The National Park Service recently announced the availability of new web-based resources on installing green roofs on historic buildings. The Green Roofs on Historic Buildings site “includes basic descriptions of green roofs and how they are constructed, special considerations for installations on historic structures, and guidance for meeting the Secretary's Standards for Rehabilitation,” according to the NPS announcement. Here is the site introduction:
Historic buildings are great opportunities for sustainable development and are regularly being rehabilitated to incorporate green design features while still preserving their historic character. One way of increasing the sustainability and energy performance of a building is to install a green roof.
I take issue with the last sentence, but I’ll get to that. First, let’s quickly define what a green roof is: It is a building rooftop that includes a growing medium and plants. That’s the simple definition. In practice, however, the range of intensity can vary from sparse installations with 2-3 inches of growing medium and groundcover, to “intensive” roofs with many feet of soil and mature trees. Some green roofs use xeriscaping (plants that do not require supplemental irrigation) but the basic idea is to load dirt, flora, and water on top of a building. Done well, there are many benefits. Done poorly, they can be expensive mistakes.
While the NPS website serves as a useful introduction to green roofs, it does not address the many underlying claims about the merits of green roofs. As such, preservationists should use the NPS site as a starting point when researching the various options available. Here at the Green Lab, we have seen more than a few green roofs – some done well, some designed so poorly that they’ve given the entire industry a black eye. Owners of older and historic buildings should diligently research all available options in order to make informed decisions about what will improve the energy performance of the building, but not adversely impact its historic character.
While the NPS site is a good place to begin, preservationists should go a step further and ask the following questions:
- Should everyone be installing green roofs?
- What are the alternatives to installing green roofs?
- Do the purported benefits outweigh the risks?
There is no question that green roofs, when installed and maintained correctly, can extend roof life; mitigate the environmental impacts of the built environment by detaining storm water to reduce peak flows; reduce the “heat island” effect of dark, heat-absorbing materials; and provide habitat, beauty, and even opportunities for urban agriculture.
But here is the downside: Green roofs are expensive and many of the benefits can be achieved through more cost-effective strategies.
The NPS website cites a 2006 University of Michigan green roof study as its data point on the economic and energy benefits. That particular study compared a building with a green roof to a building with a poorly insulated black-colored roof. (Dark-colored roofs absorb heat in the summer and poorly insulated roofs release heat in the winter, thereby requiring increased energy consumption as mechanical systems work to make up for the poorly designed envelope.) A more effective evaluation of the benefits of the green roof would include comparisons against well-insulated, reflective roof assemblies.
The NPS website asserts that a green roof “increases the R–value (a measure of the resistance of a material to heat flow) of the roofing system, along with reduced temperatures on the roof lessen HVAC loads, resulting in energy cost savings.” Again, this may be true if the base case is a really bad roof. However, I consulted with several energy efficiency experts who suggest that 3 or 4 inches of dirt is the equivalent of R-1 insulation – about the same as a single-pane window – if it's dry and R-0.5 if it's wet (which it is all winter). According to these experts, the best way to insulate a roof is to add insulation in the roof assembly, which is much cheaper than installing a green roof. One analysis indicates that four inches or roof insulation plus a “cool roof” reflective membrane would save vastly more energy and money.
Storm Water Management
A properly functioning green roof can serve to detain storm water, which decreases the peak intensity on the storm sewer system. However, most studies compare green roof performance and impacts on municipal water systems to the standard practice of directing storm water directly into the system. An alternative – once again, cheaper and less worrisome than a green roof with regard to impacts on historic character – is a detention vault, or tank, hidden from view in a parking garage or basement. Detention vaults effectively release water into the storm sewer system at controlled rates, and can be used to store irrigation water for use in dry months – a double benefit.
Evapotranspiration, Cool Roofs and Heat Islands
Once again, the Michigan study is cited to make the case for green roofs to reduce rooftop temperatures and cool incoming air (for the mechanical system) – yet the example used for comparison is a poorly insulated black roof. In practice, reflective roofing materials can achieve the same effect as green roofs in reducing rooftop temperatures (with the caveat that reflective rooftops can negatively affect adjacent sites). Additionally, a large portion of the country (West Coast, Southwest, and drought-prone regions) will require irrigation to keep green roof plantings alive during dry months, which is a sort of artificial evapotranspiration – mechanically irrigating a green roof to allow water to transpire through the roof plantings, with less overall benefit and greater cost compared to a well-insulated, reflective roof.
The takeaway here is that green roofs are viable strategies for improving the quality of the urban environment, but the preservation community should carefully consider alternatives that might achieve greater benefits for lower costs, with less risk to historic buildings. The benefits of added beauty, urban habitat for bees and birds, and storm water detention are often worth the added costs associated with green roofs. However, one of our jobs at the Green Lab is to ensure that preservationists clearly understand all of the available options in order to make the right decisions.
Much of the Green Lab’s research has focused on entire districts of buildings, and there are often economies of scale for district energy efficiency, energy generation, and water management strategies that in combination have benefits greater than the sum of individual building strategies. A network of green roofs and detention facilities could reduce the overall burden on centralized municipal infrastructure while delivering environmental and economic benefits to all of the buildings in the district. Historic buildings should be considered in the context in which they might contribute at the district scale.