In our coastal area of northeastern NC, stucco is not a common choice for exterior home finishing. However, you do see the occasional stucco home (or building), and it's important to know the difference between traditional stucco and synthetic stucco (also called EFIS, which stands for External Insulation and Finishing System).
What is the difference between traditional stucco and synthetic stucco?
Traditional stucco is essentially a layer of fine-grained concrete that is applied over a waterproof paper barrier and galvanized wire mesh, which creates a rock-hard layer around the outside of the home. The outer layer of traditional stucco acts as the primary barrier against the elements on the exterior of the home. The waterproof paper and metal flashings underneath act as a secondary barrier should any moisture get behind the facade - the secondary barrier would then direct water back to the outside. Traditional stucco is considered "breathable" since it is a porous material. Due to the material's porosity combined with the "weeping gap" that is created by the layered installation system, water that gets behind/into traditional stucco is able to move freely back out of the wall.
Synthetic stucco (EFIS) is very different, and the most important difference is that EFIS does not "breathe." Once it gets behind the EFIS layer, moisture vapor cannot escape through the stucco coating. EFIS is essentially a non-porous, plastic coating (hence, the "synthetic" name) that is applied over a board attached directly to the sheathing. There is no "weeping gap" behind EFIS like you would see with traditional stucco installations. The lack of breathability is the biggest problem with this finishing system - it can result in high moisture build-up on the interior structure, which can lead to wood rot and mildew/mold. These problems are especially predominant in areas with high humidity or heavy amounts of rainfall.
How can I tell which kind of stucco I'm seeing?
There are a few types of "tests" you can perform to determine if the stucco is traditional or synthetic, which are listed below. However, if you are still unsure, the only way to know for certain is to ask a professional inspector or engineer.
Just as it sounds, the knock test consists of actually knocking on the structure with your hand as you would knock on a door. If it sounds hollow or feels somewhat soft, there is a good chance it's synthetic stucco. Traditional stucco will sound and feel like you are knocking on rock or concrete/brick.
If there are any penetrations in the stucco, like where exterior light fixtures, electrical receptacles, gutter straps, doorbells, etc., look inside the holes. If you see styrofoam-like boards, that is a telltale sign of EFIS. If you see metal mesh or flashing peeking through, it's most likely traditional stucco.
For this test, you'll need to have access to the bottom side of the cladding, near the foundation. Put your hand under the bottom edge of the stucco and feel for gaps. EFIS will protrude out away from the foundation by 3/4 of an inch or more. Traditional stucco will not be as thick.
Again, none of these tests are a perfect way to determine the type of stucco, but they can help you gain some insight to make an educated guess. Sellers and agents are supposed to disclose the presence of EFIS (or previous presence) if it is known; however, many sellers and/or agents would not know if EFIS had been replaced with a different type of siding by a prior owner. Be sure to rely on the professionals if the absolute identification of EFIS is a major concern for you.