Success With EIFS, continuedFiberglass mesh. There are
many types and grades of fiberglass mesh. It's all made with
glass fiber and an alkaline-resistant polymer coating, but the
weight of the glass and the thickness of the protective coating
vary. So does the way the fabric is made: Some mesh is woven
(Figure 3), and some is non-woven.
3. Woven reinforcing mesh (top left) serves to
stop the cement and acrylic base coat from cracking. A
coating protects the mesh against the alkaline cement,
but mesh may still be weakened by moisture and chemical
attack. A worker trowels the mesh into the wet base
coat (bottom). For good coverage, the authors recommend
applying the base coat in two layers: one layer before
troweling in the mesh, and a second layer immediately
afterward. Exposed mesh (top right) tends to lose
strength over time.
Heavier mesh is stronger and stiffer than lighter mesh, and
mesh with a thicker coating generally holds up better against
the alkaline chemistry of cement compounds in the EIFS base
coat. We prefer non-woven mesh because its coating is more
continuous: The woven mesh can have gaps in the coating where
the fibers contact each other in the weave. Unfortunately, it
is difficult to find mesh manufacturers willing to fabricate
Manufacturers generally offer a "standard" 4-ounce mesh
(weighing 4 ounces a square yard), which is actually their
lightweight mesh. They also offer higher-weight mesh types that
they describe as medium-impact, high-impact, and so on. There
is also a very light 2.5- to 4-ounce "detail mesh" that is used
for corners, V-joints, and the like. It's common for installers
to use the 4-ounce standard mesh in the field and use detail
mesh wherever they need to bend around corners (ironically, the
corners and joint areas that are most likely to crack often
receive the lightest-weight mesh).
We usually recommend a minimum 6-ounce mesh. For the first
floor, or at least the first 4 feet of the wall — really,
in any high-traffic area where there are likely to be lawn
mowers, weed trimmers, snow shoveling, and other activities
that can damage the wall — we call for high-impact mesh.
You can't overlap the high-impact mesh, so you butt the pieces
together, then cover the whole thing with an additional layer
6-ounce mesh. With that reinforcement, EIFS is almost as
strong as some stucco systems, particularly a one-coat
For corners, some manufacturers provide specially formed
pieces made of the heavier mesh. If you have to wrap a corner
with lighter mesh, you can double it up.
The mesh strength is important, because it is the only thing
keeping the surface lamina from cracking and allowing water
penetration. Contractors may try to convince the owner that the
4-ounce standard mesh is sufficient, but our experience is that
the 6-ounce or heavier mesh achieves better results.
Base coat. Base coats are
usually formulated with an acrylic resin combined with Portland
cement. They may also contain silica sand and lime or lime
byproducts. Most manufacturers offer a choice of several base
coat types. The applicator can either add water to a dry mix,
or add just the dry cement to a premixed wet formula that
already contains water, the acrylic resin, and other
ingredients. In our experience, the wet mixes are far superior
to the dry mixes. We never specify a dry mix.
The proportions of cement and acrylic in the base coat
formulas have been an issue in recent years. European
manufacturers traditionally use no more than 25% to 33% cement
by weight in a base coat formula, but U.S. manufacturers
typically use 50% cement. High cement content makes the base
coat harder but also more brittle and more prone to cracking.
In addition, the highly alkaline chemistry of cement attacks
and weakens the fiberglass mesh, which also promotes
Some U.S. manufacturers have developed lower-cement-content
base coat formulas recently, but so far, testing has not shown
these modified mixes to perform any better than the original
mix. So the best advice we can give at present is to use any
manufacturer's standard wet mix, but not the dry mixes.
Provide a complete secondary weather barrier with
a water-shedding drainage plane and
• Provide sealed movement joints at
• Provide sealed isolation joints around all
• Provide kickout flashings at roof
• Provide pan flashings at windows.
Try to find and use high-density, low-porosity EPS
• Use wet-mix base coats, not dry mixes.
• Use 6-ounce or heavier mesh, non-woven mesh
• Use high-impact mesh on the ground floor and
in traffic areas.
• Use silicone sealants for joints.
Do not break EPS board over sheathing joints. Keep
board joints away from window corners.
• Apply base coat in two layers for adequate
thickness (ideally between 1/16 and 3/32 inch
• Backwrap all panel edges with mesh and base
coat. Do not leave exposed mesh.
• Apply sealant to base coat, not to finish
• Coordinate trades so that flashings and
weather barriers are installed in the proper
sequence and each trade knows its
Sealants. Joint sealing is
an issue in all construction, but especially with EIFS. EIFS
has been promoted as a maintenance-free cladding, and it's true
that the EIFS itself often doesn't need much maintenance. But
sealant joints do need maintenance, and they usually don't get
any. Building owners often don't notice when the sealants fail,
and sometimes they just don't care enough to maintain them
But with EIFS, sealant joints are difficult to maintain even
when someone does want to. To repair a caulk joint, you have to
scrape it out or grind it back, and that can damage the EIFS
So the bottom line is, you need to use a sealant that will
last without a lot of maintenance. You can choose
from sealants made with acrylic, polyurethane, polysulfide, or
silicone. For several reasons, silicone is the best choice.
First of all, silicone has a service life many times longer
than other sealants can expect. It's a stable mix that normally
doesn't break down from ultraviolet rays in sunlight, and it
will tend to retain its properties for decades to come.
Silicone has been on the market for many years, and our company
has rarely if ever seen it break down.
Second, silicone has better properties to begin with. In
particular, it's very flexible, and it stays flexible at cold
temperatures. In very cold weather, the EIFS panels shrink, the
joint has to enlarge, and the sealant has to flex; but urethane
or acrylic sealants typically get hard in cold weather, while
silicones remain flexible.
Silicones, unfortunately, tend to attract dust and dirt, and
most silicone sealants can't be painted. On the other hand, you
can get silicone sealants in colors to match just about any
EIFS color coat or paint. Use 100% silicone sealants, not the
"siliconized acrylic" varieties.
Materials are important, but most EIFS problems are caused by
incorrect installation. The major manufacturers all have
installation specifications, which we are generally comfortable
with. They also offer installation details that over the years
have gotten quite specific and comprehensive. When you do EIFS
work, it's important to study those details carefully and
follow directions conscientiously.
One common flaw is a base coat that is too thin and does not
fully cover the mesh. In one broad survey of 80 buildings in 34
states (conducted in the 1980s), our company found that 92% of
the current projects had base coats that were too thin. On
projects up to three years old, only about 50% had thin base
coats; and on projects between three and eight years old, about
40% had thin base coats. Basically, the more recent projects
more commonly had thin base coats. Apparently, as the market
became more competitive, people had stopped trying to apply the
material according to the instructions. Contractors reduced the
amount of material they applied in order to cut costs.
A base coat that is too thin will not properly resist water
penetration or protect the reinforcing mesh. And if mesh is
exposed to sun and rain, it will lose strength. Specs typically
call for a base coat thickness of 1/16 inch (ours call for 3/32
inch). It's important to enforce the thickness requirement.
However, a base coat that is too thick (5/16 inch or more) has
a tendency to crack because the lamina becomes too stiff.
We recommend applying the base coat in two layers (Figure 3,
above): First apply a thin layer, and then partially embed the
mesh in the wet coat; later apply a second layer over the
partially embedded mesh and trowel it smooth. The second layer
fills in the pin holes in the first layer and covers the mesh
For an installer who is trying to do a good job — a
nice smooth layer of adequate thickness — this two-step
application is just about as fast as trying to get good
coverage with a single layer, and you get a much better
product. It's especially important at transitions and joints,
because exposed mesh and poor coverage at those points will
reduce the performance of sealant joints.
Joints in the EIFS System
Whether they're part of a drainage system or a barrier system,
EIFS walls seldom leak in the middle. Most leaks occur at
joints and transitions. In our extensive studies, more than 97%
of all water damage was found under windows; around decks; at
wall-roof intersections; around penetrations for plumbing,
electric, and the like; or at failed movement joints. Window
leaks alone accounted for more than half the damage. Correctly
installing and sealing all those joints will greatly reduce the
amount of water intrusion.
EPS board joints. EIFS
walls have a large number of naturally occurring joints where
EPS board pieces butt together. Those board joints are covered
with continuous base coat and reinforcing mesh, but they are a
possible cracking point for reinforced laminas. Stresses are
high at window corners, so don't break the EPS board at the
edge of a window — instead, cut the window corner out of
a continuous piece of board. Also, don't break the EPS board
over a joint in the sheathing, and it's best not to break the
sheathing itself at windows or other openings.
Movement joints. EIFS
itself doesn't require many movement joints. Unlike traditional
stucco, which needs a joint every 10 to 15 feet to allow for
thermal expansion and contraction, EIFS can have a jointless
run of 50 or 75 feet, so you seldom need expansion joints on an
However, EIFS does need joints between floors in wood-frame
construction. The EIFS itself doesn't move, but the wall plates
and band joists of wood-frame houses develop significant
cross-grain shrinkage. So an EIFS house should have a joint at
every floor, as shown in Figure 4, to allow the EIFS to move
instead of buckling or cracking.
4. The EPS board in an EIFS system can buckle
when floor framing shrinks, causing cracks in the
surface lamina, unless a movement joint is built into
the system. There should be a 3/4-inch gap between the
sheathing and EPS board, located at or above the
midline of the band joist. A strip of peel-and-stick
membrane should lap over the drainage medium on the
lower floor, and under the drainage medium on the upper
floor. The expansion joint between EPS boards should be
sealed with backer rod and silicone
The other thing we've found is that when two large EIFS
panels connect at a narrow point, the resulting stress
concentration can cause cracking at the narrow section (Figure
5). At such locations, we like to install an expansion joint to
allow wall areas to move independently.
5. Window corners and narrow intersections
between large wall sections are points of stress
concentration where cracking may occur. If possible,
the design should allow for isolation joints to relieve
stress at such locations. In any case, the secondary
weather barrier must provide protection for wall
materials beneath the EIFS.