Wet-Applied Weather Resistive Barriers
Work is taking place now in various technical organizations to qualify WRBs: In other words, to define what properties make for one that will function properly. This month's column is an explanation of some of the key properties of WRBs.
Wet-applied WRBs come in many forms. Some are highly cementitious, while others are resinous or paint-like. Some are applied with a trowel, while others are applied using a roller or by spraying. There are also WRBs that come in sheet form (i.e. paper-like materials, building paper) and film form (thin-synthetic paper-like films in rolls, such as Tyvek) but these non-wet types are not the subjects of this article.
WRB's need to be able to be applied on a variety of substrates that have very different properties. For example, a roller-applied material that would work on plywood might not work on a coarsely textured surface like concrete block. Similarly, a highly cementitious WRB might work well as a trowel-applied material on substrates, like block but unsuitable for wood-based substrates, such as plywood. Thus, WRBs with different compositions and application methods are available.
Water preventionSince a WRB's main function is to protect the substrate and prevent water from getting into the stud cavity, first a WRB must be waterproof. But what does the word "waterproof" mean? For purposes of a WRB, waterproof means the ability to shed liquid water. But the definition does not end there. It also means being non-absorbent. One of the reasons a WRB is needed is that many substrates are water sensitive. Thus, simply not allowing liquid water to get through the WRB is not enough. If the WRB becomes soggy and if it is in direct contact with a water sensitive material, such as some substrates, clearly the substrate material could be affected. This is important, because in an adhered EIFS system, the sheathing acts in a structural capacity in handling wind forces. If, for example, the presence of the WRB diminishes the strength of the sheathing due to water retention by the WRB itself, then the structural capabilities of the whole wall could be affected.
However, waterproof does not mean being absolutely impervious to moisture in all senses of the word "moisture." Moisture exists in several forms, including as a gas (water vapor). In order to be functional as a water barrier, a WRB needs to be "continuous" in the sense of not having holes or openings in it that would let water through. This continuity is achievable when a WRB is wet-applied but obviously not when applied as a sheet or film, as sheet and film WRB's have seams or laps. However, the shingle-like style of application of sheets and films overcomes the fact that sheet and film WRBs are not joint-less.
WRBs are supposed to aid in preventing water from getting into the stud area. But if water does get into the stud cavity and if there is a vapor barrier on the inside of the wall, the vapor needs to be able to get out in some way. Clearly, the vapor barrier would thwart drying towards the indoors. The paths for drying toward the outdoors are limited in an EIFS and include going through the WRB. This means that the WRB needs to be water vapor permeable but also has to be able to shed water. This dual mode of handling "water" can be achieved by designing the chemistry of the WRB material to allow water vapor (a gas) to pass through it but not letting liquid water go through. If the WRB is not vapor permeable, then water that does get into the stud cavity will have trouble getting out. Thus, the wall could remain damp. And if water continues to enter the wall, the moisture level could rise to dangerous levels, resulting in mold, rot and other undesirable effects.
In a nutshell, the WRB should not be a vapor barrier. If it is, it could cause any water that does happen to get into the stud cavity to be trapped there, unless other drying mechanisms are present. Being a continuous film, a wet-applied WRB needs to have high vapor permeability on its own. It actually needs to be able to "breathe" water vapor better than a sheet or film material. Thus, side-by-side comparisons of vapor permeability data of sheet or film WRB's, vs. that of continuous wet-applied WRBs, can be misleading.
For example, in the case of a sheet or film WRB, a lot of moisture trying to get outdoors by going "past" a sheet or film does so through the laps at the edges of the sheet or film. Hence, the permeability of the sheet or film itself is not the only vapor movement factor that applies. However, with wet-applied WRB's, the only way for vapor to escape past the WRB is through the WRB. Hence, the permeability of wet-applied WRB's is critical. Simple permeability tests of the wet-applied WRB are not enough to ensure that the wall "works" from a drying-out standpoint; the wall needs to be looked at as a system, under a range of climate types, seasons and wall configurations.
Easily adheredAnother characteristic necessary for wet-applied WRBs is that they adhere tenaciously to whatever they are applied-to. This means essentially that the type of resin and other materials in the WRB must be compatible with-and strongly bonded to-the surface to which it is applied, such as wood or a paper facing or glass matt-faced gypsum board. One of the desirable aspects of wet-applied WRBs is that they can be bonded onto. This means that unlike sheet or film WRBs, one can use an adhesive to attach the foam, instead of mechanical anchors. This is a big plus for the EIFS installer and manufacturer: they get some extra business and also get to control the application of this critical wall component.
However, this, in turn, means that the WRB must also be able to be adhered-to by the EIFS attachment adhesive. Thus, the WRB must bond well to substrates, and be bondable-to. In addition, the fact that the EIFS foam is attached with adhesives eliminates the problem of the zillions of small fasteners holes that must go through a sheet or film WRB. In a sense, the use of fasteners through a sheet or film WRB limits the effectiveness of such WRBs by putting holes in a material that shouldn't have any: the WRB itself.
Keep in mind that it is also important that the WRB, when used in conjunction with an adhered EIFS, must itself be internally strong. This is because the wind forces being transmitted through the EIFS foam into the EIFS attachment adhesive eventually ends up going through the WRB too. These wind forces must be resisted by the WRB, lest the EIFS come off the wall. For example, some types of coatings, when they get damp, remain functional as a water barrier but lose some of their strength.
Some sheathing substrates are not normally intended to be bonded to. OSB is an example. Some substrates also exhibit a range of properties on their critical outer surface, even as they come right out of their manufacturing process. Thus, the chemistry of the WRB needs to be able to adhere to surfaces that may have a range of properties, as well as possibly some type of contamination on it, such as dust. A good WRB should also be able to work under many foreseeable substrate surface conditions, such as dampness of the sheathing and weather problems, such as low temperatures or extreme heat.
Weather toleranceI am sometimes asked if a wet-applied WRB can be left out in the weather. This question usually involves a project that is behind schedule, and the WRB is seen as a way to "protect" the building until the weather improves and the rest of the EIFS can be installed. The weatherability of WRBs is thus a practical consideration, and needs to be taken into account in the design of a WRB. For example, the types of resins used in a wet-applied WRB that is touted as being able to endure outdoor exposure, might be different from one that is intended to be immediately covered up by the EIFS.
WRBs are most often used over sheathing substrates. Sheathing obviously comes in sheets and there are seams between the sheets. These seams move slightly as the building itself moves; the seams move parallel to the seam, as well as open and close. This is why gaps are left between sheathing pieces with some types of sheathings that swell appreciably with temperature and/or moisture level changes. This movement results in stresses building up in the WRB coating. This, in turn, means that the WRB coating must be able to withstand these stresses without cracking. Cracking is not tolerable as it could form a path for moisture intrusion. Many WRBs are therefore either a rubbery material that can withstand joint movement without cracking by being stretchy, or by being a reinforced material that can withstand the forces by virtue of their own inherent strength. WRBs are not intended to reinforce the sheathing or make up for weaknesses in the supporting wall structure. Hence, there's not much need for the WRB to perform any better structurally than the substrate it is attached to.
WRBs also need to be compatible with whatever materials are present where the WRB stops. For example, peel and stick membranes are often used at openings and they need to be able to stick tenaciously to the WRB, and create a watertight seal.
A WRB needs to be easy to install. This includes not just vast, flat areas but especially where the WRB starts and stops. These start-and-stop areas are especially critical, as they often occur at openings, a common origination point for water intrusion. Thus, such areas need to be especially well designed and installed relative to the WRB. Sometimes, these transition details can be complicated; download some of the EIFS manufacturers details off the Internet to see what I mean. For example, spray-applied barriers are easy to apply to complex geometries. In contrast, film and sheet barriers can sometimes requires installation details that border on wrapping a Christmas present or can be simplified only by using lots of sticky sealant tape.
In the end, a WRB needs to remain functional for the life of the wall. This means it cannot deteriorate or fall apart. Since the WRB is not normally subjected to direct outdoor weather and should not be exposed to water for long periods, this is not too difficult to achieve. In other words, since the WRB is not in a water immersion environment, it need not be able to remain viable under continuous moisture presence but clearly it needs to stay intact when in the presence of incidental moisture. Many wet-applied WRBs are much like EIFS basecoat adhesives or paints in their composition and hence are more than capable of withstanding the "weather conditions" in their protected location within the wall.
As you can see, designing an effective WRB requires balancing a number of requirements into a single product that is functional, installable and viable cost-wise. There is currently no national consensus standard for how to qualify a WRB as being proper for use in an EIFS application. However, work is underway to do so at ASTM and in other technical forums. Once such a standard is available, the use of wet-applied WRBs are likely to increase because the WRBs can then be listed as being an integral part of an EIFS under the rapidly expanding use of the new International Building Codes.