The Finish Line: What Does the Mesh Do?
February 22, 2008
You’ll love this. I once had this funny story told to me. It’s about the “need” to use reinforcing mesh in EIFS base coats. It goes like this:
In an effort to get his price down in the competitive residential market, a GC added a note to his bid that he would include the EIFS reinforcing mesh at no extra charge. He was not talking about heavier-than-normal mesh, but about using any mesh at all. The implication is that using mesh was an option. If you like working with lawyers, try not putting any mesh in your next EIFS project. There’s a word that describes what will happen: doom.
WHAT MESH DOESReinforcing mesh in EIFS serves several purposes, but its most essential function is not¸ as is commonly believed, to provide impact resistance–although it does that too.
The main function is to prevent cracking at the joints between the insulation board pieces. If you leave the mesh out, you will get cracks at every board joint, and thus have an ugly, leaky wall. The reason the mesh is needed is that insulation, by definition, is at one temperature on one side and at another temperature on the other side. Because the length of a piece of insulation changes with temperature (heat makes it bigger), when the outside of the EIFS is cold and the inside face is warm, the outside surface shrinks and the joint between the board ends tends to open up. Something is needed to resist this tendency to open up, and mesh performs that function-it reinforces the joint. EIFS coatings, alone and without mesh, are neither strong enough nor stretchy enough to do this.
The mesh also serves to help control the shrinkage of the EIFS coatings as they cure, avoiding fine cracks, like lath does in traditional stucco. The practical benefit of the need for mesh in EIFS coatings is that the mesh also provides additional impact resistance. As we can see, mesh serves a number of functions. It’s a critical, non-optional component.
MESH MATERIALSThere are a number of materials that might conceivably work to reinforce an EIFS base coat, but the key factor is cost. Glass is cheap. Sure, using carbon fiber mesh would give a virtually bullet-proof base coat, but at what price? Metal mesh will not stay flat in a thin base coat, and would rust if made of steel. The options are limited.
A good reinforcing mesh material should not be stretchy yet should bend easily around corners-more on this “bending” matter later. Glass mesh is an obvious candidate-just like the glass cloth that is used in fiberglass boat hulls, only more “open” so the base coat adhesive can flow through it to the foam.
There is a basic problem though: most EIFS base coats contain alkali materials that are inherent in the Portland cement in the base coat material. Alkalis will slowly degrade ordinary glass. The glass would eventually lose its strength, and cracking of the EIFS lamina would ensue. There are specialty glasses that are inherently alkali resistant, but they are expensive and their use is pretty much reserved for specialty applications, such as containers to hold acid and alkali solutions in laboratory chemistry glassware. So, ordinary glass-the type used for bottles-is used, and is then coated with a plastic film to protect the glass from the alkali.
COATINGSHere’s an easy way to demonstrate the “alkali attack”: Fill a Mason jar one quarter full of Portland cement and three quarters with water. Seal it, then give it a good shake to make a cement liquid mixture. Let it sit for a week or so, and then pour out the liquid. The discoloration of the wetted area is the alkali etching the glass. The alkali is creating micro cracks in the glass surface, weakening it. The same thing would happen to uncoated EIFS mesh if the base coat contains Portland cement.
There are several ways to apply the coating. One way, which is the more effective and more expensive method, is to coat each mesh strand, and then weave it. The second method is to dip the woven mesh in a bath of liquid plastic coating. The problem with the latter method is that the point where the mesh strands cross each other is uncoated, and this is the area where high forces are generated in the mesh due to the thermal movement of the EIFS lamina. Yet another way to “coat” the mesh is by “weaving” the mesh by laying the mesh strands on top of each other-in other words, not “weaving” them-and using the coating to glue the mesh strands together. This is called non-woven mesh.
There are various types of coatings (different coating chemistries) that will protect the mesh, and these coatings need to be compatible with the base coat and the foam. Some coatings, if improperly formulated, can attack the foam, leading to de-bonding of the base coat.
MESH WEIGHTTo be short, the amount of glass in the mesh determines its strength: more glass = stronger mesh. It’s not quite that simple, as, for example, non-woven mesh of a given weight is stronger than woven mesh. Mesh is specified by weight, and the weight includes the plastic coating.
There is a limit, however, to how much glass you can put into a layer of mesh in a base coat. Thus there is a limit as to how much reinforcing is possible, for impact resistance purposes. The mesh must stay firmly bonded, via the base coat, to the foam. If the mesh is very dense (heavy) the holes between the mesh strands are small, thereby giving limited “locking” of the mesh to the base coat. It’s similar to having too many rebars in a concrete column. The way around this is to add multiple layers of heavy mesh. But this has other implications outlined below.
The type and amount of coating is very important and has been the subject of problems in the field and litigation. The coating needs to be effective in prohibiting the alkali attack, but also chemically compatible with the mesh and foam. Some mesh coatings do not work well with EIFS foam, and can “eat” the foam, as the coating is not chemically compatible with the foam. This can cause the lamina to come loose, and the coating to be rendered ineffective, causing the mesh to weaken and the lamina to crack.
If you are thinking of buying non-OEM inexpensive overseas aftermarket reinforcing mesh, keep in mind that it may void your warranty and may also cause the lamina to crack.
The amount and stiffness of the plastic coating itself can also be varied, and this affects the ability of the mesh to bend. Heavy coatings make it tough to work the mesh around tight curves, such as drip grooves and aesthetic reveals. However, the more coating that is present, the better protected the glass will be.
NONMETALLIC MESHAttempts have been made to use nonmetallic mesh in EIFS. The rationale is that such materials, especially some plastic meshes, can be unaffected by the alkali in the base coat. This is a huge basic advantage. The problem is that the plastic mesh melts at low temperatures, and has trouble passing code-required fire tests. The mesh is what holds the EIFS together in a fire and prevents the fire from spreading. In addition, plastic meshes are not as stiff and strong as glass, and do not reinforce the base coat as well as glass. So, at this point, plastic meshes are rare in EIFS base coats.
BASE COAT COMPOSITIONMost base coats in North America have Portland cement in them. This means the alkali attack issue is a key technical matter. If you use a noncementitious base coat, this problem sort-of goes away-but not completely. For example, in Europe where the cement/resin ratio is often 80/20 (in the USA it is often 50/50) the alkali problem is less, and leads to a longer lasting EIFS lamina. The increased resin level, however, increases resistance to water penetration, adhesion, and flexibility.
It’s been said a million times before, but bears repeating: Mesh is not a screed. But, the mesh does need to be fully encapsulated by the base coat adhesive. This means, in practical terms, that the pattern of the mesh should not be apparent to the touch, even though the pattern may be visible. It is possible for the mesh pattern to be visible even though the surface of the base coat is smooth and the mesh is fully embedded. This “pattern showing” phenomenon is caused by different drying rates of the base coat at the mesh strands versus at the mesh openings. This “full embedment issue” is why some EIFS producers and contractors go the extra mile and insist on a skim coat over the original base coat, to be positive the mesh is well-embedded. This also means that a layer of base coat needs to be applied first, and then the mesh embedded into it-do not push the base coat adhesive through the mesh. Then, whatever extra base coat material is needed on top of the mesh, to fully embed the mesh, needs to be applied.
DETAIL MESHDetail mesh is the term used to describe lightweight mesh used for forming corners, terminations, and joints. This mesh needs to be extra flexible so that it can be bent around tight corners. To allow this, detail meshes use lightweight mesh and a thin coating. This lighter coating means the mesh is less protected from alkali attack, but does make the mesh usable in tight quarters. Keep this in mind when you see cracks at the bottoms of drip grooves, aesthetic reveals, and tight corners. Pre-bent mesh “angle” pieces of heavy mesh are available for outside corners where heavy mesh is needed for impact resistance.
When leaks occur in an EIFS building, some people think the EIFS lamina itself is leaking. This is very rarely true. Even the thinnest EIFS lamina sheds water. But the use of multi-layer base coats does increase water penetration resistance. This resistance refers to liquid water. It also has the reverse effect of not allowing water vapor to easily escape. EIFS walls with thick base coats “breathe” less easily, and thus are slower to dry if water gets into the wall. So when specifying multi-layer base coats, it’s a good idea to talk with the EIFS producer and see if adding extra base coat layers affects the breathability of the wall. The newer EIFS with Drainage gets around this issue as the water vapor can dissipate through the drainage cavity.