EIFS reinforcing mesh is made of glass. Why glass? Glass is used because it does not stretch much. The technical term is that it has a “high modulus.” The modulus of a material is how much it stretches in relation to the “pull” applied to it. A related characteristic that applies to mesh is brittleness. Being brittle means that when a material does break, it does so all of a sudden. For example, a piece of soft steel will stretch and eventually break but its does not “snap.” Glass, on the other hand, breaks instantaneously, and with little warning: it shatters. If you ponder why beer bottles smash to bits when you tip one over, that’s why. Compare a brittle, glass beer bottle to one of those elastic, plastic two-liter cola bottles and you’ll see the difference. The fact that a material is brittle is OK, as long as you keep that characteristic in mind when you’re using it.
The other reason glass is used is that it is very strong. This may sound crazy but glass is much stronger than steel. The problem is that this extreme strength cannot often be used because of its brittleness. Thus, the actual working strength is reduced many times, to make sure that it never breaks.
Why is a low stretch material used in EIFS? It’s simple: to keep the edges of the EIFS insulation boards together. Think of it this way: If the mesh were stretchy, the EIFS insulation board joints would open and close as the foam gets hot and cold, due to normal swings in outdoor air temperature. This opening and closing of the joints is not good for the mesh, as it tends to wear out the mesh by it constantly being pulled-on. Try this experiment to see what I mean (I use this in EIFS seminars):
Go get a husky rubber band. Hold it by the ends. Pull on it. It stretches, right? Try stretching it 1 inch and notice how much force it takes to stretch it that much. Now, hold the rubber band 1/4 of the way from each end, and pull on it. It takes more force for it to stretch 1 inch, right? Now, hold the rubber band in the middle, with your thumb and index finger touching each other. Try to stretch it now. It won’t budge. Why? Because there is no “rubber band” between your fingers to absorb the force you’re applying to it. If you pull enough, you can break the rubber band.
EIFS insulation boardThe same thing happens, on a smaller scale, between the edges of EIFS insulation boards. The trick is that glass hardly stretches at all, because the EIFS insulation board edges are abutted tightly together. So, the EIFS insulation board edges remain together and the EIFS lamina does not crack.
Now, what happens if there is a gap between the ends of the EIFS insulation boards? This situation is a bit akin to holding the rubber band 1/4 of the way from each end. This constant “working” of the glass tends to wear it out and can lead to the glass rupturing. This basic type of behavior, in engineering terms, is sometimes called “fatigue,” just like athletes who get “worn out” by simply playing too hard. Once the glass ruptures, there’s nothing to hold the lamina together, and a crack appears. This can lead to appearance problems, water entry, and getting to know your lawyer better.
The above description explains why glass is used in EIFS reinforcing mesh: it’s stiff (as opposed to stretchy) and strong. But there’s a hidden problem with glass. It has to do with EIFS basecoat and the fact that many basecoats use Portland cement.
Try another experiment. Go find a drinking glass that is made of glass. Make sure it’s one that is OK to throw out. Put some tap water in it and a handful of Portland cement. Give it a good mixing and put it away for a week. After the week is up, pour out the remaining liquid. Take a look at the glass. It’s turned gray where the water was. The alkali in the cement has etched the glass. Feel the etched area. It’s slightly grainier than the smooth, unetched glass areas.
The same thing happens if uncoated glass is used in EIFS reinforcing mesh. The reinforcing mesh is etched with zillions of microscopic fissures. This weakens the mesh in a manner not unlike scribing a sheet of window glass with one of those hand held glass-cutting wheels. Glass, being a brittle material, has “notch sensitivity,” which means that small nicks in the surface can propagate easily, like other brittle materials, such as stucco. Once weakened by being etched, its strength and longevity are reduced.
You may ask: Where does the alkali come from? Well, it’s in the Portland cement by nature, but becomes part of a liquid solution due to the pressure of water near the lamina, such as humidity or rain. Hence, the alkali can then migrate around within the basecoat and get right up inside the mesh.
There are various types of glass. The type of glass used in EIFS mesh is ordinary glass, like that used to make bottles. There is a type of glass, called alkali resistant, that it immune to this alkali problem. This type of glass is used for special glass applications where extreme resistance to chemicals is important. A common example is test tubes used in laboratories that must be capable of withstanding all sorts of harsh chemicals without weakening. The problem with AR glass is that is costs a fortune, and thus is not viable, price-wise, for making reinforcing mesh for EIFS.
As an interesting point of fact, you can nick the glass in EIFS reinforcing mesh by using the edge of your trowel to press the mesh into an aesthetic reveal. The sharp edge of the trowel cuts into the mesh, nicking the surface. This is one of the reasons why cracks sometimes occur at the base of aesthetic reveals: the mesh has been weakened during the application process. The moral: Use a trowel with a contoured plastic insert to press the mesh into the groove. It won’t nick the mesh and will create a more uniform looking groove, too.
Plastic meshSo what about using plastic mesh? Perhaps you have seen this on the market. Plastic mesh can work and has the highly desirable attribute of being unaffected by alkali. That’s the good news. The bad news is that it is stretchy. However, this property can be worked around in the design of an EIFS but there’s another issue that is hard to handle. The problem is that plastic weakens substantially in a fire, while glass does not. This lack of fire resistance is a key issue as it’s the glass that holds together the EIFS lamina in a building fire, and thus protects the EIFS foam. There’s no way around this high temperature sensitivity of plastic, so the use of plastic mesh in EIFS has, so far, been limited in a few residential types EIFS applications, where combustibility is not an issue.
Most EIFS reinforcing mesh is made by dipping mesh that has already been woven, in a bath of plastic resin. The resin keeps the alkali away from the glass. The resin is tinted to whatever color the buyer wants. Keep in mind that just because a mesh has no color, that it does not mean it is uncoated. Some coatings are clear, and let the natural color of the glass, off-white, show through. It’s also possible to buy mesh where each mesh strand is coated with a plastic resin before it is woven. This makes for a stronger mesh but is more expensive.
In terms of using EIFS on buildings, the above discussion of the mechanics of EIFS reinforcing mesh applies to using mesh on EIFS, and on non-EIFS applications. For instance, sometimes an EIFS wall comes up to a non-EIFS wall. The EIFS basecoat is brought over past the end of the EIFS wall, and onto the adjacent wall, which might be concrete or block. Cracks can occur at the interface between the EIFS and the look-alike EIFS coating because the two adjacent materials, over which the EIFS is supposed to bridge the gap, move at a different rate, and thus impose high forces on the mesh where the two materials meet. How to solve this problem? Put in a caulking joint, or, if you’re a risk taker, put some extra reinforcing mesh at that location to make it stronger. No guarantees on the latter, but you might get lucky.
I’ve heard the comment that one only really needs the mesh at the insulation board joints, as this is where the only forces that need to be resisted are found. This is not true. One needs to look at the EIFS on a broader basis.
The EIFS basecoat, especially if it uses Portland cement, shrinks a bit when it cures. To make this point, notice how the mesh doesn’t show when put on in the afternoon, but next morning it does. That’s the shrinkage at work. Thus, the mesh serves to control the shrinkage of the adhesive-mesh matrix, which, in turn, helps reduce cracking of the basecoat.
Although meshing only at the insulation board joints works for solving the board joint expansion/contraction problem, it does not solve the more basic problem of the EIFS lamina not being particularly strong without the mesh. If you leave out the mesh in the field of the EIFS insulation board, the impact resistance is almost nonexistent. This is not viable on any real wall, as you could literally poke your finger through it.
Thus, the reinforcing mesh in an EIFS serves many purposes. Simply follow the normal ways in which it is supposed to be installed, and it’ll be fine. In the competitive EIFS market, people seem to be always looking for a price advantage. This includes removing things from the EIFS, to lower the price. Well, I can assure you, the modern types of EIFS used in North America has been honed to their most basic viable, composition, and everything that must be in it, such as the reinforcing mesh, is required in order for the EIFS to function properly. Now, if someone could find a way to put micro fibers in the EIFS finish, so that an entire, thin, flexible and strong EIFS lamina could be put on in one pass, then we’d really have something.