1. Urban Density and Urban Heat Island Effect – From anthropogenic heat to Heat Island Reduction (HIR)
Large cities and metropolitan areas are hotter than surrouding suburban and rural areas. Urbanization of ever increasing areas is replacing natural vegetated surfaces, such as grass and tress, with non-reflective, water resistant impervious surfaces that absorb high percentage of the incoming solar radiation.
The UHI mainly depends on the modification of energy balance in urban areas which is due to several factors:
- urban canyons
- thermal properties of the building materials
- substitution of green areas with impervious surfaces that limit evapo-transpiration
- decrease in urban albedo
Roofs constitute about 20-25% of the urban surface.
Also, the physical properties of non-reflective surface worsen the already compromized situation of urban configuration and localized microclimates, that can lead to the formation of “local hot spots”.
2. The roofing system – Can a building positively effect its surrounding environment ?
Several elements of the built environment, including roads, parking lots, and obviously buildings, can significantly affect global warming and environmental pollution. The research explores the implications related with the roofing assembly selection, focusing not only on the building scale, but also assessing the impacts on the urban environment, to further understand the role of the roof within sustainable design and to define accessible guidelines for its evaluation.
- which direction has to be taken when selecting a roofing system ?
- What are the related performance a design should aim to and what is the trade off ?
3. The designer choice – How to address the trade-off?
With respect to heat island mitigation, the efficiency of green roofs is not as high as the one provided by white roofs.
Although when comparing roofing solutions, there are benefits and environmental services performed by green roofs that have no counterparts from impervious roofs. Besides direct effects, such as water runoff control and water quality improvements, there is also direct air quality improvements, urban biodiversity, noise reduction, and building amenity value, including to urban residences overlooking the green roofs.
Several studies seem to agree that if global warming is a major concern, white roofs should be the preferred choice.However, is water a less iportant resource than energy ?
Stormwater management may be a decisive factor in favor of green roofs, particularly in the presence of strict local stormwater regulations. Therefore, if the local environment is a primary interest, green roofs should be preferred.
And if green roof is selected for its environmental benefetis, Is it effective to combine it with a solar roof to increase the energy performance as well ?
4. A possible answer – Cost/Savings analysis
White roof are the most cost effective strategy, even without accounting for their heat island mitigation virtues. Black hot mop roofs can be made white at no additional costs, the cost premium of white versus black as virtually disappeared.
On the other hand the numerous positive effects on the environment and the physical properties of buildings of green roofs are difficult to clearly determine. For the green roof industry to survive in the long run, it is critical to demonstrate conclusive evidence that extensive green roofs are able to provide a short and long-term return on investment.
A negative aspect of the economic evaluation of green roofs is that, addressing life cycle of a roof, is not always possible to identify who accrues the costs and benefits: in the private sector for example, the construction cost is paid by the building developer, while tenants or owner pay the operation and maintenance costs. Improved habitat, pollution reduction, and also improved stormwater management benefits are mainly perceived at the municipality level.
5. It is also a matter of time – Roof time line
A green roof is not really considered established until the second year. The plants and further vegetation growth is expected over the next few years after installation.
By contrast, the white roof starts out with the highest albedo it will ever have, and this is well-known to degrade over time as soot and weathering processes begin to darken the surface.
Therefore, time is an other variable that needs to be taken into account: costs and benefits do not happen at the same time, and they are usually associated either with annually (long term) or one time considerations.
6. Incomes/Outcomes – E[m]ergy accounting
The proposed diagram intend to define the “roofing system” architecture to provide a comprehensive analysis of the different sources and flows that support the system, and the benefits that constitute its outcomes, which are not easily comprehensible in a small scale analysis.
A negative aspect of the economic evaluation of green roofs is that, addressing life cycle of a roof, is not always possible to identify who accrues the costs and benefits: in the private sector for example, the construction cost is paid by the building developer, while tenants or owner pay the operation and maintenance costs. Improved habitat, pollution reduction, and also improved stormwater management benefits are mainly perceived at the municipality level.
Moreover, time is an other variable to be taken into account: costs and benefits do not happen at the same time, and they are usually associated either with annually (long term) or one time considerations.
The complexity of the system suggest emergy accounting as the preferred mean to address the total amount of sources that drive the process (energy, material, labor, waste,..) defining a common background for the evaluation of the several roofing systems researched.
The complete study is available at the following link:
niccolò benghi 