Structure as Finish: The Pros and Cons of Leaving Off Layers
Consistent with the environmental goals and theme of the Deramus Education Pavilion at the Kansas City Zoo, Bob Berkebile and Tom Nelson of BNIM Architects in Kansas City initially included a number of high-end finishes. Surfaces specified for this $16 million project included 100% wool carpet and a floor made from discarded stone fragments from a local marble company.
But due to a sudden leap in construction costs in the region near the completion of design, the project ended up over budget and many of the finishes were either downgraded or removed entirely. Noting that “everything that could readily be removed was eliminated during value engineering,” Berkebile now strives to use finishes that are integral to the structure right from the start.
Berkebile’s creative response to the problem of making functional and aesthetic surfaces integral to the structure is not really new. Many buildings, especially architectural landmarks and high-end custom homes, make a statement by exposing dramatic structural elements. Exposed structures hark back to antiquity and continue today in many indigenous building systems. The elegant simplicity of having one material perform multiple functions still appeals to many designers.
The opposite approach, of using separate materials for structure and finish, also has a logic, however. Materials that are designed to perform one basic function are more likely to fulfill that function well than those that are doing double-duty. The need for thermal insulation in building envelopes also increases the likelihood that separate finish layers will be needed to contain and protect the insulation. And keeping distinct functions separate can be helpful when the building is adapted to different uses in the future.
Environmentally conscious designers and builders often expose structural elements in their designs for a number of reasons, including design aesthetic, cost, indoor air quality, and reduced material use.
This article explores the pros and cons of such an approach, and offers examples that maximize the benefits and minimize the drawbacks.
The Rise and Fall of
Most preindustrial building systems consist of one basic material, such as stone, earth, or logs, exposed both inside and out. Depending on the material, the thickness of the walls and roof, and the climate, these buildings may be more or less comfortable and durable. Some stone and earth buildings with thick walls have survived intact for centuries. In hot, dry climates they can be remarkably comfortable with their enormous thermal mass. Log cabins offer better insulation than stone, making them more acceptable in the colder climates, though they are notoriously leaky and require regular attention to air-sealing as the logs dry and shrink over time.
With the growth in human population and the advance of industrialization, most of these indigenous building methods are no longer viable on a wide scale for several reasons. First, they tend to require large quantities of high-quality, structural material, whether in the form of whole, straight logs; large, high-quality stone; or agricultural-quality soils for adobe. (In parts of China the government is reportedly seeking to convert from adobe to concrete as a standard construction material due to the scarcity of cropland.) Second, they are rarely capable of providing the level of thermal insulation that we came to expect in the 20th century, meaning either that comfort is compromised or large amounts of energy are required for heating and/or cooling. As we’ve often noted in
EBN, efficient use of materials is not generally a good choice if it comes at the expense of inefficiency in building operation. Third, these construction methods are typically very labor intensive, which makes them prohibitively expensive in industrialized countries (but not among owner-builders or in countries where labor is readily available and money is not).
As these simple, monolithic structures have been replaced by more complex, layered ones, materials and products designed to create finished surfaces have emerged. On the exterior, a wide range of cladding and siding materials must provide weather protection while also expressing a building’s public character. Ironically, though no longer used as a structural material in North America, bricks are still popular as a face veneer, both for their durability and for the image of solidity that they project. On the interior, surfaces such as plaster and gypsum wallboard are intended as a substrate for additional finishes of paint or wallpaper. In many buildings the finish surface is necessary largely to contain and protect insulation, thus unconditioned buildings without insulation, such as garages and porches, are excellent candidates for an exposed-structure approach.
To counter the additional cost of multiple layers, many finish materials are cheap, of marginal quality, and may introduce problems of their own. These problems include poor durability, offgassing of volatile organic compounds (VOCs), or toxic byproducts of their manufacture. Low-end carpets tend to be high in VOC emissions, for example, and cellulose-based ceiling tiles can harbor mold growth if they are exposed to overly humid air. For some designers, the attraction of exposed structural elements stems from an interest in avoiding these problematic materials. High-end finish materials that minimize these drawbacks can be budget-busters, so the solution emerges of using no finish layers at all.
A number of common building systems combine the functions of structure and building skin. At the more economical end of the spectrum, concrete masonry units (CMUs, commonly known as concrete blocks or cinder blocks) are widely available with a finished surface. Split-face CMUs are cast in a double mold and then put through a splitter to provide an uneven, textured surface where the two blocks were connected. Split-ribbed blocks are similar except that the connection between the two blocks isn’t continuous, so a ribbed pattern is created when they are split apart. Ground-face CMUs are honed to a smooth, polished surface. Glazed concrete blocks are also available, although these are typically only exposed on the interior.
CMUs may be used structurally, with steel and concrete reinforcing at regular intervals in the cores, or as infill or cladding around a steel frame. Although they are popular for use in commercial/industrial buildings (including big-box retail) and in low-budget schools, they can also be used more sensitively. While CMUs can be used to create an entire wall, providing structure and both interior and exterior surfaces, such walls are difficult to insulate effectively. Insulation inserted inside the cores of standard CMUs is compromised by the thermal bridging effect of the concrete webs. Appropriate thermal and moisture protection for most climates can only be achieved with additional materials behind the block, and a layer of interior finish—or exterior finish, if the block is exposed to the inside—is needed to cover the insulation.
Tilt-up concrete construction uses concrete panels that are cast flat on the ground or on a deck, and then tilted into place once the concrete has cured sufficiently. The panels may be attached to a structural steel frame or bolted to each other to create their own structure. Polyethylene form-liners are sometimes used to create a texture in the exposed surface of the concrete (usually the outer surface). These are available in different forms to simulate various surface materials. The textured concrete surface may be painted or left exposed (if additional weather protection is not required). With integral pigments many colors are possible, although getting a uniformly consistent color on a large job can be challenging. Using coal fly ash to replace much of the Portland cement can reduce the wall’s permeability to moisture while also creating a beige color instead of the typical gray.
Like tilt-up panels, pre-cast concrete elements can also be specified with a wide range of surface finishes. For large commercial projects the possibilities with customized precast concrete are limitless. Smaller projects may be limited to certain stock options, such as the Superior Wall foundation system, which is designed to be exposed on the exterior and insulated from within, allowing at least some of the concrete to remain exposed as an above-grade finish. Mike Trolle of Homes for Health & the Environment (of Ridgefield, CT) used Superior Wall for a garage that is largely above grade. He reported that the brushed concrete exterior finish looks similar to stucco and should be maintenance-free.
Cast-in-place concrete can also be designed to provide both structure and skin, using the form-liners described above or other techniques during the pour to create acceptable finish surfaces. Exposed-aggregate concrete, in which round stones are used and the cement and sand is washed away from the surface soon after the pour, is one example of such an approach.
Many earth-building technologies have been modified to varying degrees to take advantage of modern technologies. For example, air-dried adobe bricks, which are made from sand, clay, and straw, are commonly
stabilized today. Emulsified asphalt is usually added to the mix at concentrations of 3% to 5% by weight for fully stabilized adobe; less than that for partially stabilized.
Architect Nader Khalili of the California Institute of Earth Art and Architecture is promoting the practice of ceramic dome structures that are shaped out of clay and fired from the inside, creating a durable and beautiful finish. Another traditional building method, rammed-earth construction, involves compressing earth in layers between concrete-style forms. Rammed earth is now often stabilized with cement and reinforced with steel, especially in earthquake-prone areas.
A more innovative approach practiced by Napa, California builder David Easton involves the use of gunite technology to shoot stabilized earth through a large hose against a single form (see
Depending on the degree to which these materials are stabilized with additives such as asphalt or cement or protected from the elements with large roof overhangs, their suitability may extend beyond the relatively dry climates where they are most common. The high thermal mass of these systems makes them comfortable in areas where very hot or cold periods are short enough to be dampened by the lag time for heat transfer through the materials (see
Vol. 7, No. 4). Some builders have modified these earth-building methods to incorporate insulation materials, but these methods tend to increase the cost and complication of the system significantly. As a result, earth-building systems are best suited for climates that don’t experience long cold spells.
Cob construction uses earth and straw placed and shaped by hand, as has been done for thousands of years. Cob uses more straw than adobe, so it has somewhat better thermal insulation characteristics, but is still far from the performance we’ve come to expect from modern insulation materials.
Landmark buildings with large budgets may still use traditional materials, such as stone, to provide both structure and interior or exterior finish. Such buildings can be very impressive and enduring. As Steward Brand notes in
How Buildings Learn, they are “high-road buildings” that will endure if people are committed to maintaining them. On the downside, they are very difficult to modify and are therefore not as adaptable to changing uses over time. They also require special attention to issues of thermal and moisture protection to meet modern expectations without huge energy demands.
On a more modest scale, the sandwich walls of load-bearing straw-bale buildings are an example of insulation (the bales) contained between thick layers of plaster that help support the structure. Insulating form systems using cementitious blocks, such as Durisol®, Faswall®, and Rastra®, can provide an almost-finished surface, although a layer of plaster or stucco is usually desired for aesthetic reasons. In these systems it is not, strictly speaking, the structural concrete itself that creates the skin, but rather the formwork that contains the concrete.
At the opposite extreme conceptually from concrete and earth are tensile-fabric structures, including everything from lightweight and portable camping tents to the huge and durable roof systems of the Millennium Dome in the U.K. and the Denver International Airport’s main terminal. While it can be difficult to insulate these structures effectively, a double-skin system with trapped air does provide some degree of thermal protection, and solar gain can be controlled by coatings on the fabric. Interestingly, the Millennium Dome was originally envisioned as a temporary structure with a two-year service life. But when political and environmental pressures forced a change from a PVC-coated polyester from Germany to a domestically produced teflon-coated fiberglass, the entire vision of the project changed. The teflon skin has an expected life of at least twenty-five years, and the entire Dome is now seen as a much more permanent installation.
On wood-frame houses a cost-saving approach that has been around for a while is the use of structural sheathing that doubles as siding. Joe Lstiburek and Betsy Pettit of Building Science Corporation in Westford, Massachusetts are considering this approach for a project in southern California, where rain is minimal and structural requirements are severe due to the threat of earthquakes. Their approach requires coating of both sides and all four edges of the sheathing, and careful flashing at all openings and joints.
Regardless of what it is made of, special care must be taken with any structure-as-skin system in wet climates. Building systems that rely on a single barrier to keep water out are prone to failure, especially when there are pressure differentials, such as wind or building mechanical systems, pulling moisture in. A well-designed system with multiple layers not only adds some redundancy, but it also offers an opportunity to equalize wind-generated pressures within the wall, thereby reducing the driving force inwards.
Structure as Interior Surface
Many of the systems and technologies mentioned above that combine building structure and skin can also be used to create finished interior surfaces.
There are variations on these systems that are more specific to interior finishes and interior structural elements, such as exposed floor slabs. Depending on the needs of the space and its aesthetic, a polished and sealed concrete slab may provide an acceptable floor. A wide range of very sophisticated finishes may be achieved with integral colors and stamped-out patterns.
Designers experienced in these methods note that it can be difficult to protect the surface of a slab during the construction process, so integrally colored or stamped concrete floors are often created by adding a thin “topping slab” onto the existing structural slab. “You simply cannot run a construction project on the structural slab for a year and expect it to appear decent enough to finish nicely,” notes Anne Whitacre of Zimmer Gunsul Frasca Partnership in Seattle. As a result, the finished concrete is really a separate finish surface rather than an exposed structural element. The cost of this approach is comparable to low-end flooring materials such as vinyl composition tile.
On smaller projects or within a limited area, it may be feasible to protect a finished slab during construction with a used carpet turned upside down, for example. If the slab is isolated from the foundation, it may also be possible to pour and finish a slab-on-grade after most of the construction has been completed.
Another approach to exposed structure as interior finish is to use metal or concrete decking as an exposed ceiling. In homes, such exposed ceilings typically require insulation from above—sound insulation if there is another floor overhead, or thermal insulation if there is a roof. In large buildings that typically run utilities above a dropped ceiling, the services can either be exposed overhead or contained within a raised floor system. Exposing the ducts and wiring conduits overhead requires extra attention to their design and installation so they work visually, and extra work to keep them clean. There may also be fire code issues with exposed structural members. Access floor systems have their own set of costs and benefits (see
Exposed structural elements may be psychologically reassuring to occupants if they appear to be robust and stable, or exciting if they appear risqué. Many construction methods dating back to antiquity are most notable for their structural arches, columns, or wooden beams (such as the vigas used in earth construction).
Traditional European Tudor construction is known for exposed “half-timbers” in rectangular and diagonal patterns on the exterior. Traditional Japanese framing is also generally hidden in the walls, except for a prominent wooden center-post and an exposed roof structure—sometimes using whole round logs with bends and curves that are assembled using sophisticated joinery to dramatic effect. Such craft is also evident in the exposed structure of Japanese temples, where even squared-off timbers are carefully assembled so that the posts are always standing right-side up, the way they grew.
Old American post-and-beam houses, with varied degrees of sophistication, commonly hid the structure behind split-wood lathe, horse-hair plaster, and lime paints. Traditional post-and-beam construction has been revived in North America in the last thirty years, and in these more recent homes the structure is usually celebrated. Larger structural elements are often completely exposed on the interior, with the entire building wrapped in insulated panels. In these designs the posts are commonly stouter than structural requirements alone would dictate to provide the desired aesthetic and psychological reassurance of stability.
In larger buildings, exposed structural members are used in innumerable ways. In addition to the aesthetic possibilities, structural members that are exposed to the interior can provide useful thermal mass. In climates with large diurnal temperature swings, using outdoor air to cool the structure down at night can greatly reduce the amount of cooling needed during the day. In a recent renovation to the Martin Luther King Jr. Civic Center in Berkeley, California, for example, ceiling tiles were removed in many places throughout the building to exposed the concrete slab above. According to consulting engineers Ove Arup and Partners, this thermal mass, combined with sophisticated window controls, allows the building to rely entirely on natural ventilation for cooling.
Many newcomers to the idea of structure-as-finish assume that by leaving off materials they will always save money. This is certainly possible, but not necessarily the case. Often, the additional care and attention required during construction to build a structure that is attractive enough to leave exposed more than offsets the labor and material savings of the avoided finish materials. “For the most part, if you wish the structural elements in a building to be left exposed, you are asking concrete workers, iron workers, and even rough carpenters to exhibit a degree of fineness to their work which is not typical of their trades, and it will influence cost,” says Anne Whitacre. Barry Smith, of Simons Construction Ltd. in the United Kingdom, concurs: “Unless you are planning to surface-mount utilities and services, a finished-structure building takes a lot of pre-planning—you have to know where
all of your services are going before you start. To fast-track something like this is very difficult.”
Depending on the building’s function, size, and location, special measures may be needed to address fire protection concerns. “Wrapping the structure in gypsum wallboard is an economical means to provide fire protection,” says Muscoe Martin of Susan Maxman Architects in Philadelphia. If that wallboard is omitted, there may be additional expense for alternative (less conspicuous) fireproofing of structural members, partitioning of the space into smaller zones, and/or additional sprinklers.
Of course, a key factor in this equation is the quality and cost of the finish materials that would be omitted: “We are forever being told by contractors that a two-by-four lay-in ceiling with ‘standard’ fluorescent troffers is cheaper than no suspended ceiling with exposed ducts, conduit, and pendant light fixtures,” notes Martin. While this may be true, it doesn’t begin to account for the fact that uniform, direct lighting from the standard system is a low-cost solution that is not likely to create a particularly pleasing environment. In fact, one argument for avoiding a dropped ceiling is to allow better daylight penetration from perimeter lightshelves. Even without daylight, direct-indirect ambient lighting using hanging pendant fixtures is usually preferable to pure downlighting.
Even very tight tolerances can be achieved at reasonable cost if the building is designed carefully, with constructability in mind, suggests architect Huston Eubank of the Rocky Mountain Institute’s Green Development Services. Eubank notes that prefabricated components may cost more if produced to very tight tolerances, but that the extra cost can be recouped due to the ease of assembling such well-made components.
There are so many different ways that structural elements can be treated as finished surfaces and accents that there is no simple answer to the question of whether it is a good idea. If layers of finish can be omitted without adding to the embodied energy or other environmental burdens of the exposed structure, that is a benefit. But it is also important to consider issues such as future adaptability and reuse—an exposed-structure building may be more flexible for future uses, or it may be less so, depending on the design. Conversely, layers of finish might require replacement for a future use of the space, but having them there might allow the underlying structure to be more generic and adaptable.
In terms of future reuse of the materials when the structure is eventually removed, Smith notes that “structural finishes are much better for eventual deconstruction and recycling because you don’t end up with a composite of structural material and finish materials, so you have far less to strip out before demolition.”
High-end solutions can, and often do, highlight exposed structural elements. If structure-as-finish is to be a cost-saving approach, however, it may require challenging some user expectations regarding the appearance of a building or interior space. In the words of Amherst, Massachusetts architect Bruce Coldham, “Challenging public taste is what we as designers do all the time, but it is best done with a full understanding of the pros and cons.” As with so many design approaches, the best advice may be simply to provide some useful questions to ask. The answers remain up to the designers. Stewart Brand concludes: “There is no universal answer. There can be advantages either way, so it all comes down to execution.”