Stucco and Porosity

Stucco is a material made of aggregates, a binder, and water. Stucco is applied wet and hardens to a very dense solid. It is used as decorative coating for walls and ceilings and as a sculptural and artistic material in architecture. Stucco may be used to cover less visually appealing construction materials such as metal, concrete, cinder block, or clay brick and adobe.

The difference in nomenclature between stucco, plaster, and mortar is based more on use than composition. Until the latter part of the nineteenth century, it was common that plaster, which was used inside a building, and stucco, which was used outside, would consist of the same primary materials: lime and sand (which are also used in mortar). Animal or plant fibers were often added for additional strength. In the latter nineteenth century, Portland cement was added with increasing frequency in an attempt to improve the durability of stucco. At the same time, traditional lime plasters were being replaced by gypsum plaster.

Traditional stucco is made of lime, sand, and water. Modern stucco is made of Portland cement, sand, and water. Lime is added to increase the permeability and workability of modern stucco. Sometimes additives such as acrylics and glass fibers are added to improve the structural properties of the stucco.

Stucco is a blend of Portland cement, lime and aggregates, none of which is negatively impacted by the presence of water. In fact, the mix that makes Portland cement plaster will strengthen over time when exposed to water. The benefits of cement stucco make it a favored cladding for building exteriors because it is highly water resistant, noncombustible, pest resistant, durable, low maintenance and capable to be seamless even in radius surfaces.

One myth that continually pops up is that cement stucco cladding is porous.

POROUS: Defined as a material having small holes that allow air or liquid to pass through.

AIR: The International Energy Conservation Code (IECC) under section C402. Assemblies, item number 2, lists a Portland cement/sand parge, stucco or gypsum plaster having a minimum thickness of 1/2 inch (12mm) as an air barrier assembly compliant to the IECC. Cement plaster is therefore not porous to air per the code.

WATER: While cement plaster will absorb water, the water will not pass through properly mixed and applied cement plaster. Independent third party studies confirm this fact:

FEDERAL TESTING LABS (1996): Test Report No. 96-3484. This independent test used mixes compliant to ASTM C 926. Application was 3/4 inch thick basecoat (scratch and brown) with no finish coat applied. All panels were 3/4 inch thick cement and moist cured for 24 hours. Water testing was done 29 days after cement application, simulating a moderate wind-driven rain per ASTM E514. The water volume was 112 gallons per hour and sprayed continuously for two hours at the same location on the panels. The back of the plaster cement was exposed and no panel exhibited any sign of water seepage during or after the test. Similar tests were conducted in the 1970s in southern California with similar results. The Federal Lab test was sponsored by the Northwest Wall and Ceiling Bureau, and the full report is available by contacting the SMA or NWCB.

NCMA RESEARCH AND DEVELOPMENT LABORATORY (2007) Project No. 05-466B. The test was to determine water resistance of cement plaster (stucco) over masonry units. Thirteen walls were constructed with various cement thicknesses over standard Concrete Masonry Units (CMU) One wall had no stucco applied to the CMU and was a control wall. Mixes were all ASTM C 926 compliant and water testing was done per ASTM E 514. The walls with skim coats of 1/8, 1/4 and 1/2 inch cement plaster resisted water penetration for up to four hours. The control wall, no stucco applied, leaked in 30 minutes.

Hurricane. The NCMA then decided to test at levels representing a hurricane condition. The wind was increased to 155 and 180 mph. The results concluded that all cement plaster resisted water penetration at low to moderate wind pressures. The 1/8 inch skim coat kept water out at lower wind speeds. The 1/2 inch cement coatings resisted water entry even at 155 mph, and only minimal water leakage at 180 mph after 24 hours of continual water spray. The full report is available from the Florida Lath and Plaster Bureau or the SMA.

Both tests for water and the code statement on air verify that properly mixed and applied cement plaster (stucco) is not porous.



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