Stucco and wood perform differently with regard to moisture. Stucco needs to breathe to allow water vapor to pass through it and away from the home, while wood must be protected from exterior water damage.
Exterior stucco is strong, fire resistant, and can last a lifetime on a home. But it's the very features that make it so durable that require forethought before updating a stucco finish.
Stucco Basics Traditional stucco is a construction technique that has been used for centuries. It is formed by troweling on layers of plaster over a framework. Over the years, different mixes of minerals have been used to create the plaster. Today, Portland cement most often the key ingredient in a stucco exterior.
On a typical home, stucco plaster is troweled on in two or three layers, each at a thickness of about 3/8 inch, over metal lath attached to wood framing. A water-resistant building paper separates the plaster and lath from the framing. Plaster can also be applied directly to masonry or concrete walls.
Stucco is finished with a 1/8-inch hard coat or cementitious mix that may have powder or acrylic color added as well as an aggregate for texture and variation.
Stucco's Strength and Permeability In its hardened state, stucco is permeable and can breathe, allowing water vapor to escape rather than getting trapped behind the surface. Thanks to this breathability, stucco is able to resist rot and fungus.
To further keep stucco moisture free, perforated flashing called a weep screed is installed at the base of the exterior walls where the sill plate meets the foundation. This channels any water that enters through outlets or cracks down to the weep screed, and out and away from the surface.
Painting Problems with Stucco Painting stucco with latex or acrylic paint changes the permeability properties of the stucco system. "Everyone has found out through error that painting stucco is not the way to go," says Dennis McCoy, owner of RAM Builders of Lindon, Utah, a remedial contractor with a special focus on repairing or replacing stucco. "We see the difference in the dry rot we find which is 20 times worse behind painted stucco."
While paints are permeable to some degree, they provide a more moisture-resistant film than a cement-based thin coat. That means that while less moisture may pass in from the outside, the vapor that transfers from inside a house may get trapped in the walls. Terry Collins, concrete construction engineer with the Portland Cement Association in Skokie, Ill., suggests that breathability is sometimes looked at as a regional issue. For southern climates, where there are fewer problems with the freeze-thaw cycle and the heating season is short, non-cement paints won't be as problematic, unless there is an external moisture source such as bad flashing or a leak in the roof. In northern climates, the issue of breathability is particularly important. There's warm moist air on the inside, cold dry air on the outside. If moisture hits a nonbreathable layer in cold weather, it sets up a dew point inside the wall that will deteriorate the framing and insulation.
Some fracturing is also likely in a stucco system, especially in extreme climate conditions. Any flaw or fracturing makes proper coverage difficult. Stucco is composed of peaks and valleys with places of very thin materials that pinhole the system, making it near impossible to achieve waterproofness.
For those who choose to use paints or have acquired a home with painted stucco, "there definitely are maintenance issues," says Ivan Burgand, director of sales for Sider-Oxydro of Hawkinsville, Ga., which specializes in construction industry-related products. Those issues include the paint surface "bubbling," the need to regularly repaint, and necessary touch ups.
Experts also suggest checking with the paint manufacturer to see if there are issues with the paint adhering to stucco. Removing paint from stucco typically requires sandblasting or high-pressure water blasting.
Refreshing a Stucco Exterior Homeowners do have several options, however, in refreshing a stucco finish.
Recoat Recoating provides a long-term finish but does require prep work. The surface has to be cleaned so the new finish will adhere and permeable bonding agents, specialized for the climate, should be applied. Recoating also gives the homeowner the opportunity to try additional texture or new colors.
Limewash A coat of limewash is the traditional finish for the glowing "old world" stucco look. Limewash is a mix of lime putty and water that sets slowly. As it does, it absorbs carbon dioxide and produces calcite crystals that give the characteristic look. Similar to a stain, it penetrates the stucco yet allows it to breathe and release moisture. U.S. Heritage Group in Chicago is one company that custom produces these traditional limewashes.
Cement-based thin coats "Paints" made of Portland cement work well with stucco because they are made of the same material. Because Portland cement starts to cure as soon as it comes in contact with water, these "paints" come as powders that are mixed with water before application. They can be scrubbed, sprayed, or rolled into the surface and can come premixed with pigments for a variety of colors.
Fog coat This is a light spray-on application of a cement-based slurry that does not change stucco's ability to breathe.