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