Stucco

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If and when surface bond masonry becomes a popular way to put up masonry walls, it will be especially suitable for stucco exteriors, since the surface bonding material can be mixed with the stucco (which is precisely what contractors are doing). But whether blocks are mortared or dry- stacked, stucco over them (or over metal lath on wood for that matter) is one of the cheapest, and in my opinion, prettiest of low-maintenance exteriors. We used to live in a stuccoed house. When it was about 25 years old, I brightened up the white stucco exterior with a new coat of cement paint, but the walls didn’t need it for any maintenance reason. Applying stucco by hand is hard work, but the rough textures require little plastering skill to achieve. In fact, professionals use machines to blow on stucco through a hose.

A new or clean concrete block wall will take stucco very well without need of metal mesh or lath. Wet the wall first for good bonding. The base coat should be about /8 inch thick and left plenty rough. A second coat should be about ¼ inch thick and textured to suit your preference. The concrete plaster can be tinted with various colors to save painting.

With the new surface bonding cements, stucco offers increased ad vantages in addition to not having to mortar the walls. Surewall, for example, offers seven colors in its surface bonding cement and its finish coat to avoid the need for painting: Antique White, Natural White, Pale Yellow, Pearl Gray, Sandstone, Suntan, and White Jade. It also offers six textured finishes, from Light Lace to Travertine. Who ever thought humble stucco would hobnob in such snooty company?

Stucco over Exterior Insulation

Of greater significance than pretty colors and a nice finish is the new insulative qualities of these materials. The products being pioneered by the surface bonding companies allow for better insulation of masonry walls, and this insulation is on the exterior of these walls, where it's most effective.

A coat of surface bonding cement is first applied to the raw masonry wall. Then, using special adhesives and mechanical fasteners, polystyrene insulation panels are attached to the wall, and one or more coats of stucco are applied over the insulation. Lathing with metal or glass mesh is not necessary. Because of the resiliency of the insulation board and the adhesives used, the wall can move without cracking the surface coating. (Control joints are required in most cases.)



Cutaway of stucco over exterior insulation over masonry wall.

Manufacturers claim that this type of insulation increases heating and air-conditioning efficiency because the heavy masonry building mass in side the insulation envelope remains at a more stable temperature relative to outdoor fluctuating temperatures. This increased ability of masonry to act as a heat sink makes the insulation method especially interesting for passive solar systems, where heat from the sun needs to be stored effectively for nighttime winter use, and cold nighttime temperatures stored for daytime summer use. In fact, builders of passive solar homes pioneered the use of surface bond masonry on the masonry walls they built into their homes.

Two inches of polystyrene board stuccoed to the outside of a standard concrete block wall gives a total R-value of 10.56 (counting the R-value of the stucco and the block along with the polystyrene).

Other recent refinements in exterior insulation and stronger plasters make stucco look even more attractive for the future. Manufacturers like Penbar, Inc. (2808 North 2 Street, Minneapolis, MN 55411), apply their acrylic-improved concrete to extruded polystyrene rather than the expanded kind. Extruded foam has a tighter cell structure, less moisture permeability, and more compressive strength. Moisture could become a crucial problem in exterior insulation board and stucco finishes because, to use the words of the experts, “water may get behind the insulation, and when it freezes and expands, the system can delaminate from the substrate.” (“Delaminate from the substrate” means, in plain English, that it can come loose from the wall.) Extruded polystyrene has an R-value of 5 per inch, exceeding expanded polystyrene.

Penbar calls its exterior insulation/stucco system “the energy envelope,” touting the superior advantages of applying insulation on the exterior wall over insulation on the interior wall. Its exterior insulation can be applied to masonry, wood, even metal. Special adhesives glue the insulation board to the wall. Fiberglass mesh is then fastened to the insulation board with galvanized nails. Then, special mechanical anchor assemblies go through the mesh, the insulation, and into the wall to anchor all solidly.

A bonding agent is paint-rolled onto the mesh, the fiber-reinforced base coat of concrete is then applied, and finally, the finish coat is put on. The last operation is to spray on a clear or colored sealant. Penbar’s stuccos are available in varying shades of red, yellow, buff, orange, green, brown, and black, mixed integrally into the cement. The resulting stucco finish is advertised as being maintenance-free.

Dryvit (Dryvit System, Inc., P.O. Box 1014, One Energy Way, West Warwick, RI 02983) was used over brick on two Rodale Press buildings to add insulative value and to spruce up the exterior. The finish has held up well for the three years that it’s been there. In the process, at least 6 inches of insulating foam was applied to the outside of the brick, and the large, warehouse-like buildings are now energy efficient. Fuel bills have gone down appreciably. And Roger Moyer, a former Rodale employee who worked in one of the buildings, says that the appearance of the buildings was completely changed: they went from run-down brick to neat and modern. And as a bonus, he adds, the Dryvit has made the buildings quieter because the foam and stucco exterior provides more of a sound barrier to outside noises than did the plain brick.

Being what economists call a contrarian, I can’t help bringing my mental attitude toward the economy into the realm of philosophy and architecture. I am pessimistic about the long-term endurance of exterior insulation boards glued to house walls, even though there appears to be no problems. The improved stuccos will certainly last because even the unimproved ones do. But in 20 years will “de-lamination from the substrate” become a problem? It is a fair question to ask, I think. When I ask it, I’m assured my worries are unjustified.

Stuccolike Materials for Patching, Anchoring, Waterproofing

There are other improved, super-strong concretes on the market that can be used for stucco or, if too expensive for that, are perfect for patching, anchoring, and thin-coat waterproofing. Kan Kote, Inc. (P.O. Box H, Parker, PA 16049), has been around for a decade in the professional builder’s market, but only recently for the general consumer. Thin mixtures will waterproof and seal masonry and stone, even wood. With the addition of sand, it makes a cement with strength enough to endure considerable flexing and movement. Another stucco product, Advocote (Coatings International, 5123 Woodlane Circle, Tallahassee, FL 32303) will, like Kan Kote, stick to just about anything and has remarkable tensile strength. I bent a piece of sheet metal to which it was stuccoed over 45 degrees from straight before the stucco cracked. Another stronger-than- concrete product is Rockite (I-Jardine Products Company, Inc., 2186 Noble Road, Cleveland, OH 44112).


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Updated: Thursday, March 5, 2009 18:06