Exterior Insulation Finishing Systems are relatively lightweight and mostly composed of expanded polystyrene (EPS) foam, which when hit with direct impact, can give way to punctures or dents. This isn't to say EIFS is a weak material, as it can resist inclement weather and other external forces. Certainly, on the other hand, manufacturers of competing products might attempt to exploit this as an issue that EIFS cannot overcome, labeling it as a poor choice of material and mentioning this as a reason to choose their own materials over EIFS. When a client, home or building owner comes to you with this concern, a bit of information is all you need to adequately defend the strength and impact-resistance of EIFS.
Use heavier mesh if necessary
There are a few measures one can take to increase the impact-resistance of EIFS if desired or necessitated by certain factors. The easiest and most obvious is by increasing the weight of the reinforcing fiberglass mesh that EIFS incorporates within its components. There are several grades of heaviness available for this mesh, and utilizing different strengths of fiberglass mesh can go a long way depending on whatever the purpose of the EIFS wall might be. If it's going to be used on the side of a house, regular-strength mesh is more than adequate. If it's going to be used on the wall of a country club building that tends to get whacked with a stray golf ball now and then, heavier mesh used in vulnerable areas will protect it from impact. The same recommendation stands if the EIFS is somewhere near grade, such as next to a walkway or path where people will be walking frequently (or in Ontario, where snow shovels might often nick it) and the risk of the EIFS being bumped is higher than normal.
While the main purpose of the mesh is to allow for the shifting of the building without breaking or separating the foam pieces, it also offers this significant impact-resistance.
It is extremely important to use mesh made of the proper material. Reinforcing mesh used for EIFS applications can be made of either plastic or fiberglass, but fiberglass is definitely king. Plastic mesh melts easily when exposed to extreme heat, offers poor impact resistance and basically only exists because the alkali properties in the Portland cement, the material that normally encapsulates the mesh in EIFS applications, eats away at fiberglass and another option had to be created: plastic. To compensate for this, much of the fiberglass reinforcing mesh available is coated in plastic anyway, making coated fiberglass mesh the best option that has no negative consequences when compared to plastic mesh, which has plenty.
All of that said, the impact-resistance qualities of an EIFS wall are not solely the job of the reinforcing fiberglass mesh to defend. Base coat thickness and resin levels, the more of each the better, thicker foam and flexible sheathing will also provide extra impact-resistance. It's important to note that strength is not the only important factor, but springiness also: the more "give" a material has, the more likely it is to bounce back and the less likely it is to crack, shatter, or puncture.
Testing impact resistance
The "Perfotest" method that was developed in Europe relies on a spring-loaded bar being shot at the EIFS. If it bounces back, the EIFS is adequately impact-resistant, and if it creates a puncture, the wall is not strong enough. However, there are relatively few people who will walk up to an EIFS-clad wall and poke holes in it, and most impact-resistance tests developed for EIFS represent unrealistic situations the wall would never face in real life. Many designers will also specify the type of reinforcing fiberglass mesh that should be used as well as the minimum strength or heaviness of mesh they recommend.
All in all, the reason any wall cladding system needs to be somewhat impact-resistant is that it avoids a situation arising where it has to be replaced or repaired. However, one of the better qualities that EIFS brings to the table is that if there is indeed a puncture or dent in the surface, it's far easier and cost-effective to repair, and rarely if ever does it necessitate replacement because it only is affected near the surface where the impact took place, not deep within the material. This is especially useful to know if a small portion of the EIFS has to be cut away to see inside of it, which might be the case when a contractor or EIFS installer needs to learn more about the specific preexisting material that was used on a wall they're working on or repairing.