Tech Talk: Exterior Finishes for Concrete Homes
By: Ed Sauter, Director, Concrete Foundations Association
This is the final in the series of articles dealing with RCF (Removable Concrete Form) homes. We have described the energy benefits, interior finishes and a host of other topics. This article will deal with the final, and most visible, aspect of the concrete home, the exterior finish.
The prior article dispelled the myth that the interiors of concrete homes are cold, damp, and dark. Insulation, wide windows and openings, and all the attributes that make frame homes desirable are also possible with RCF construction—with the added benefits of durability, quietness, low maintenance, fire resistance, and energy efficiency.
When you drive by a RCF or almost any other concrete home today you probably will not know that the home is concrete—unless the designer wants to express the concrete as part of his or her design concept. Brick, siding, EIFS, and other finishes are combined to produce modern designs, Victorian motifs, Colonial designs and literally every other form of expression commonly seen in today’s homes. The concrete home does not need to look like concrete—on the inside or the outside.
The type and installation method for the exterior finish of your concrete home is dependent on the type of insulation system utilized in the construction. While the thermal mass inherent in concrete homes aids in their energy performance, most locations in the United States will still require an insulation system to maximize performance. Typically 2 to 3 inches of insulation is sufficient and it can be positioned on the interior as described in last month’s article, the exterior of the concrete, or sandwiched between two layers of concrete.
Exterior insulation systems
Several systems position insulation on the exterior of the structure. The insulation is typically placed against the face of the exterior form and is held in place by a form tie with a plastic sleeve that enables the form tie to be broken off behind the insulation and removed, thus eliminating thermal bridging. Most of the plastic sleeves incorporate a flange that serves the dual purpose of holding the insulation against the wall and providing an attachment mechanism for exterior finish anchorage.
The simplest and most common form of exterior finish is an Exterior Insulation Finishing System (EIFS). EIFS systems are typically composed of a multiple coat cement-based product applied to a rigid polystyrene insulation, extruded (XPS) or expanded (EPS) polystyrene base. Since the base is already attached, EIFS is a natural and economical choice if the home is in an area where EIFS makes sense. There are several EIFS systems on the market. Make sure you research the warranty, requirements, and other aspects of the system before proceeding. As with most things, you get what you pay for. A reinforcing mesh or other type of fiber grid is required for most EIFS systems. One of the advantages to these systems is that it is relatively inexpensive to impart depth or detail to the foam covering, even layering or building outward from the primary surface. This type of detail can add architectural appeal to a home.
If the RCF exterior insulation system has attachment flanges that can be used to secure nailing strips, providing a continuous base for any of the conventional siding materials available. The requirements of the siding manufacturer may dictate what, if any, additional materials might have to be applied under the siding. Thin sidings may require a backer board of some type while self-backing or structural grade sidings may be able to span between anchors. Check with the siding manufacturer to be certain of the requirements.
A third type of finish is traditional masonry—most notably brick or stone. Anchorage to the flanges supports wind suction forces and prevents sagging of the material. The weight of a traditional masonry system will require either a wider foundation wall or footing, or a ledger angle on which the masonry can rest. It can be laid up in a traditional manner.
Sandwich insulation systems
Growing in popularity is a method of positioning the insulation between or “sandwiched” between two layers of concrete. There are different options and requirements for exterior wall finishes. The simplest and most cost-effective system would, of course, be exposed concrete. While that sounds harsh and non-residential in character, it is a very modern approach to materials. There are also several finish options to consider if the “modern” approach is not desired.
First, many forming systems actually have an embossed or cast pattern in the surface of the form. This texture or shape is imparted to the wall and can be painted after the concrete has cured. Brick is the most common form-embossed pattern but some other options, such as stone or adobe brick, can also be found. The wall can be painted a uniform color or, in the case of some of the brick patterns, the wall can be painted to match a mortar and then a paint roller used to highlight the projecting areas (the bricks).
Another option with concrete exteriors is the use of a form liner. Simple, one-time use vacuum-formed plastic liners are manufactured to simulate almost all common building materials. Other materials, such at textured plywood siding, can also be used as form liners. Be creative, concrete arrives at the site in a near-liquid state and will take on the shape of virtually anything that can act as a form, or a form liner. Once the forms are removed, the resulting concrete can be painted or stained.
Yet another option is sandblasting. This is an inexpensive operation that can soften the harsh tone and crisp edges of the concrete and expose the aggregate used in the mix. Sandblasting finishes can range from light, which will barely expose the aggregate giving an even appearance to the concrete, to a deep sandblast that makes individual stones visible. Most ready-mix plants have a variety of aggregates that can be used in their concrete mixes ranging from river gravel to crushed stone—each will yield a different effect.
There are also “thin-brick” options. These systems use real, kiln-fired brick, 1/2-inch thick, set in a receiver that holds the brick in place while the concrete is cast. The bricks are manufactured to a very close tolerance so they will fit in the receiver. The receiver and its base are attached to the traditional vertical forms. A wide range of brick textures and colors is available.
Lastly, if you are so inclined, you can cover the concrete with any of the same materials used to cover the insulation mentioned in the first part of this article but with some steps saved not having to provide support over the insulation.
Most of the RCF homes constructed in the United States use traditional wood framing for the roof structure. Even when the ceilings are cast-in-place concrete the roof forms are wood. There are several reasons. The complex roof forms typical in most of our homes are quite difficult (and thus expensive) to form in concrete. Similarly, the steep pitches prevalent in today’s homes are difficult, if not impossible to cast-in-place.
If you are looking for the all concrete home, roof included, you need to keep the roof form simple and the pitch low. Homes of with sloping concrete roofs are being built daily in Mexico and other regions of the world where concrete housing is more widespread. Precast another option for sloping roof shapes. Pitch is not as great a concern but complexity is. If you go the route of a concrete roof you still must decide on a finish. It could include raw concrete, painted concrete, cement or clay tile, or a system with nailers, deck, and traditional asphalt shingles.
The options for the exterior expression of a concrete home include not only all of the options you see on traditional wood-frame houses but some new options provided by the concrete itself. You can build in traditional styles, ultra modern, or replicate virtually any historical style. It is being done daily in the commercial concrete market—it simply takes someone with some imagination and a builder willing to be creative.
For more information on RCF concrete homes contact the Concrete Homes Council at (319) 895-0741 or visit their site at concretehomescouncil.org.