As background, we all know that the reinforcing mesh used in EIFS is made of glass. The type of glass is the ordinary type used for drinking glasses. We also all know that many EIFS basecoats use Portland cement. Portland cement is alkali (caustic). Caustic materials can etch glass. The etching causes the surface of the glass to have microscopic fissures. These fissures act as a point at which a crack can begin. Being a brittle material, cracks in glass propagate easily. In the case of EIFS, this means that the mesh loses its strength and the EIFS lamina may crack. Cracks can let water in, along with lawyers.

A plastic coating is put on EIFS reinforcing mesh to stop the alkali from getting to the glass. The alkali is transported in a liquid solution by the dampness of the adhesive. Thus, the presence of the coating causes the mesh, but not the glass itself, to become alkali resistant and thus it retains its strength. Often the plastic coating, which is naturally "clear" in color, is tinted to show its presence and identify the meshes source: blue for Dryvit, yellow for Sto and so on. The fact that there may be no "color" to the mesh does not mean that there is no coating; some coatings are clear or white.

Plastics

To help make the mesh more flexible, a softer grade of protective coating can be used. The softness of the coating is determined by many factors, one of which is the presence of chemicals called plasticizers. Plasticizers do what the name says, they make plastic more "plastic." In other words, more bendable.

Herein lies the problem: Some plasticizers do not want to "stay put" within the plastic coating. They want to slowly migrate around within the plastic coating film. This would not be bad except that some types of plasticizers can attack EPS foam. They can literally melt it. Although there is not much plasticizer present, it can be enough to unbond the basecoat from the EPS insulation. The result is a loose lamina, which often presents itself as bulge or blister.

Higher temperatures make this plasticizer migration problem worse, because increased temperature speed up the migration chemical reaction. Hence, if one wants to guess where this problem will appear first, try looking on the south or west side of an EIFS building that has a dark color finish. This will be the area with the warmest lamina.

If it's suspected that the blisters that have appeared are due to mesh plasticizer migration, another way to get a clue is to stand back and look at the wall. Often, there are bands of debonded lamina at regular intervals. The location often coincides with that of laps between the mesh pieces. The presence of the lap indicates a doubling of the thickness of the mesh and hence twice the plasticizer in that area. Hence, mesh lap areas tend to fail first.

One of the ways to tell if blisters are caused by mesh plasticizer is to peel back a section of the blistered lamina, and look at the EPS. If the mesh plasticizer is the problem, you may see a waffle-like pattern on the foam with slight indentations at the area where the mesh strands contact the foam. In fact, one quick way to see if a roll of mesh has any tendency to give off plasticizers that are antagonistic to EPS is to take two pieces of EPS and place one on each side of a piece of the mesh. Lay the sandwich on a horizontal surface and put a phone book on top (to press them together). Take this outside on a hot sunny day and let it sit for a few days. If the plasticizer is migrating, one can see the mesh "sticking" to the foam when the sample is disassembled.

Blister

Fixing an EIFS with a lamina that is debonded due to mesh plasticizer migration is full of surprises. Here are some examples:

Since the original lamina is loose, there's no point to putting another lamina on top, since the original lamina would still be unbonded.

One can peel the original lamina off and then install a new one directly onto the original foam. This works some of the time but peeling off the lamina often results in ruining the surface of the PS. The EPS has lumps and cavities, making it unsuitable for applying a new lamina, unless the EPS can be sanded.

If the EPS is mechanically attached, one will have lots of fun sanding it.

One solution is to leave the whole original EIFS in place and to attached metal lath through the original EIFS and then apply a whole new EIFS onto the lath. This solves the problem of removing the original EIFS.

One of the side stories of the mesh migration issue is this: It points, once again, to the need to have a thick basecoat. The extra basecoat adhesive helps keep the mesh away from the foam. It also points out why the basecoat adhesive is applied first to the foam and then the mesh is embedded into the wet adhesive. In other words, if you press the adhesive through the mesh, the mesh will be in direct contact with the foam.

Another sideline to the glass alkali issue is the benefit of using low cement or noncementitious basecoat adhesives. Since there less (or no) Portland cement in such adhesives, this whole issue because less critical. In Europe, where EIFS was invented, low cement basecoats are much more common, as is heavily coated mesh. The combination make for an overall much more durable basecoat.

By the way, this "foam melting" phenomenon does not occur with polyisocyanurate foam, as the basic chemical structure of poly-iso is not sensitive to the types of chemicals of which plasticizers are made.

One should be aware that dark colors alone, under certain conditions, could also cause debonding of the lamina from EPS. In other words, the manifestation of the problem is the same, i.e., blisters, but the cause is different. It is possible, under bright sun conditions and with dark EIFS finish colors, for the lamina to get hot enough to melt the EPS directly beneath it. This causes the lamina to be loose and to appear as waviness or blisters.

Another form of behavior that is related is the disappearance of the coating. Actually, the coating essentially stops protecting the mesh, leaving it wide open to alkali attack. When this occurs, it usually results in small, closely spaced and evenly spaced vertical and horizontal cracks throughout large areas of the lamina. The cracks tend to be worse at the insulation board edges, where the stresses are greatest.

The mesh plasticizer migration matter is a weird phenomenon. It will probably never be seen. Properly designed coating for EIFS reinforcing meshes do not exhibit this behavior, so there's no need to get worried.

Lastly, it's no secret that one can buy EIFS reinforcing mesh on the open market. This generic mesh is not part of any EIFS manufacturer's product per se, and thus the issue of its compatibility with other EIFS materials, such as foam and adhesives, is brought into question. One of the advantages of buying a proven EIFS product is that unusual, technical issues, like mesh plasticizer migration, are looked-into a part of the design of the entire EIFS system. You know what you are getting. Or at least if something does happen, you have a system manufacturer to talk to.