First, it needs to be said that an AR is cut into or out of the foam layer. In other words, they are a subtractive process. This is in contrast to foam shapes, which are an add-on process. Thus, the removal of material from the EIFS foam layer to create an AR causes a change or a discontinuity in the otherwise flat main areas of a wall. This basic fact has a number of implications.
Obviously, if the AR is large, it reduces the energy efficiency at the part of the wall, as there is less insulation at that location. This slight increase in heat flow through the wall at the AR is normally not important. The real issue is that the AR creates a structural weak spot in the EIFS. This occurs because the EIFS lamina is under constant stress due to thermal and physical movement, and stresses get concentrated at the AR because the forces within the EIFS must change direction at that point. This is why you'll sometimes see cracks at ARs but not nearby where there are not any ARs.
The shape of thingsThe shape affects the crack-proneness of the AR. V-shaped grooves and grooves with sharp, square edges don't handle the flow of stress, as well as grooves with rounded contours. The problem with rounded contours is that they don't look as clean and sharp as hard-edged ones.
Square-shaped horizontal ARs have the problem that the lower edge acts like a ledge, holding airborne dirt and perpetuating staining. They also can serve as a holding area for microbes and nutrients that can lead to mold and mildew. Putting a slope on the lower edge of horizontal ARs helps the groove drain better and thus helps reduce staining.
When installing an AR, it's a good idea to use one of those special trowels designed for installing ARs, like those sold by the Wind-Lock. Not only do they give a more uniform and easier-to-install groove but they also do not damage the mesh. The issue of mesh damage is important, as stuffing the mesh into the groove with a sharp-edged metal trowel can remove the plastic coating on the mesh.
The plastic coating is present on the mesh to protect it from the alkali of the Portland cement that is present in many basecoat adhesives. Without the presence of the plastic coating, the alkali would weaken the mesh. Areas with weak mesh are more prone to cracking. This is why you sometimes see cracks at the bottom of AR grooves. V-shaped grooves are especially susceptible to mesh weakening because they act like a bellows when the lamina expands and contracts. The movement is concentrated at the bottom of the groove, right where the trowel pushed hard against the mesh. The moral: take it easy on the mesh when installing it into the AR groove.
If you are using mechanical fasteners to attach the foam board, watch out where you put them-you don't want them to line-up with AR grooves. A router (and other techniques, see below) is often used to make the AR groove after the foam layer is attached. These groove-producing techniques will have trouble going through the fastener.
There are a couple of ways to create ARs. The first thing to recognize is that you want to create the AR groove in the foam on the wall after the foam is in place. It's really difficult to pre-cut ARs in foam boards off the wall and then try to get them to line up.
Routers can be used to cut small ARs but be careful of all the foam beads break loose during the routing process. The beads tend to get blown into the next county-use some type of system to contain the beads before they get blown away.
Sometimes, it's hard to find a router bit of the right shape, or is big enough, for the AR that is required. Thus it's easier to use a portable hot wire cutter with a blade of the correct shape. Hot wire cutters also make clean, smooth-sided cuts and do not cause the loose EPS bead problem.
For small, simple grooves, some people use a very sharp, thin, long-bladed matt knife and a straight edge to cut out slices of foam.
When the drawings call for, say, a 3/4-inch wide AR, remember that this is the final width of the groove, with the lamina installed. This means that you need to make the AR wider than 3/4 inch to accommodate the thickness of the EIFS lamina.
Aesthetic reveals are normally done using lightweight grades of reinforcing mesh. Forget about trying to bend heavyweight mesh into a narrow AR groove. Usually, the heavy mesh starts and stops at the AR and only the lightweight mesh goes into the AR groove.
An AR should not line up with the joints that occur where the foam boards abut. If they do line up, the chance of getting a crack at the AR is much greater.
It's a good idea to try to layout the wall so that mesh laps do not occur within the AR. If they do, it can be difficult to get all the mesh into place. Better yet, at the AR, use a narrow strip of mesh that runs the entire length of the AR, rather than trying to work a whole roll-width of mesh into an AR joint.
Aesthetic reveals are not a substitute for expansion joints that go all the way through the EIFS and that use caulking to seal the joint. Expansion joints are located where significant movement occurs, and are intended to allow for that movement without leaking, by using a flexible seal (caulking) between two areas of EIFS. Aesthetic reveals are not a type of joint that accommodates movement. If you try to substitute AR for a caulking joint, it'll almost surely crack.
Replacing a cracked AR with a real expansion joint is a lot of work, as a strip of EIFS must be removed at the joint area so the EIFS can be backwrapped. The good news is that it is often possible to repair cracked ARs by stuffing a small backer rod into the AR and then caulking over it.
Line upSometimes, ARs line up with the corners of openings, such as at windows. This can look nice but it makes it difficult to install the diagonal corner reinforcing mesh that should be used at all openings. Because the location of ARs near the opening can affect both the aesthetics and performance of the wall, this issue needs to be resolved by the wall designer before construction starts.
Remember that the minimum thickness of the foam at the bottom of the AR groove is, for most EIFS manufacturer's products, 3/4 inch and more is better. This requirement exists to avoid cracking of the lamina at the base of the groove. This can have major implications in the design of EIFS walls: It means that if you want to have a 3/4 inch deep AR groove, you'll need at least 11⁄2-inch foam on the rest of the wall. If you don't want to use all that extra foam on the entire wall, try this (make a mockup first to see if it looks OK): Make a shallower AR groove (which allows thinner foam on the rest of the wall) and then paint the groove a slightly darker color than the adjacent main wall area. This makes the groove look bigger/deeper than it really is.
It's important to realize that if cracking does occur in an AR (and especially at a horizontal AR), that water traveling down the wall tends to "hang up" at the AR, simply because the AR is an offset in the wall surface. If the water travels back into the AR and there is a crack in the AR at that point, than the water is actually channeled toward the crack, thus amplifying the potential for leaks.
Sometimes, the groove of an AR (especially a wide AR) is a color that contrasts with the main color of the wall. It is often easier to apply the same color finish to the whole wall, including the groove and then to paint over the groove using the contrasting paint color. This approach is often simpler than trying to apply a special color of EIFS finish to a small AR area.
Be careful where ARs go up under a flashing, such as at a windowsill or a parapet. Water can get blown up under the flashing at this point because the flashing isn't tight against the AR groove. One way to solve this is to caulk shut the AR groove at the flashing. Another way is to stop the AR right where it meets the flashing. In other words, by not running the AR to the very top of the wall.
From a design standpoint, I see ARs-along with foam shapes-as one of EIFS' greatest strengths. As an architectural consultant, I see these two features of EIFS as a way of producing inexpensive but good looking "architecture"-buildings that look good but don't break the budget. It's reassuring to know that I am not alone in this regard. When I drive around town and eyeball the buildings in the area, I am constantly amazed at how much EIFS there is. Especially the number of nice looking buildings that use ARs as a design feature is just amazing. It's a technique of using EIFS that I would really push if I were in sales or contracting.
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.
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