Whole Systems Analysis as a Basis For Decision-Making in Green Buildings

by Marc Rosenbaum, P.E.

Architects and engineers suggesting strategies which lead to better environmental buildings will be asked "what's the payback?" by the client at some point in the design process. My response, working on projects as a sustainable design consultant, goes like this.

Buildings are like ecosystems - complex assemblages of interwoven, interacting elements. The best buildings result from consistent attention throughout the design process to the overall performance. Although the final performance is certainly dependent on many independent decisions, disaggregating the whole into component strategies, and attempting to individually quantify their cost/benefit, frequently undervalues those strategies. Often a price tag can't be assigned to the most significant benefits.

The easiest components to evaluate from a payback perspective are building energy systems. One can predict the financial benefit which will accrue due to a more efficient lighting system, or chiller, or heat recovery ventilation. The situation gets murkier when one evaluates the benefits of building envelope upgrades. Increased insulation levels or higher grade glazings often aren't attractive investments when their payback is calculated. Here we can apply 20 years experience with superinsulated homes - when the envelope is upgraded to levels significantly beyond the usual payback threshold, the cost of the space conditioning system can drop dramatically. As a system, the envelope plus mechanicals cost the same in the base case and the upgraded case, with the substantial cash flow benefit of the reduced energy costs. An 1800 ft2, superinsulated, active solar home I designed in the early 1990s in Hanover, NH has averaged less than $200/year for back-up electric heat and hot water, and yet cost no more than typical custom homes with similar level of finish.

In commercial and institutional buildings, envelope upgrades can often be paid for by elimination of perimeter heating systems. Energy conserving mechanical system upgrades, such as enthalpic heat recovery ventilation ( a heat recovery device that recovers 75-80% of both heat and moisture), can be paid for by the resultant reduction in the size of the heating/cooling system, since ventilation may be the peak load in densely occupied buildings such as schools. The ensuing energy savings are tremendous, and the building comfort is improved due to higher relative humidity in the winter and lower relative humidity in the summer.

Many measures I recommend in my work as a consultant helping people create environmentally friendly buildings have multiple benefits, some quantifiable, others not. On a recent university building, the staged oil-fired boilers heating the building were located in an adjacent building, replacing the aging boiler in that building as well. This measure was accepted when the construction manager determined that the cost of the piping and controls needed to distribute heat to the new building was similar to the cost of building a new chimney and installing fuel tanks in the new building. However, I believe there are several other benefits of this measure:

  • the existing building's fuel use will drop due to the more efficient new boilers
  • there will be no combustion products or fuel storage in the new building, making it safer from a fire risk and indoor air quality perspective
  • the piping linking the two buildings begins a campus-wide distribution system, preparing the school for an eventual renewably-fueled central heating plant
  • avoiding a boiler room in the new building frees up at least 100 ft2 of space - the value of this space at $110/ft2 is $11,000!

The financial return of measures which make the building more durable are even harder to quantify. One of my clients had removed 25% of a metal roof on a recently occupied 30,000 ft2 building to retrofit peel-and-stick membrane as a band-aid fix for severe ice dams. Drawing on my experience troubleshooting these common building failures, I work with the architect and structural engineer to design roofs that are airtight, without thermal bridges (places in the envelope where highly conductive materials, such as steel or masonry, penetrate the insulation), and, if they are pitched roofs, with functional venting. Energy savings may result from these measures, but how do you value the absence of ice dams and their attendant damage? Similarly, use of a more costly and durable siding material which permits longer intervals between re-painting yields direct financial benefits, but the environment also wins - less paint has to be manufactured, and applied, so fewer pollutants will be produced and emitted over the service life of the building.

Other measures which may have unattractive paybacks (based on energy savings alone) prevent cold surfaces and the attendant risk of biological contamination. A good example of this is perimeter and underslab insulation in slab-on-grade or occupied-basement buildings, which prevent condensation in these vulnerable areas, eliminating the likelihood of mold. There are many other measures which protect occupant health. Some increase cost beyond the business-as-usual approach, others don't. How should we value the benefit of not creating a sick building? Better yet, how can we value the benefit of producing a building which enhances the occupants' well-being, with superior comfort and air quality, and the connection to the outdoors? Studies publicized by the Rocky Mountain Institute and others are beginning to indicate that the primary value of energy-saving strategies such as daylighting is in increased occupant productivity.

Sometimes a strategy proposed with environental quality intentions has a benefit which affects the entire project viability. The use of composting toilets on a new law school building has allowed the client to project a campus-wide, net reduction in water use on the small town water system that serves it, enabling the school to get a water supply permit in the face of a water hook-up moratorium. In another small, mixed use project, the use of composting toilets and graywater treatment may allow the owner to build on area previously held for on-site sewage disposal, which will place the building in a location which will dramatically improve its ability to harvest sunlight for daylighting and energy generation.

The use of resource-efficient materials is another area with hard-to-quantify benefits. The benefits often don't accrue to the building owner directly, they are shared instead by everyone. Examples include using materials with significant recycled or waste stream content, or not using wood from old-growth forests. In fact, many measures which address the three principal environmental threats that we are facing (global climate change, ozone depletion, and loss of biodiversity) increase building cost without direct payback to the owner. Henry Thoreau was remarkably prescient when he asked, " what is the use of a house if you don't have a tolerable planet to put it on?"

In order to have a "tolerable planet ", we need to go beyond conventional ways of thinking about costs and benefits, and create buildings which honor the Earth and its inhabitants. In nature, the whole exceeds the sum of the parts. Buildings are complex, and the only way to justify many green building strategies is to evaluate the whole building, and compare it to what would have been built without the green building intelligence. As long as a building meets its programmatic and budgetary goals, I feel it is a mistake to focus limited design resources on payback analysis - who wants to price a building they aren't going to build? Instead, let the design and consulting resources flow to where it is most needed, helping create the best building possible within the constraints, by exploring a true team-based, systems approach to green design.

- Marc Rosenbaum