When EIFS was first introduced to the North American market, there were a lot of questions about its strength. This particularly revolved around the use of EIFS on lightweight stud walls with gypsum board sheathing substrates. This concern proved to be unfounded and since then EIFS has been used on some very large buildings, including in areas where strength is an issue (windy areas like the Gulf of Mexico). What follows is some useful information about strength issues and EIFS substrates. This concern proved to be unfounded and since then EIFS has been used on some very large buildings, including in areas where strength is an issue (windy areas like the Gulf of Mexico).
NONSTRUCTURAL CLADDING VS. SUPPORTING WALL
EIFS is a nonstructural material. It does not support the wall. This makes it
more like a curtain than strong materials like concrete block that can be used
to support the building. The wall that supports the EIFS may be structural,
including having load bearing structural studs.
BUILDING SITE LOCATION
The physical location of an EIFS application has a bearing on the amount of
strength it should have. Hurricane areas, for instance, require stronger
attachment than areas that are not windy. The building codes have maps that
indicate the minimum wind forces to which walls must be designed. Wind forces
act in two directions: they push the cladding into the wall (positive force)
and they also try to pull the EIFS off the wall (negative or suction force).
The latter mode is more of a concern than the former, as the cladding can
Wind forces are higher at the corners of buildings than in the center of large
wall areas. The suction forces are highest on the backside (away from the wind
direction) than on the front side. Wind forces increase with the height of the
building and the area that gets the most wind action is often the downwind
parapet area at the corners. This is why you often see flashings torn off the
edge of roofs during storms.
Thicker applications of EIFS foam insulation make the EIFS stiffer and stronger.
This is due to the reinforced composition of the EIFS coatings and substrate.
However, this strength increase due to the use of thicker foam is not taken
into account when designing EIFS cladding to resist wind.
COATINGS VS. FOAM VS. ATTACHMENT
The foam insulation is the weakest material in EIFS. Attachment by using
adhesives, which spreads out the wind forces over the entire foam layer, are
much stronger than a small number of mechanical fasteners (washers and screws).
Adhesive patterns, such as closely spaced notched trowel stripes, are stronger
than the traditional ribbon and dab pattern.
Buildings adjacent to each other affect the movements of wind over each other.
This is due to the channeling effect of the wind moving between buildings, such
as on streets and alleys. On tall buildings in built-up areas, wind tunnel
tests and computer models are used to predict what the wind forces will
actually be. This force is factored into the plans for the building, no matter
what the minimum tolerances required by the building codes are.
SOLID SUBSTRATES VS. SHEATHING
Heavy, stiff, solid walls-such as concrete and block-make the best substrate
for EIFS. They do not move much and provide a rigid surface onto which to
attach the EIFS using adhesives. The use of lightweight stud/sheathing
substrates for EIFS is not as common outside North America.
MOISTURE ON SHEATHING
The presence of moisture can affect the performance of sheathing. It can weaken
the ability of the sheathing to resist wind pull-off forces. This damage can
occur before the EIFS is installed and can be due to water leaks in the
sheathing. This is especially true of older paper-faced gypsum sheathing and
moisture sensitive wood-based sheathings, such as oriented strand board. Thus,
it is important to protect the sheathing during the initial construction
process and to do regular maintenance so that water does not get into the
FRAMING AND DEFLECTION
The stiffness of framing is important. This stiffness is a measure of a
property known as deflection. Deflection is the in/out movement in the plane of
the wall due to wind. Deflection is usually expressed as a percentage of the
distance between the attachment points for the framing-known as the span. EIFS
is a fairly flexible material and for most EIFS the maximum allowable
deflection is 1/240th of the span.
The stiffness of the framing is related to the depth of the framing, its shape
and the thickness of the metal. A 6-inch stud is much more than 50 percent
stiffer than a 4-inch stud.
If mechanical fasteners are used to attach the foam to a structural substrate
like plywood, you can locate the fasteners where you want and use as many as
you need. With nonstructural substrates like gypsum board, the fasteners need
to go into the studs. This limits their number and spacing.
Since plywood and other wood-based sheathings are combustible, they can’t be
used on large commercial buildings where building codes call for noncombustible
materials. To remedy this, adhesives are used on commercial
When initially introduced to the North American market, there was concern about
the resistance of the thin EIFS coatings to being damaged from being struck by
hard objects. At that time, only lightweight reinforcing mesh was available.
This problem had been addressed in Europe, where EIFS was initially developed,
by the use of heavy mesh. The heavy mesh was imported to North America and
solved the problem for most buildings.
However, the heavy mesh did not completely solve the problem-especially on
lightweight walls using studs and sheathing where breaches in the wall could
cause problems. In cases of buildings such as banks, the penetration resistance
comes from the supporting wall, which is usually masonry or
Thicker EIFS basecoats improve the impact damage resistance as does the use of
the more flexible, cementless basecoat adhesives. Higher density foam also
helps but can contribute to the tendency of the EIFS lamina to crack. This is
why low-density, springy EPS foam is used.
This is the movement of a wall going out of square along its own length. It is
caused by forces pushing against the wall along its face, such as high wind or
earthquakes. Glass reinforcing adds substantial resistance to racking but this
is not taken into account when using EIFS. Instead, the sheathing, straps or
other forms of bracing on the studs are used.
Sometimes, the substrate to which EIFS is attached changes from one material to
another. A classic case is where stud framing abuts a more rigid substrate like
concrete. The difference in springiness of where the two substrates meet can
cause the EIFS to crack. This is why EIFS producers require an expansion joint
where the types of substrates change.
PUBLISHED STRENGTH DATA
EIFS producers publish strength data on their products with tolerances given
various attachment methods, substrates and stud spacing. This information is in
their catalogs as well in product approval documents issued by code agencies
such as the International Code Council. W&C
Robert Thomas is a nationally recognized EIFS consultant, based in
Jacksonville, Fla. He was the manager of technical services for a major EIFS
producer, is the author of several books about EIFS and chairs the ASTM
committee on EIFS.
The Finish Line: Know Your EIFS
December 1, 2011