Water Vapor and Permeability and EIFS
Over the years, I have come to feel that most people in the construction community do not understand permeability. It is not taught to want-to-be architects and even with experience, few people come close to understanding what permeability means. This month's column is about how this important topic relates to EIFS walls. Hopefully, it'll help answer questions that come up in conjunction with the all-to-common subject of leaks and mold/mildew.
Point of viewFrom an EIFS perspective, the term "permeability" is a way of describing a material's ability to pass water vapor through it. Keep in mind that water vapor is an invisible gas, not a liquid. Of course the term permeability can be ascribed to liquid water, as when water is placed on the top of a dry sponge and permeates down through the sponge to the other side. That is not what we are taking about. What we are taking about is more like (although technically not the same thing) when the invisible smell of garlic permeates a house while cooking.
Materials are permeable to water vapor to various degrees. Glass is not permeable at all; it is impermeable. So is metal. Wood is somewhat permeable, as are many coatings. Cellular materials (like foam insulation) and fibrous materials (like fiberglass) are permeable, too. EIFS coatings are partially permeable. The physical form of a material can often give some idea of how vapor permeable a material is.
Obviously, "open" materials like carpeting let vapor fly right through. However, when it comes to coatings, appearance is often not much of an indication. Coatings are a good example. Some coatings-epoxies for instance-generally have low permeance, i.e., they are high impermeable. Acrylics, on the other hand, have a range of permeability. The types used in EIFS are partially permeable. Thus, you need to know what the permeability for a material is based on some objective standard. Luckily, there is an ASTM test that produces a numerical result.
Permeability is normally described in terms of amount of "water-per-area-per-time-unit-per-unit-of-vapor-pressure difference." In English, this means "grains per hour per square foot per inch of mercury." By the way, grain is a unit of measure-1/7000 of a pound, to be exact. The term "inch of mercury" is an oddball way of describing the amount of vapor present on side of material vs. that on the other. It's the type of unit that is employed to describe, for instance, atmospheric pressure readings taken from a barometer and used in weather forecasts. In any event, describing permeability is a mouthful. A common measuring unit of permeability is simply "perms." Enough tech talk.
The degree to which moisture can permeate a material depends on the properties of the material and its thickness, and the conditions on either side of the material. A simple analogy would be a solid concrete wall with no finish material on either side. Concrete is not impermeable itself, and depending on how hot and humid it is on either side, the water vapor in the air permeates through the concrete at different rates. The greater the difference in temperature and humidity from one side to the other, the more the vapor is driven through a material.
EIFS is a laminated system. The finish (basecoat and insulation) are in direct, continuous contact with each other. The only way for water vapor to permeate through the field of an EIFS wall is to go straight through it. Of course, if there were joints in the wall, the water vapor "rather" would take the path of least resistance and go through the joint, rather than through the EIFS. However, sometimes the vapor has no path to relieve this pressure difference other than through the EIFS itself. Thus, the properties of EIFS materials become important.
EIFS is fairly unique as a wall cladding in that it can have very few joints. This means it has very few paths for moisture to leave, other than going directly through the EIFS. This helps explain why EIFS walls, if they get wet inboard of the foam, have trouble drying.
It's important to note that barrier and drainage EIFS behave differently in regard to vapor flow. Because barrier EIFS are in close contact with the substrate, water vapor that wants to get to the outdoors from the substrate area must go through the EIFS itself (assuming there are no other easy paths, such as joints, through which the vapor can flow). Water vapor flow by permeation is a slow process. In contrast, vapor that flows through "open air" migrates quickly. In the case of drainage EIFS, which has a cavity between the foam and the substrate, vapor that gets to that point in its outboard journey from the substrate area, can flow directly outward via the drainage cavity. The difference between this "mass flow" of vapor through a cavity vs. that of permeation can be hundreds of times quicker for drainage EIFS. This is good for a wall, as drying occurs more quickly.
The other sideThere's a flip side to this coin, however. Drainage EIFS has a weather resistive barrier on top of the substrate. This material is used to protect the substrate from the water that might occurin the EIFS drainage cavity. WRBs can take two basic forms: sheet materials and coatings. Two common sheet materials are building paper and Tyvek (and similar materials). Coating-type WRBs are usually proprietary trowel- or spray-applied materials. EIFS producers, and others, make them coating-type WRBs. Coating-type WRBs can offer the distinct advantage that EIFS foam can be adhesively bonded to the WRB itself. Such is not true with sheet type WRBs, since they are nonstructural and/or the coating won't stick. But the plot thickens, in terms of permeability.
Sheet-type WRBs have seams or overlaps between the sheet pieces. No kidding. In contrast, coating-type WRBs, in order to be effective, must be continuous and unbroken. Although a coating-type WRB may have a good permeability rating (allows water vapor to easily pass through it), in the case of coatings permeation is the only way for moisture in the stud cavity to get past it. In contrast, sheet-type WRBs allow vapor to pass through them both by permeation and by going "around" the WRB via the sheet overlaps. This is an important distinction, because if moisture does somehow get into the stud cavity, it needs a way to get out, lest it could possibly damage the wall due to moisture retention.
There is a trend toward using trowel-applied WRB in drainage EIFS. Clearly, there are many advantages to this approach. One advantage, from the EIFS contractor's point of view, is that the EIFS contractors get the work for installing the WRB, and then gets to put the EIFS on it. The thorny issue of permeability of the wall as a whole still remains in relation to essentially sealing off the middle of the wall with a partially permeable layer. One of the mantras of wall and roof design is that you only want one vapor retarder, if any, in a given assembly. This allows moisture to leave in two directions. Placing two or more barriers, especially on the inside and outside faces, complicates this vapor flow matter in terms of the self-drying ability of the wall.
To resolve the issue of wet walls, what is needed first is to keep the water out in the first place. That's a tall order, and the courts therefore have said that some type of backup system is required. Enter drainage EIFS. Drainage EIFS primarily handles water at the perimeter of the EIFS in the sense that the field of an EIFS wall does not leak (and yet that is where the drainage plane is located). It's an odd twist of fate that the drying ability through the field of the EIFS thus becomes important. This is where permeability comes in.
What is needed in EIFS walls is an inherent tendency to dry out. This is especially true if moisture gets into the wall inboard of the EIFS foam. It's even more important if somehow moisture gets into the stud cavity. EIFS coatings and foam are partially permeable materials. Accurately predicting the drying characteristics of an EIFS wall assembly, or the occurrence of condensation, is a complicated technical task. The word "accurately" is the key word in this equation, because there is much more going on in a wall regarding water vapor than simply whether of not there are ventilation paths, and whether or not the materials are permeable. For example, the ability of the wall's materials to absorb, retain and give off water vapor is an issue. So is the outdoor weather.
Where I live in Seattle, it's damp and humid all winter. This means that water in a wall does not have much incentive to leave, as it may be as damp inside the wall as is it outside. Sophisticated computer techniques can be used to predict water vapor performance of walls. Unfortunately, the information needed to do the calculations, and the knowledge of how to run such programs, is beyond all but a handful of experts. Many EIFS producers have the engineering horsepower to do basic analyses of this sort but it's fair to say that on most EIFS projects this issue is never looked at. Thus, a more universal solution, which is inherent in the design of EIFS wall assemblies, is needed.
Additional research is needed on how these permeation issues operate in real EIFS walls. Then, the development of the next generation of EIFS, that address these issues under a wide range of end-use conditions, needs to occur. Fortunately, work on the research end of this issue is already underway. EIMA has partnered with the U.S. Department of Energy to do extensive computer simulations of moisture aspects of EIFS. The testing of actual EIFS wall system mockups around the United States is augmenting this analytical approach. The mockups will be instrumented to generate data that will hopefully correlate with the theoretical computer simulations. Results will be a few years in the making, but should provide useful information about how EIFS walls work under real world conditions of moisture flow.
As for now, if you are interested in learning more about vapor flow issues, a good source of basic information is the ASHRAE Handbook of Fundamentals. ASHRAE is the American Society of Heating Refrigerating and Air Condition Engineers, a national technical association. You can also find a discussion of this subject in architectural manuals, such as "The EIFS Design Handbook," and in various papers by published by such technical organizations as ASTM.