Below is a list of things that were on our "wish list." It's a pretty ambitious roster but if there were such a material, it surely would solve a lot of substrate-related problems and would make for much better EIFS wall assemblies.
Wish listMaterial cost. Obviously, the sheathing must be affordable in the range of what is reasonable in the world of EIFS-types of wall constructions. There are lots of very durable, strong substrates but most of them cost several times that of popular substrates such as those based on gypsum materials. This is probably the single biggest factor, as the EIFS wall market is price sensitive, as opposed to high-end wall systems, such as real stone and custom curtainwall, which sell for many times the price of EIFS, and for which specialty sheathings that cost many times that of common EIFS sheathing substrates is not a big factor in the overall price of the wall.
Installation labor cost. Some strong and durable substrates are, to be blunt, a pain to work with. Some must be sawed rather that scribed and snapped. Some substrates are also quite heavy, making handling them an ordeal. This raises labor costs and is a turn-off to contractors.
Durability. One of the main reasons we undertook the sheathing substrate project was that a lot of the then-popular substrates simply did not hold up well when left exposed on the wall or in storage. They also didn't fare well if water got into the wall. This obviously could lead to major performance problems, such as the adhesive letting go from the surface of the substrate. There are many substrates that are inherently water resistant, notably those with mineral or Portland cement cores but these tend to be heavy, expensive and hard to handle. Recent advances in gypsum-based sheathings, with treated cores and non-paper facings, has done a lot to address this problem and is one of the most important changes to EIFS walls in decades.
Fire resistance. Although on wood-frame construction you can use combustible (wood-based) sheathings and meet the codes, in commercial buildings the substrate must be noncombustible. This means the sheathing must be fundamentally inorganic in composition, as opposed to organic (wood and/or paper-based). Also, the sheathing must be capable of being used in fire-resistance-rated wall assemblies (one-hour, two-hour, etc.), hopefully as a direct replacement for gypsum products in the dozens of already tested fire-rated assemblies.
Attachment. The sheathing must be capable of being attached without fuss. This means using nails, and especially drywall-type screws. Some high-performance sheathings are rigid and brittle, and require pre-drilling of fastener holes, a major labor factor. Because of their brittleness, such sheathings also have what is sometimes called "edge distance" problems, meaning that the attachment fasteners have to be located a large distance from the edge, lest the sheathing crack when the fastener is driven home.
Weight. Some of the mineral- and Portland cement-based sheathing require several people to lift, thus slowing production. Sometimes, the overall weight can be a factor of the EIFS wall assembly, although EIFS itself is so light that this extra weight does not usually amount to much in terms of the extra loads it puts on the supporting wall structure.
Sheet size. The sheathing should come in sizes that work well with standard spacing used in the North America market, such as stud spacings. There are lots of nice sheathing from overseas but they come in odd, metric sizes, which means a tremendous amount of cutting and waste.
Availability. There are some killer substrates available from outside of North America but that does no good if the average designer or contractor cannot specify or purchase them. Since construction in the U.S. is on a vast quantity basis and spread over an enormous geographic area, clearly the substrate would have to be well distributed in order to be truly useful on a majority of projects.
Bending. The "ideal substrate" should be bendable to some extent. Normally this is not much of an issue but some currently popular substrates can be warped, and this would be a helpful characteristic when doing the occasional curved wall. Many of the mineral- and Portland-cement-based sheathings cannot be bent at all, while some of the wood-based types are good in this regard.
Strength. Clearly, the substrate must be strong enough (bending strength) to resist wind loads-including high loads on tall commercial buildings. It would also be nice if the sheathing had a lot of shear strength, which would enable it to be used for bracing.
Adhesive compatibility. Obviously, the sheathing would have to be compatible with the attachment adhesives used in EIFS so that the system could be bonded to the substrate using adhesives.
Water resistance and water barrier. The sheathing should be stable enough that if water gets into the wall, it does not fall apart. A perfect substrate would be able to act as the water barrier on its own, thus eliminating the need for a separate water barrier. Dream on.
Screw-ability. A perfect substrate would allow normal, threaded EIFS mechanical fasteners to be screwed directly into it, as is common in residential EIFS applications, where the EIFS is mechanically attached to OSB and plywood. The current crop of gypsum-based sheathings fall short in this area and a lot of the hard, brittle, mineral and Portland cement-based sheathing doesn't work well in the sense either.
Stability. The sheathing itself needs to be stable. This means it should not swell too much as its moisture content changes, nor should it move a lot when its temperature goes up and down. Gypsum-based sheathings are good in this regard. There are some interesting wood- and mineral-based sheathing around but they tend to be unstable enough that the induce stresses into the EIFS, which can lead to cracking.
The ideal substrate
The above list is not all-inclusive: There are lots of other characteristics that would have to be addressed for the "ideal" sheathing. But it's obvious when reading the above litany, that there's no such thing as an ideal EIFS sheathing substrate. It's also apparent why the current gypsum-based sheathing with treated cores and non-paper facing are popular: They meet most of the criteria above.
As a person who regularly works with clients who have an interest in "the ideal sheathing substrate," I will predict that the next generation of sheathing will be composite in nature and use mostly natural, inorganic materials for bulk, and will use man-made (synthetic), high-strength materials for strength and durability.
The first person who can come up with a cheap, durable, readily available, etc., sheathing is going to get the Nobel Prize. In the end, "the ideal sheathing substrate" project that I was involved with for my employer turned out to be an academic exercise, since no such material exists. However, it did have value in the sense that it acted as a checklist of things to look for when sheathing producers approached us with what they thought was an ideal substrate.
Luckily, the ideal sheathing problem that did exist a decade or so ago is largely gone and the EIFS now has more pressing issues to deal with, such as quality control, inspections, insurance and image.