Against Green Roofs, as Such

Against Green Roofs, as Such

By: Jerry Yudelson — Tuesday, October 25, 2005

Source: Columnist

The green building of the future is taking shape: following the European model, it will have photovoltaics and/or a green roof (note that these two “eco-goods” can be in conflict, because the rooftop PVs will cause shading for the green roof), rainwater harvesting and reuse systems, low-flow water fixtures, no irrigation, zero-VOC finishes, contain no PVC or similar products, have a low-energy roof (note again the potential conflict between PVs, green roofs and Energy Star® roofs with high emissivity, as specified by LEED), use far less energy than a conventional building, etc.

What I want to explore in this article is how “techno-fixes” such as the currently popular green roofs and photovoltaics may be taking us away from intelligent thinking about sustainable design and green buildings. For example, take a building with a 30,000 sq.ft. roof, four stories high, or 120,000 sq.ft. total area, an average size for a LEED-registered building. Assume for a moment that PVs could cover 20,000 sq.ft. of roof area and produce about 160 kW (peak) power, at 8 watts per sq.ft. output. In a typical northern climate, that would yield about 240,000 kWh per year of gross energy production (at 1,500 kWh per kW of peak power rating), and about 200,000 kWh per year of net energy production, assuming 15% losses through the inverter. Let’s do the math. That same building will use about 15 to 20 kWh per sq.ft. of total energy, or 1,800,000 to 2,400,000 kWh equivalent per year. Saving 10% of building energy use would equate to the entire output of the PV system.

Now let’s look at costs: a 160-kW PV system will cost about $6,250 per kW installed (more for a smaller system), or about $1 million. Which is better: to spend a million dollars on a PV system to supply about 10% of a building’s energy use, or to spend far less (as any good mechanical engineer would tell you she can do) to save the equivalent amount of energy? The embodied energy of the PV system is also likely to be far higher than the embodied energy of the energy-efficiency measures (better glazing, higher-efficiency HVAC systems, reduced plug loads, etc.), because it is made from semiconductor-grade purified silicon using a variety of highly toxic chemicals.

In fact, just working to reduce lighting and plug loads by 1.25 watt per sq.ft. would reduce the average LEED building’s power demand by 160 kW, equaling the entire power capacity of the PV system and exceeding its output by 33% (2000 average office working hours vs. 1500 assumed for the PV system at peak output). How much extra would it cost to outfit an entire office that replaced CRTs and “tower” computers with laptops, used T-5HO lamps instead of T-8s, bought Energy Star office equipment, etc.

So here, we have a basic energy conundrum: reducing demand is always far more cost-effective than increasing supply. (There may be financial and tax incentives for PV systems that would change the relative economic calculations in a given locality, but these inducements can always go away, and there are in many places the same financial benefits for saving energy, no matter what the source). The average LEED-certified building gets about four energy points, representing about 30% savings (in LEED v. 2.1). Focusing on integrated design, engineers and architects could easily move this up to 40% savings, sometimes even higher.

Take another example: commissioning a building has been shown repeatedly to get 10% to 15% more energy performance out of a building, at a cost of less than $1.00 per sq.ft. in a typical building. So for $120,000 for our example building, we could get the equivalent energy output of the entire $1 million PV array. Would it be responsible for engineers to design a PV system for an un-commissioned building? Hardly. Does it happen? Now and then.

To give an example of the power of integrated design and collaborative problem solving, our mechanical and electrical engineering firm recently worked with GBD Architects of Portland, Oregon, on design of a 400,000 sq.ft. mixed-use health care and medical research building in urban Portland, which models at 62% savings vs. the LEED version 2.1, ASHRAE 90.1 standard. We were able to SAVE 12% of the initial HVAC budget through this approach, effectively getting a super-high-performance building with first-cost savings on energy, enough to afford 60-kW of PV sunshades on the entire south face of the building and a complete rainwater harvesting system, with more than a million dollars left over. Isn’t this the definition of good design? Saving money while boosting performance? (See the chapter on “Tunneling Through the Cost Barrier,” in Hawken, Lovins and Lovins 1999 book, Natural Capitalism.) Why can’t we do with buildings what the entire world is doing with other manufactured products, such as cars, chips and washing machines, i.e. lowering costs and boosting performance?

Now let’s go back to green roofs, our first example. Green roofs have many benefits as amenities in cities. They help reduce stormwater peaks, give visual relief to higher-up building occupants, create refuges for office workers (if the liability and security folks will actually let people be on them) and create some bird habitat. They also cost $15 to $20 per sq.ft. more than just about any other green measure. On tall buildings, this cost (per square foot) is obviously reduced by the number of floors, but it’s still a $450,000 to $600,000 amenity for the average 30,000 sq.ft. roof. You can buy a lot of energy conservation with that, as well as a rainwater reclamation and reuse system that will capture 100% of the stormwater in most American temperate climate zones and re-use it for toilet flushing and irrigation, saving more money on building water purchases.

Let’s close the loop: there are good green building technologies and approaches that have strong cost vs. benefit profiles. These should be employed first. For example, siting the building trumps everything else. A suburban green office building is going to have a high energy use profile, even with 40% energy efficiency, a green roof, PVs and only a code-amount of parking, owing to the commuting energy use of the occupants. Conversely, a 20% efficient urban building renovation will have a much lower energy profile, with or without a green roof or PVs.

The most extensive study to date of an urban district, the 2004 Lloyd Crossing Sustainable Urban Design Plan study by Mithun architects and planners for the Portland Development Commission (PDC), available from the PDC web site,, shows us that urban buildings need to get to “Factor Six” to “Factor Ten” levels of efficiency (83% to 90% savings vs. current codes), even with rooftop PVs, to begin to reduce current impacts to “pre-development” (natural) levels of solar energy utilization and carbon dioxide emissions. This level of building performance should be our mission as sustainable architects, engineers, builders, building owners and developers, not just adopting the latest “technology du jour” from Europe and calling it “green.”

August 7, 2005

Bill McNeese of Agriboard comments:

I agree that there are multiple ways to reduce load and agree that that should be the first stop in building design. Once the connected load has been optimized then it is opportune to evaluate pv or other onsite power generation opportunities.

Green roofs are the coming thing and certainly have their place but when cost of a roof is factored into the # of points issued a green (garden) roof runs a similar cost/benefit analysis as pv. I suspect there is room for both.

Where implementation of solar technology will become main stream is when new designs in pv that include transparent films that can be incorporated into clerestories and windows (they are available in ASIA now). It should be noted that these technologies are not far off. Also, it should be noted that next gen pv products are anticipated to product between 12 and 20 watts per sf. This doubling of power generating capabilities will no doubt need to be factored into the equation of cost/benefit.

One other item in case you haven’t noticed, our conventional power delivery system is becoming less reliable especially during peak usage. The concept of micro generation (distributed/self generation) is the inevitable model for reliable power in the future. What is occurring to foster accelerated development and demand is the high cost and diminishing supply of fossil fuels, aging (read deteriorating) infrastructure and incentives from federal, state, and local governments. It will not be far in the future before a visionary governor (like Arnold) gets legislation passed requiring self generation in new developments.

I embrace both of these technologies and encourage LEED to offer more points for this entire area so that all portions of building effaces are addressed. Just picking one because it gets you points is the wrong approach.

Agriboard, Inc. William J. McNeese, Vice President, Sales

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