The insulation is actually a component of an EIFS that serves several functions and this month's column will give an understanding of what the foam really does, and why certain types work better than others.
The key component of an EIFS that distinguishes EIFS from other stucco-like materials is the presence of the insulation layer itself. Without the foam, it is not an EIFS, it's simply an uninsulated coating system. Clearly, the foam insulation provides thermal insulation, in other words, reducing the transmission of heat or cold. Obviously, thermal insulation comes in many forms, including fibrous types, foam plastic types and ceramic types. Although all these types are used in EIFS, foam plastic is far and away the most popular used in North America.
More fiber?
Fibrous insulations, in the form of semi-rigid boards, exist commercially and one would think that they might be adaptable for us in an EIFS. In fact, they can be and sometimes are but to a very limited degree. The problem with fibrous insulations is that they have little strength, especially perpendicular to the plane of the insulation sheet. Another big issue is that coatings don't bond well to them. Although fibrous insulation can be treated to allow the direct application of coatings to their surface, the fluffy, uneven nature of their surface makes creating a flat wall difficult. To get around this, the thickness of the coating must be substantial and this produces a heavy coating that is difficult for the fibrous insulation to support. To handle this excessive weight, the coating must then be supported independently from the insulation.In Europe, for instance, there are fibrous insulation-based EIFS products that use wire lath that it held against the surface of the insulation by mental pins that go into the substrate. In a sense, the coatings, which are stucco-like and are hanging on the wire ties, and the insulation acts more like a spacer than as a support for the coatings? Essentially, the coatings "hang" like a curtain, and float free from the insulation, somewhat in the manner that traditional stucco floats free of the sheathing due to the presence of a bond breaker layer-namely the building paper.
Fibrous insulation also has the problem that water can pass through it, both in liquid and vapor forms. This means that if cracks develop in the coatings, that liquid water can get directly at the substrate and can migrate within the insulation layer. This makes for a wall that has limited ability to deal with water intrusion. The "open" consistency of fibrous insulation also allows water vapor, a gas, to wander freely within the insulation; this can cause condensation to be induced, as there is little impeding the vapor from traveling toward the cold side of the wall.
Strong stuff
Solid ceramic insulations have their own issues that make them difficult to use in an EIFS application. There are, in fact, a number of commercialized ceramic based EIFS products available, and they are mostly used in urban areas where the use of combustible types of insulation, such as foam plastics, are not permitted due to fire code restrictions. For instance, sheets of "foam glass" insulation can be used to make an EIFS that is incombustible, a real advantage in densely populated areas, or in tall, critical use buildings.The main stumbling block for ceramic insulation is their stiffness; they do not "give" much-they are brittle. This is not a desirable trait in the sense that it tends to make the outer EIFS coatings crack prone, as any movement in the insulation layer is transmitted directly to the coatings. The effect is somewhat like applying traditional stucco to a concrete block wall: If the block wall cracks, the stucco will likely crack along with it.
Foam plastics are an inbetween sort of material: denser and easier to bond-to than fibrous insulations, while softer and more "giving" than the rigid ceramic types. EPS foams have an advantage over extruded polystyrene or polyisocyanurate foams in that they are more "springy," which explains their widespread use in packaging. The only real issue with foam plastics is that they can be made to burn. This can be a big issue on certain types of buildings, for instance, on tall buildings where the occupants are infirmed and could not be evacuated during a fire.
Fortunately, when protected by a reinforced coating with limited combustibility, and attached to durable substrate, tests show that the common types of EIFS used in North America have little propensity to perform poorly when subjected to fires. This is the reason why the building codes permit the use of EIFS on buildings that would normally have to employ walls made of noncombustible materials.
For use in EIFS applications, the requirement that the insulation layer have some "give" to it is often under appreciated. This softness provides a way to isolate the movement of the outer EIFS coatings from the supporting wall. Given that by its essential external location, the outer surface of an EIFS is at the same temperature as the outdoor air, and the substrate is closer to the indoor temperature. This temperature different induces movement and the insulation layer allows the coatings to move a bit, independently from each other. This goes a long way to making the outer EIFS coatings remain crack free. In EIFS applications, it's also nice to have an insulation material that itself is not water absorptive, thereby providing an extra barrier to water intrusion, should the outer EIFS coatings crack.
For EIFS applications, it's also good if the insulation has some inherent strength. This makes a difference as the outer EIFS coatings are subjected to wind forces, and somehow those forces must be resolved back into the substrate, lest the wall be pulled apart in a storm. Materials like foam plastics are strong enough in all three directions and can either be glued or mechanically attached to the substrate. Weak materials, especially those that have directional strength properties, such as fibrous insulations, are particularly difficult to deal with from a wind-engineering standpoint. This is one of the reasons why a fiberglass-based EIFS system, developed and marketed briefly in the '80s, was limited to low rise residential applications.
Various attempts have been made to develop new types of insulation materials that provide all the desirable qualities mentioned above, while eliminating the bad properties. This has met with little success. I have been involved with some of this work and the problem is one of the basic properties of the types of materials that might make sense for use in an EIFS, and commercial issues, such as cost and availability. For example, flexible ceramic materials are a bit of oxymoron.
Similarly, plastics that do not burn do exist, such as silicones but are hard to bond-to, and are expensive. The real deal breaker though is cost. This is because construction materials are used in such large quantities that the unit cost must be low; EPS, for example, is dirt cheap compared to the exotic insulating tiles used on the underside of the space shuttle. For the near term, unless someone gets a Nobel Prize for developing some ingenious new form of insulation material technology, it's likely that the forms of insulation that are popular for EIFS now, especially EPS, will remain so for decades.
