In the early days of the North American EIFS industry there was a relatively meager selection of non-solid substrates (solid substrates being concrete, masonry, etc.). In other words, good sheathing board substrates were hard to find. The selection was basically OSB, plywood and Exterior Grade Gypsum Sheathing (EGGS), meeting the ASTM C79 specification, and sometimes called “brown board.” When it came to making prefab panels and commercial (noncombustible) construction, EGGS was basically all there was for several decades. Although not intended to be used as a substrate for adhered claddings, thousands of projects, including some huge ones, were built and are successfully performing with EGGS as the substrate.
The basic issue with EGGS was the need to keep it dry. This required careful scheduling regarding weather, careful detailing and good craftsmanship. While I was working for an EIFS producer, we were anxious to locate a higher-performance sheathing, especially for tall buildings. So we undertook what came to be known as The Ideal Substrate project. We scoured the globe and located many good sheathings, but all seemed to have a least one or more deal-breaker problems. What follows is a list of some of the criteria we were looking for. If you are considering using a high performance sheathing as a substrate for EIFS, these are some of the things to keep in mind.
Cost: This was the biggest single factor in determining better substrates. EIFS users were conditioned to certain pricing, and some of the really nice sheathings from Europe were quite expensive. After adding the cost of shipping, these got way out of line. Unless one found a good, cheap sheathing overseas from a country with very low labor rates and inexpensive raw materials (not Europe), it was hard to compete with products made in North America.
Availability: Some of the imported sheathings were priced right and worked well, but had no distribution in North America. This made them essentially nonexistent from a practical standpoint.
Weight: In doing field trials of various types of mineral sheathing, there was a lot of griping by workers about the weight of some of the sheets. Back pains and the need for extra gear to position the sheathing panels made workers reluctant to use heavy sheathings.
Not Brittle: Peanut brittle is called “brittle” because it snaps suddenly. Some of the mineral sheathings cracked easily because they were brittle and stiff. This was an especially big problem at the edges. This could even happen when just moving them around. Some even required pre-drilling of holes, which raised labor costs.
Easy To Cut: The ability to use a matt knife and simple saws to trim and make holes was a big advantage for gypsum-based products. There was also the problem with mineral boards of having to constantly saw them (rather than score-and-snap), and the issue of all the dust that is created when sawing mineral boards.
Leaving it Exposed: This was a critical factor. Even some of the wood/cement boards did not hold up well when left to the weather. Some absorbed too much water and had problems during freeze-thaw exposure.
Dimensional Stability: Some of the mineral boards were not stable, as they moved too much when subjected to temperature changes, and when subjected to changes in moisture content. This could lead to cracks in the EIFS. It was even more of a problem if you tried to apply a direct-applied finish system (without foam).
Sheet Sizes: A lot of the imported boards only came in odd metric sizes, making them incompatible with North America stud spacings. This required a lot of cutting and also created a lot of waste.
Fire-Rated Assemblies: We were hoping to find a board that could be substituted directly for drywall to make fire-rated assemblies (one-hour; two-hour; etc.). A lot of the board could not do this and this would lead to huge expenses to go through all the UL fire-rating tests.
Nailed and Screwed: We wanted a sheathing that could be screwed and also nailed, such as on wood-frame construction. Some of the cement boards were brittle and cracked when hammered. Also, some did not permit beating the nail until it was flush. The lack of being flush created problems with finishes like paint, but with EIFS it was not a big issue as the foam insulation can ride over a nail head, and the EIFS attachment adhesive makes the foam stand-off slightly from the substrate anyway.
Wind Loads: Most of the sheathings were sufficiently strong. The ability of the sheathing to withstand high wind loads did not actually become a major factor. By adjusting stud spacing and screw spacing we could get wind capacity at least as good as gypsum sheathing.
EIFS Adhesives: We were hoping that the then-current crop of EIFS attachment adhesives would be compatible with the sheathing, but it turned out that the porosity of many boards required a special adhesive. This was not a huge problem, but did necessitate some formula-tweaking.
Structural Holding Power: We were looking for a sheathing that one could attach normal things to, like handrails and light fixtures. Most were strong enough in terms of bending strength, but didn’t hold screws or nails very well. Thus the same technique for attachment was needed as for gypsum sheathing: Going into a stud or framing, or through-bolt a surface-mounted object.
Mold and Mildew: Although this was not a huge issue back in the ‘80s when The Ideal Substrate project was undertaken, it is a real big deal now. Major strides have been made in recent years on this aspect, and now there are gypsum boards that, despite having organic content, do not support mold and mildew.
Noncombustible: This is essential for commercial construction. There are, for instance, some wood/mineral composite sheathings that, for practical purposes, do not burn at all. Even though they were not purely inorganic, they could pass the code requirements in the same way that EGGS does, despite EGGS having paper facings.
Water Leaks: A lot of the mineral boards did quite well in this regard, and this was a major area of concern. Although some would soak up a good bit of water, when dried, they retained their original properties. Some of the wood/cement boards had a lot of wood in them and swelled and fell apart.
Warp-able: We wanted, ideally, to be able to bend the sheathing around gentle curves. Gyp board is springy compared to cement boards, and can be warped. Many of the cement boards snapped when warped, and could not be kerf-cut to allow warping, without splitting.
Is There An Ideal Substrate?It’s obvious that there is no Ideal Substrate, but some of the glass matt-faced gyp boards, and some of the mineral boards, come pretty close. For the present, the advent of glass matt-faced gypsum boards has solved this “better than ASTM C79 sheathing” issue, and is used most of the time for commercial projects.
Other new types of sheathings are coming into the market, providing more selection and even higher performance. It basically comes down to the price of the sheathing itself–it’s not hard to design a “super sheathing” if cost is no object. The other key factor is availability. Various types of mineral-based sheathings are available from around the world, and getting your hands on some of these sheathings is an issue, as widespread distribution is not yet in place.
So the search goes on for “The Ideal Substrate.” Interestingly, back in the ‘90s, lawyers came upon The Ideal Substrate study during the discovery phase of a major court case. They made a lot of noise about it. I told the lawyers that we were being responsible and that the continual search for product improvements does not mean there is something wrong with the way things are being done. It’s called progress.