I was at a public hearing recently where one of the most "heated" topics of discussion was the value of green or reflective roofs to energy conservation in building design. Unfortunately it seemed that both sides of the discussion lost one critical aspect of the argument.
The builders representative was quite adament that he could meet energy star compliance for a building using traditional materials and an increased amount of insulation to ensure that winter heating and summer air conditioning did not escape. He argued that green roofs increased the structural requirements for a building dramatically, but never really addressed reflective roofs.
The proponent for the green and reflective roofs noted how these features would provide their own benefits for reduction of internal energy consumption, but ultimately agreed that similar internal savings could be achieved with increased insulation.
Lost in the entire discussion, and I mean never raised at all, was the benefit of the green or reflective roofs on the environment as a whole. The missing discussion was on the concept of urban heat islands. It is a well recognized concept that black asphalt raises temperatures on hot sunny days (on cold winter days as well given ice will melt on asphalt on sunny days when it is below freezing). Black asphalt shingles or other asphalt roofing materials will do the same. Exactly how many degrees may be debatable, but I have seen figures that these urban heat islands could raise temperatures by as much as ten degrees. US EPA suggests about a five degree daytime and twenty-two degree evening temperature diferential. The following illustration from the EPA provides a dramatic picture of this temperature impact of heat islands:
The builders representative was quite adament that he could meet energy star compliance for a building using traditional materials and an increased amount of insulation to ensure that winter heating and summer air conditioning did not escape. He argued that green roofs increased the structural requirements for a building dramatically, but never really addressed reflective roofs.
The proponent for the green and reflective roofs noted how these features would provide their own benefits for reduction of internal energy consumption, but ultimately agreed that similar internal savings could be achieved with increased insulation.
Lost in the entire discussion, and I mean never raised at all, was the benefit of the green or reflective roofs on the environment as a whole. The missing discussion was on the concept of urban heat islands. It is a well recognized concept that black asphalt raises temperatures on hot sunny days (on cold winter days as well given ice will melt on asphalt on sunny days when it is below freezing). Black asphalt shingles or other asphalt roofing materials will do the same. Exactly how many degrees may be debatable, but I have seen figures that these urban heat islands could raise temperatures by as much as ten degrees. US EPA suggests about a five degree daytime and twenty-two degree evening temperature diferential. The following illustration from the EPA provides a dramatic picture of this temperature impact of heat islands:
Black surface trap and absorb heat, when the sun goes down, these surfaces are very slow in releasing heat. A US EPA report illustrated that the temperatures for heat islands are at their highest towards their centers and tapers off towards the outer edges where trees and other landscape changes may occur. The largest differential in temperatures, center of heat island to edges or adjacent rural areas, occurred about four hours after sunset.
If we take this concept to site design, we usually find most suburban plazas and malls with large masses of parking, often surrounding the building. The building is thus cast as the center of the heat island. The building can be constructed for maximum energy efficiency (protecting against loss or heat or cooling) but if the site is not designed and constructed for maximum energy efficiency, the heat island that is created will be increasing the overall energy use of the property.
Look back at the original discussion about roofing materials. Black absorbent roofing materials are reported by the EPA as being up to ninety degrees higher than the surrounding air temperatures during the daytime. The green and reflective roofs are intended to reduce heat absorption, and could therefor reduce ambient day and night temperatures. This will reduce the heat island effect of the site, and thereby reduce the energy consumption. These constructions standards can, and should, be matched with increased efficiency in the insulation, providing a double benefit.
Heat islands, and therefor the green roof concept is important to consider in that higher ambient air temperatures increase energy needs; thus creating more air pollution (from the need to generate more electricity; higher levels of ground level ozone (caused by the chemical reaction that takes place in these higher temperatures); greater health issues related to both the heat and the air pollution levels; and water quality issues as run-off temperatures, and ultimately the water bodies to which the run-off is directed, have higher temperatures. These results clearly illustrate issues, beyond simply ensuring the energy efficiency of a building, that argue for the use of alternative roofing materials. This is so even when greater insulation may be able to protect the building itself from the surrounding heat.
Unfortunately not all states provide planners with the tools to intervene in building decisions. In Massachusetts planners are specifically precluded from making any requirements that step into the areas covered by the state building code. That places us at a disadvantage for directly influencing some building decisions. I am almost certain if we were to place a requirement in zoning that only reflective roofs were used in commercial projects, the requirement would be overturned as outside of our jurisdiction. We would be making the right choice, we just do not have the tools available to us. We will just have to look at the issue and find ways to back door these standards.
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