There is embedded trim and then there is flashing. What's the difference? Flashing is not bonded into the EIFS coatings but may be in loose contact with the coatings. Embedded trim is bonded into the EIFS coatings. Flashings are separate external-wall elements while trim becomes an integral part of the EIFS. Because embedded trim is intimately connected to the EIFS, it needs to be compatible with the properties of the EIFS. One of the issues that sometimes causes problems when using embedded trim with EIFS is that EIFS producers do not make trim. Some producers sell trim as part of their EIFS materials package but more often than not trim is bought locally from building supply distributors as a separate item, such as caulking. Some producers are picky about which specific trim design they want used in the EIFS systems, while other let you choose your brand. Given these variations on how trim is used with EIFS, it is not surprising that people in this industry have various opinions about using trim with EIFS. Here are mine, written as a set of "rules."

Basecoat thickness

Many types of trim become an integral part of the basecoat; once the basecoat has dried, the trim is "in there" for good. This integrating of the trim into the EIFS coatings occurs both by bonding of the adhesive to the trim and by the mechanical gripping of the adhesive through holes or undulations in the trim. In typical PB EIFS in North America, the basecoat is thin-sometimes as thin as the trim itself. A thick basecoat, as used in Europe, where the use of trim is much more common, gives more "guts" to the basecoat and helps the trim stay put. In Europe, trims are available in materials not used in North America, such as composites (fiberglass) and stainless steel. Rule #1: To properly bond the trim into the basecoat, the basecoat needs to be several times thicker than the overall thickness of the EIFS reinforcing mesh, at the perimeter of the EIFS-where the trim is located.

Resin content

Since keeping the trim in place and avoiding cracks relies at least in part on the adhesion of the EIFS to the trim itself, promoting good adhesion is helpful. Rule #2: A resin-rich basecoat adhesive works better than one that has a higher Portland cement level, simply because resin-rich adhesives stick better and are more flexible.

Thermal expansion

The term "thermal expansion" has to do with the change in size of a material as a result of its temperature. Keep in mind that most materials also change their dimension as a result of other factors, such as moisture content and stresses. Almost all materials get bigger as they get hotter. Trim (metal or plastic), EIFS foam insulation, and EIFS basecoat materials (adhesive and mesh) all have different rates of thermal expansion. So do trim materials.

Check out the list below. Note the wide range for these common materials. The point is that to help the trim stay in place and to avoids cracks, it helps if the trim has an expansion coefficient near that of the materials it is in contact with, such as the basecoat adhesive. Note that these numbers are a function of the material's temperature, which is related to heat in the sense that the temperature of the EIFS and trim depends on the air temperature, color and other factors. Rule #3: Pick a trim material that has thermal properties similar to those of the adjacent EIFS materials. EIFS producers can give you the thermal expansion coefficient values for their products.

Brick 3.1

Oak 3.0

Glass (plate) 5.0

Cement 6.0

Steel 7.3

Concrete 8.0

Copper 9.3

Aluminum 12.3

Zinc 16.5

PVC plastic 24

NOTE: All above numbers are multiplied by 10-6. For example, aluminum moves 0.0000123 per inch per degree Fahrenheit (i.e., 12.3 x 10-6).

Foam thickness

Many trim designs wrap around the edge of the foam. This means that different trim pieces are required for different foam thicknesses. It may also mean that the trim may have to be "trimmed" when the foam thickness changes, such as at an aesthetic reveals or at a foam shape. This "trimming process" can be tedious, if not impossible. There are some novel trim designs that can be used on a range of foam thicknesses, such as the opening image of this column, offered by Wind-Lock. This design does not wrap all the way around the edge of the foam in the normal "J" bead fashion but rather only comes to the end of the foam piece, at which point the basecoat is brought around from the face of the foam and bonded to the trim. Rule #4: If you want to use trim, try to limit the number of foam thicknesses on the job so that you don't have to deal with a multitude of trim profiles.


Extreme climate can affect trim. Large temperature variations and extremes of sunlight (sunny climates and high altitudes), salt laden air (seaside), and caustic air (desert) can deteriorate certain types of trim. For example, extreme cold can make some plastic trims brittle, and hence allow cracking. Harsh oceanfront exposure can eat away at some metal trim materials. Huge temperature swings exacerbate the thermal expansion issue. Rule #5: Match the trim material to the job site conditions.

Sealant adhesion

If using trim at sealant-type expansion joints and the sealant is supposed to adhere directly to the trim, make sure it will. Sometimes, a primer and/or a particular type of sealant is needed to get good adhesion. Rule #6: When using trim as part of a sealant joint, make sure the sealant is compatible. EIFS producers and trim producers can tell you what sealants will stick to their materials.

Window heads

One of the best uses for trim is at the bottom of walls and at window heads, especially in conjunction with EIFS with drainage. Trim can provide a way of catching and routing to the outside whatever water might be flowing down the drainage plane within the wall assembly, while also providing an attractive finished edge.

On commercial buildings, make sure the type of trim used at window heads can withstand a fire. Plastic melts. So does aluminum at building fire temperatures. The code-required fire testing that allows EIFS to be used on noncombustible (commercial) buildings uses a wall mockup where the fire exits through a window opening, and the trim at the window head needs to have a lot of guts to withstand the heat. Rule #7: When using EIFS on noncombustible walls, make sure the type of trim you want to use is code-approved. Steel and normal backwrapping can withstand building fire conditions at window heads, at least for code testing purposes.

EIFS as proprietary systems

EIFS producers vary considerably in their views on using trim in their systems. Some require it with some systems, some won't allow it at all and some say "it depends." This is, I suspect, due to reservations about being sure it will work. In other words, their concerns relate to product performance, which in turn affects warranties.. So it's not just an "I'll decide if I want to use trim" issue on the part of specifiers and contractors. Remember, EIFS are proprietary systems. Rule #8: Check with the EIFS producer and discuss their views about using trim on a specific EIFS product and your project.

Use trim?

As for my work in assisting clients in using EIFS on specific buildings, I look at each job individually. I am especially concerned about what the EIFS producer thinks and about the local EIFS contractor's experience with using trim. The local climate and the building's design are also factors. I would love to see some more research into this aspect of EIFS, as it has the potential for addressing some of the thorny-wall assembly issues that are plaguing the wall products industry in general, such as providing a more sophisticated termination of the EIFS that could help reduce water intrusion. Trim can also make for a nice looking wall that can be easy to construct by eliminating backwrapping.

You can see from this basic discussion about trim that it is one of those aspects about EIFS that is not standardized. Unlike insulation board patterns and mesh lapping and dozens of other EIFS techniques that are common to almost all brands and styles of EIFS, trim is one of those brand and project-specific aspects that need attention on a project-by-project basis.

In my experience, embedded trim seems to work best in moderate climates, with light finish colors, on flat walls that have a minimum of changes in foam thickness and few joints, reveals or foam shapes. Trim seems best suited at the bottom edge of walls, and at window heads, especially EIFS with drainage systems. A thick, resinous basecoat is also helpful in keeping the trim firmly attached to the wall.