Structural engineers can be green: Five strategies to impact the sustainability of a building project

Often overlooked, the structure and foundations of a building have a significant impact on the sustainability of a building project - especially the building's future energy performance. Here are five strategies that your structural engineer can use to help: 1. Respect the building insulation envelope. The thermal effectiveness of the building envelope depends on the performance of the complete system, including any elements which produce thermal bridging, or details which allow air infiltration through the air barrier. A building's structure interacts with, and sometimes is part of, the building envelope's insulation and air barrier systems. For example, with an R-value about 0.004 per inch, steel serves as a thermal bridge, and so it should be kept out of the insulation layer as much as possible. Cold-formed steel studs, which frequently are the economic and material-efficient choice, should have continuous insulation running along their exterior surface, instead of relying on insulation between the studs. Concrete also conducts heat fairly efficiently. The exterior edges of concrete slabs lose a tremendous amount of energy to the outside, if the edge is not well insulated. Don't be fooled by the notion that this is only a small percentage of the building envelope - this strip is contiguous with the large thermal mass of the slab. 2. Consider structural insulated panels. Structural insulated panels, (SIPs), are sandwich panels with rigid insulation in the middle and oriented strand board (OSB) adhered to both faces. Structurally, the OSB works compositely with the insulation to allow the factory-fabricated wall and roof panels to span between supports and serve as both roof deck and walls - in place of stud walls. They have great material efficiency and have been used successfully in thousands of buildings. SIPs used as exterior walls compare very favorably to conventional 2X6 stud walls with fiberglass batt insulation. In addition to their energy performance, SIPs result in a reduction of jobsite waste. They generally install quicker than standard framed buildings. Many contractors do not yet have experience in this type of construction, but those who do will be well positioned to have an edge on what is sure to become a growing market. 3. Use fly ash or GGBFS in concrete. The production of Portland cement releases approximately 8% of the world's total emissions of carbon dioxide from man-made sources. Approximately one pound of carbon dioxide is emitted for every pound of Portland cement produced. Fly ash and other supplementary materials such as ground-granulated blast furnace slag (GGBFS) can be used to reduce the amount of Portland cement in concrete, by 20% or more. Both materials are by-product materials which would otherwise be discarded. Fly ash and GGBFS change the properties of the concrete somewhat - contractors need to be aware of, and plan for, their use. Their main effects are a slight slowing of curing time, which could be offset by the use of an accelerating admixture. The ultimate strength of the concrete will be higher than a comparable mix made with straight Portland cement. Flatwork with fly ash or GGBFS will bleed later and less than conventional concrete, so finishers need to be aware and plan accordingly. Curing is more critical, for the same reason. The resultant concrete will be less water permeable and, presumably, more durable than concrete with no fly ash or GGBFS. 4. Consider insulated concrete form walls. Insulated concrete form (ICF) walls use proprietary rigid insulation forms that remain in place to serve as building insulation. They have been around for several years, originally being used mainly for residential basement walls. But their use as superstructure walls is increasing. Like SIPs, their energy performance is superior to standard batt-insulated stud walls. The additional benefits include quicker construction time, with no forms to erect and strip. The insulation is useful to meet cold-weather concreting requirements in the wintertime. And as the industry has matured, several options have become available for finishing both the inside and outside faces of the walls. 5. Use frost protected shallow foundations. Traditionally, the bottoms of footings are set at an elevation four feet or more below grade, to keep them below frost depths. But if the insulation is positioned outside the foundation wall, the heat from the building is directed downward, reducing the frost depth near the building, allowing footing depths of two feet or less in most of the state - if properly designed. This is an example of a sustainable, money-saving idea that differs from the traditional construction system. Many buildings are built with vertical insulation on the inside face of the foundation wall, and frequently horizontal insulation along the outer edge of the underside of the slab on grade. Such a configuration of insulation prevents the building heat from reaching the footing bearing elevation, and does nothing to stop the heat loss out the slab edge. Reducing the foundation depth of a building by half has obvious financial benefits, as well as material reduction - and the reduction of the environmental footprint that accompanies it. In addition, a properly detailed shallow insulated foundation system can improve the energy performance of the building by reducing the thermal loss at the slab perimeter, as mentioned previously. If you ever come across the rumor that it costs more to build green think of this detail! As in all good design, team collaboration is essential for the best ideas to emerge. After all, the design and construction team is all working on the same building, likely with the same aim: To create a high-quality, economical, energy efficient building with as little environmental impact as possible. James D'Aloisio, P.E., SECB, LEED-AP, is a principal of Klepper, Hahn & Hyatt, Syracuse, N.Y.