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FAQ's > Application over ICFs
Q: How is stucco applied over insulating concrete forms (ICFs)?
 A:
Stucco is a popular and cost-effective finish. When placing plaster
over ICFs, it is recommended to treat this substrate as any other
sheathed system. It should include building paper, metal lath, and
3-coat portland cement plaster. In this way, plaster is properly
supported, yet free to move independently of the substrate beneath
it.
Some ICFs contain embedded furring strips on their face. After
placing paper over the face of the ICF, metal lath is mechanically
fastened to the furring strips. The lath supports plaster and holds
it in position while the paper isolates the plaster from the foam.
ASTM C 1063, Installation of Lathing and Furring to Receive Interior
and Exterior Portland Cement-Based Plaster, provides guidance on
the size of metal lath, number and length of fasteners, and appropriate
spacing of contraction joints.
Although plaster would adhere to the face of the ICF, the weight
of the layer is more than should be supported by foam. The plaster
could settle or shear entirely off the face of the wall. (Note that
ASTM C 926, Application of Portland Cement-Based Plaster, is silent
on (does not currently recognize) direct application of plaster
to ICFs.
Why Use Building Paper?
Currently, per the building code, a portland cement plaster is only
required to have a weather-resistant barrier (WRB) behind it, which
is satisfied by the ICF; hence, building paper would not necessarily
be required. However, in an ICF-stucco installation, the paper’s
primary function is to serve as a bond breaker and not
as a WRB between stucco and insulation, which have significantly
different rates of expansion and contraction due to changes in temperature
and moisture condition. Therefore, best practice indicates isolating
the two materials from each other to allow independent movement
and reduce stresses that might otherwise lead to cracking in the
plaster layer. By using a permeable paper, the permeability of the
wall system remains unchanged.
 If
it is desired to apply a finish directly to the foam form, an exterior
insulation and finish system (EIFS) material may be considered.
These finishes are thin, lightweight, and tough. Although the thinner
EIFS materials can be direct-applied, moisture management then becomes
even more critical. If an EIFS coating is chosen, openings (windows,
doors, etc.) must be properly detailed and constructed so that moisture
is kept out of the wall because the system is not breathable. The
Exterior Insulation
Manufacturers Association (EIMA) may provide additional information
on finishing and details.
In considering textures for plaster finishes, note that smoother
finishes of stucco tend to show cracks more readily. Depending on
the frequency and width of cracks, more water could be transmitted
through the face of the plaster.
A partial list of suppliers of stucco or EIFS materials is found
on the Concrete
Homes Web site.
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