From a sealant compatibility standpoint, the EIFS lamina is neither a nonporous or porous material, or "sealant substrate." For example, aluminum and glass are nonporous, while some types of wood and stone are porous (the term "porous" here means water absorptive to some degree). Strong, nonporous surfaces, of the type that the sealant can get good adhesion to, make the best substrate for sealants. Some nonporous substrates, such as some paints that repel water, are difficult to adhere to since the sealant cannot get a good grip on the surface without some help, such as a sealant primer. Unstable materials, such as raw or treated wood, which expand and contract a lot, are often difficult to bond the sealant to and soft, weak materials, usually make poor sealant substrates.
Moisture contentThe EIFS lamina, both the finish and the basecoat, can absorb some moisture in the sense that their moisture content changes with its exposure to water but they are water shedding enough to prohibit water from passing through, i.e., they do not "leak." This is one of the reasons why EIFS works on walls but is not well suited for roof applications.
In addition, the EIFS finish softens slightly when in prolonged contact with water. This weakens the coating, making less force required to detach the finish from the basecoat. On the other hand, a lot of basecoats have at least some Portland cement in them, and all have glass reinforcing in them, making them, unlike the finish, a structural material, i.e., stronger. EIFS finishes are essentially paints with aggregates added to give them texture and some thickness, and thickeners added to make them trowellable. Thus, the basecoat makes a much better surface to bond a sealant to.
In the early years of the EIFS industry, the fact that the sealant works better is bonded to the basecoat was not well known and bonding the sealant to the finish was common practice and acceptable. Bonding the sealant to the basecoat creates it own problems, such as the very practical one of keeping the finish away from the sealant bond-to zone. Keeping the finish out of the bond-to zone can require masking off the bond-to area, which is time consuming and expensive, and can leave unattractive areas of exposed basecoat.
It was not until the '80s that the desirability of bonding to the basecoat became fully apparent, mostly as a result of sealant joint problems on existing buildings, i.e., the sealant sometimes literally pulled off the finish from the basecoat. In fact, the finish stayed attached to the sealant and if this occurs, one can pry back the sealant and see the finish's aggregate particles still bonded into the sealant.
This behavior was due to many factors, such as sealant joint design problems (width too narrow or the sealant bead is the wrong shape), sealants that are too stiff (and thus yank the finish off the basecoat) and the moisture sensitivity of EIFS finishes. It was in the late '80s that EIFS producers and sealant producers changed their policies about bonding to the basecoat instead of the finish and now bonding to the basecoat is the accepted manner for most producers. For some producers it is the only accepted way and not doing so can affect the warranty.
The effect of the above characteristics of EIFS is that many sealants work better with EIFS if they are used in conjunction with a primer between the EIFS and the sealant. The primer being some form of liquid-applied coating that is placed on the basecoat prior to applying the sealant. EIFS is definitely not the only material that normally requires a primer, so don't think EIFS is being singled-out in this regard.
Both sides nowRemember, in any EIFS sealant joint there are two sides to the joint-one with EIFS and the other with some other material or perhaps also EIFS. The point is that different primers may be required on the two sides of the joint; the primer used on EIFS is not the same one used on Kynar paint, etc.
Most sealant and EIFS producers offer various types of primers that can be used to enhance the adhesion of the sealant to the EIFS. Usually, the primer is of the sort used for semi-porous surfaces, like concrete or stucco. Some EIFS producers offer their own primers, usually a brush-applied, paint-like material that can be tinted to match the adjacent sealant and EIFS finish. Keep in mind that some generic sealant primers are untinted (clear) but when exposed to sunlight turn yellow or brown, causing a stained appearance. This makes it necessary to be very careful about putting the primer only where it will be covered with the sealant or is otherwise not visible.
On new construction, if you get into a dialogue about how to handle EIFS sealant joints, consider the following:
• When choosing a sealant for use on a specific EIFS, look at the EIFS producer's list of acceptable sealants (and primers). Usually, there are several choices and most are commercial grade products. These "approved sealant" lists are published and many are available on the Internet.
• Sealants that work well with EIFS generally have a low modulus of elasticity, a technical term that essentially means "soft and stretchy," have good wet and dry adhesion to the EIFS and do not get too stiff when they get cold. Some sealants that stick like mad to EIFS, such as acrylics, do get "hard" (stiff) when they get cold, and hence don't work well on joints that experience a lot of movement. Silicones and urethanes are usually good choices, while a lot of cheap, household-type, blended sealants are not usable for working, expansion-type joints found often on commercial buildings.
• Follow the recommendations of the sealant producer and that of the EIFS producer for the particular sealant and EIFS product at hand. Keep in mind that this may vary from one EIFS producer to another and may result in the need to change sealant types if the brand of EIFS used is changed. The type of sealant and the primer, if any, should match between the EIFS producer and the sealant producer. In other words, they should both agree that a particular EIFS and sealant combination will work together.
• If you want to use a sealant that has not been tested with a specific EIFS, you should test it first to see what combinations work well. There are a number of standardized tests for this purpose, such as those offered by ASTM. Be especially careful to test the sealant-EIFS combination under both dry and realistic, high moisture conditions, as this is usually the condition where the bond is the poorest.
• Make sure the contractor who is installing the sealant on the EIFS understands what EIFS really is and installs the sealant properly. This includes using the right sealant type (and primer, perhaps) and making the joint the correct width and shape. Some sealant contractors treat EIFS joints as if they were concrete or stucco, which they are not.
On existing construction, if a survey reveals that the in-place sealant joints have the sealant bonded to the finish and not the basecoat, don't panic. Keep in mind that some EIFS producers did allow this practice and that some do not now require (but do prefer) that the sealant be bonded to the basecoat. Another way to look at this sort of situation is simply to say "Is it working?" and if it is, then "So what is the problem?" Thus, the condition at hand may not be the best way to do a sealant joint but if it is not leaking, "So what?" In other words, it's not "defective."
Sealant issues are ever-present on existing buildings and in my experience, the leading sealant problems are pretty basic, namely:
• There's no joint at all, where one should be.
• The sealant is some crumby product that should not be used with EIFS.
• The sealant has simply reached the end of its life.
• The joint width is way to small for the movement that is taking place.
• The sealant was never installed.
• And especially, that the edge of the EIFS is not wrapped by the basecoat, and they tried to bond the sealant to the foam, which will not work. Period.
Testing shows that bonding the sealant to the basecoat is the way to go, and all it takes is going through the tedious process of removing and replacing defective sealant joints to convince you to get them right the first time, i.e., use a proper, high-quality sealant (and primer), design the joint correctly, and install the sealant right. That will ensure getting the most longevity out of the joint before replacement is necessary.