The Finish Line: Drainage Efficiency
The origin of the EIFS with drainage goes back to the 1990s. The idea of adding drainage came about due to a rash of water intrusion problems on houses in the southeast. Water had gotten behind the EIFS and ruined the supporting wood structure, causing a flurry of lawsuits and repairs, and sparking the interest of building code officials.
The odd thing about this debacle was that the EIFS itself was not leaking but water was getting into the wall around the edges of the EIFS at terminations and openings. Thus, it was the wall assembly that was leaking, not the EIFS.
However, EIFS got blamed for the problem. The obvious solution, to some people, was to add a cavity behind the EIFS to allow water to work its way harmlessly back outside. The idea of EIFS with drainage had thus been conceived but not yet born.
THE EARLY DAYS OF EIFS
For many years, building codes have required a separate, secondary water barrier in exterior walls. For example, with stucco, it’s the stucco itself that is the primary barrier, while the building paper (on top of the sheathing) is the secondary barrier. These secondary barriers are often known as water resistive barriers.
Water resistive barriers are needed with many claddings, since the field of the wall is often impossible to seal completely. For example, consider brick veneer, which has miles of mortar joints. It’s impossible to make sure every mortar joint won’t leak. It’s different with EIFS, as there are no joints.
EIFS was thus an exception and for years the code authorities accepted that EIFS was a true barrier system and that water could not get through the coatings. The impenetrability of the EIFS coatings is true, and thus EIFS was exempt from having to have a drainage capability or a WRB.
With the advent of the water intrusion problems, this assumption was reassessed, and the conclusion was that a WRB and drainage was indeed needed with EIFS, too. The requirement for drainage and a WRB, however, was limited to buildings with supporting walls that were moisture sensitive. This basically means wood frame buildings, and not steel and concrete structures. The requirement for using EIFS with drainage is now written into residential building codes.
Traditional EIFS, as developed in Europe and used there today, is a barrier-type wall. Such walls do not lend themselves to adding drainage behind the EIFS. In Europe, EIFS is installed almost always on masonry or concrete walls, making the wall structure itself less sensitive to leaks. When considering adding drainage to an EIFS wall, the question immediately arose of “How much drainage is enough?” It turns out that even a traditional barrier EIFS, with the foam insulation up tight against the substrate, does drain-somewhat. The question then became “How efficient does the drainage need to be?” The answer was to build a mock-up and test it.
A DRAINAGE PLANE
For efficient drainage, an open cavity of some sort is needed for the water to flow through. Many means for doing this were developed, including using foam board with vertical drainage grooves cut into the back, and using a spacer (often a plastic, lath-like mesh) to separate the foam from the substrate. A slick, simple variation on normal EIFS adhesive attachment methods is to use thick vertical beads of adhesive. This creates stand-off ribbons of adhesive that produces a cavity, and is widely used.
Along with the cavity, some way was needed to protect the substrate (usually OSB or plywood) from water. This took the form of materials applied over the sheathing and included various types of sheet goods (papers and wraps), as well as trowel-applied coatings. Papers and wraps had the problem that the EIFS attachment adhesive would not bond to them. Thus, mechanical fasteners were needed.
Mechanical fasteners have their own issues, and many contractors prefer using adhesive to glue the foam to the wall. In the case of EIFS with drainage, the fastener screws put holes in the WRB, reducing its effectiveness. The same problem occurred when using plastic lath as a spacer: the adhesives didn’t stick well enough, and the lath fasteners created holes in the WRB.
Using a liquid-applied WRB allowed adhesives to be used to attach the foam, and also kept the work in the plastering trade. EIFS producers could thus supply the WRB coating, and the EIFS contractors could apply the WRB using traditional plastering techniques.
It turns out that a cavity of about 1/8 inch or wider is needed to achieve decent drainage. To see what kind of drainage design worked, a test method was developed. A 4-foot-wide by 8-foot high wall mock-up (including studs, sheathing, WRB, drainage cavity, and the EIFS) is built. A slot is cut into the top, all the way back to the WRB. The bottom edge is left open. Water is sprayed into the slot and allowed to drain. The test specimen is then allowed to dry out. The amount of water that passes through, minus the amount sprayed, is the amount retained. (The test apparatus is shown right). This test method is a laboratory test. It is not done on an actual building’s wall, although it might be possible to do so.
It’s obvious that bigger cavities support better drainage. However, large cavities allow air circulation between the insulation and the supporting wall, thereby reducing the effectiveness of the external location of the EIFS insulation.
There is not a lot of bulk water left inside a test specimen that drains well. Most of it is hanging onto the foam and WRB as droplets, as it gets hung up trying to work its way through the vertical grooves in the foam or through the cavity space.
This drainage test method was a variation of a test originally developed by an EIFS producer. Eventually, the test method became an ASTM test method and is now referenced in the building codes.
Initially the test method had no pass/fail criteria: the required percentage of efficiency was not defined. The thinking of not defining the efficiency was to allow local authorities to specify how much efficiency they thought was needed. This did not work, as the authorities wanted a hard number. But what should this number be?
THE MAGIC NUMBER
Extensive discussions occurred over many months with the EIFS industry, research and trade groups. It turned out to be quite difficult to assign a precise drainage efficiency number to a wall, as there was little scientific data to generate a meaningful number. Also, the amount of water that is put into the test specimen in the test is far in excess of anything that would normally occur in a real wall-the amount of water applied is at the upper limit of rainfall ever recorded in intense storms. In addition, the presence of the WRB kept the water from getting to the wood sheathing and studs, anyway.
In the end, the efficiency of 90 percent was agreed on. This was based on how much water would be retained, and also having the WRB not function. In other words, the water would be allowed to wet the sheathing and studs. To say the least, this test method is a very worse case scenario. The 90 percent number would theoretically give water retention values for wood that are close to the limit that wood can withstand without damage. In fact, most commercially available EIFS with drainage designs have efficiencies much higher than 90 percent, so the wall is well protected.
Keep in mind also that the drainage test has a 75-minute spray time and a one-hour drying out time. It is after drying out that the retained moisture is measured. In the real world, the wall will continue to dry out, so the wall doesn’t have a high level of water retention for extended periods.
PAY ATTENTION TO THE DETAILING
The bottom edge of an EIFS wall area that has drainage needs to allow the water to drain out or at least route the water to some place where it can do so. This means an open slot or some type of flashing is needed to bring the water out to the face of the wall where it can dribble away harmlessly. Thus, on EIFS with drainage walls, you often see trim flashing at window heads and at the bottom of walls. Also, normal back wrapping, where the edge of the foam has adhesive between the foam and the substrate, cannot be used. Doing so would seal the edge, preventing water from exiting. Instead, the mesh is wrapped only onto the backside of the foam, and not onto the substrate.
In the end, what really makes EIFS with drainage work is not the drainage cavity but the attention to the construction details at the edge of the EIFS. The use of carefully designed flashings, caulking joints and sealant tapes, is what actually keeps the water from getting into the wall in the first place.
Fortunately, the code mandate to use EIFS with drainage is limited to wood frame buildings. On the other hand, some designers are enamored with the concept of having a backup drainage system and want to use it on all sorts of buildings. If you want an exercise on how to design and build an EIFS wall that incorporates drainage, try doing prefab EIFS panels that have drainage. You’ll quickly see that the panel joints become a nightmare, not to mention how to do the flashings and other seals around windows. At least homes are stick built at the site, so the contractors can get access to the complex waterproofing conditions at the edge of the EIFS. W&C