Should I Choose a Green Roof of White Roof?

20080708_Chicago_City_Hall_Green_Roof

Should I Choose a Green Roof or White Roof?

This is an article that I contributed for Green-Buildings.com. To visit the article, click here

 

Many experts would agree that black roofs could be replaced by white or green roofs to provide an economic and/or environmental benefit.

Indeed, black roofs should be phased out perhaps with the possible exception of colder regions where black roofs may be preferable due to their ability to retain heat.

Research shows that green and white roofs are a better choice across the board.

However, the debate as to whether you should build with a green or white roof has become more interesting given their various characteristics and benefits.

What Are Green Roofs and White Roofs?

A green roof, also known as a “living roof”, or vegetated roof, is a roof that is partially or completely covered with vegetation and a growing medium, and planted on a waterproofing membrane. Green roofs can also contain layers of root barrier and drainage and irrigation systems.

Scandinavian Green Roof Inst.

Green roofs are generally grouped into 3 different categories:

  1. Extensive Green Roofs
  2. Intensive Green Roofs
  3. Semi-Intensive Green Roofs

Extensive green roofs require minimal maintenance since they contain plants that have native, low maintenance/self-sustaining characteristics, intensive green roofs somewhat resemble, and may require the semi-regular maintenance level of, traditional gardens and semi-intensive green roofs which aim to be aesthetically pleasing yet require regular ongoing irrigation, fertilizer and maintenance.

A white roof, or “cool roof”, is a roof that is painted with solar reflective white coating capable of reflecting up to 90% of sunlight (in contrast, conventional black roofs only reflect up to 20%).

White roofs’ benefits are measured partly according to the solar reflectance index, or SRI. A roof’s SRI is a measure of a surface’s ability to reflect solar heat. SRI measures reflectance (reflecting the sun’s rays) and thermal emittance (the roof’s ability to radiate absorbed heat).

SRI is a gauge from 0-100, with 0 being most likely to absorb and radiate heat (such as a tar roof) and 100 being the most reflective (least likely to get hot) such as a “cool roof”.

Both white and green roofs alike are successful in mitigating the adverse effects of dark impervious urban surfaces, which contribute to “Heat Island Effect” (man-made areas that become hotter than nearby rural areas).

Both white and green roofs have positive environmental effects in comparison to conventional black roofs.

Should I Choose a Green Roof or White Roof?

Credit: WalMart via Flickr

But which should you choose?

There are three factors you should consider prior to deciding to design with either a white or green roof:

  1. Economics
  2. Environmental Effects
  3. Aesthetics and Function

Economics (Short and long term)

The Berkeley Lab Report on Economic Payoffs of Various Roofs is a study that was conducted by Arthur Rosenfeld.

Rosenfeld, who started at Berkeley Lab in the 1950s, is often referred to as California’s “godfather of energy efficiency” for his innovation in the field of sustainable energy.

The basis of the study was to analyze 22 commercial flat roof projects in the United States in which two or more roof types were considered.

The research consisted of a 50-year life cycle cost analysis (LCCA), assuming a 20-year service life for white and black roofs and a 40-year service life for green roofs.

Researchers found that in comparison to black roofs, white roofs provide a 50-year net savings (NS) of $25/m2 ($2.40/ft2) and green roofs have a negative NS of $71/m2($6.60/ft2).

This study discovered that even the cheapest of green roof costs roughly $7 per square foot more than black roofs over 50 years, while white roofs save $2 per square foot compared to black roofs. Essentially, white roofs cost $9 per square foot less than green roofs over 50 years, which equates to $0.30 per square foot each year.

The annualized cost premium is just $3.20/m2-year ($0.30/ft2-year). This annual difference is significantly small enough that the choice between a white and green roof should be based on preferences of the building owner, other than cost savings.

However, green roofs save their owners more on their energy bills due to plant evaporation contributing to cooling the building in the spring and summer months. In the winter months green roofs insulate and retain heat. That being said, white roofs are also effective at keeping cool when it is hot outside and heating when it’s cold.

White roofs are most cost effective.

While it does not affect the bottom line of the building owner directly, in some states and municipalities green roofs may be eligible for tax credits, deductions or incentives. Green roofs also provide public benefits by limiting the amount of stormwater runoff that needs to be processed. The plants on a green roof absorb rain water which, in turn, reduces the amount of wastewater flowing to the sewer systems during storms.

Owners concerned with global warming may choose white roofs, which are three times more effective than green roofs at cooling the globe. Owners concerned with local environmental benefits should choose green roofs, which offer built-in stormwater management and a “natural” urban landscape esthetic. We strongly recommend building code policies that phase out dark-colored roofs in warm climates to protect against their adverse public health externalities

Zinco roof systems claim that their systems filter 50-90% of water. Pipes and drains can also be incorporated into the cost and help reduce the drainage costs too. It is estimated that a 50 percent green roof infrastructure in New York could save the city $18 million per year in storm water treatment.

It is also worth noting that green roofs can create extra recreational space, which can be attractive as amenities for tenants or value added features for owners or residents of a building.

Green Roof on City Hall of City of Chicago

As stated by Benjamin Mandel of the Berkeley lab report:

“Any roof that saves on air conditioning energy more than it loses on heating energy, like either white or green roofs in warm climates, will bring indirect climate change benefits from reducing power plant emissions. What makes the monetary comparisons difficult however, is that green roofs offer many environmental benefits that are difficult to objectively measure…”while the economic results are interesting, it also highlights the need to include factors such health and environment in a more comprehensive analysis. “We’ve recognized the limitations of an analysis that’s only economic,” Mandel said. “We would want to include these other factors in any future study.”

Ultimately, a green roof can generate a return on investment (ROI), but not for the short term investor.

Even over an extended period, the installation premiums for green roofs do not offset their long term benefits in contrast to white roofs.

Environmental Effects

As mentioned previously, white roofs reflect sunlight roughly 3 times more effectively than green roofs, They may be preferable if the building owner is particularly concerned with global warming. Conversely, green roofs offer more benefits to the local environment. Green roofs may not reflect sunlight as well, but they increase bio-diversity by improving (and not disturbing) their surroundings. Other green benefits include capturing C02, creating and supporting new habitats etc.

Shaunacy Ferro points out in her article for fastcodesign:

“Turns out, though, that a plain white roof is way better for the environment. A new study in Energy and Buildings points out that while green roofs are better than roofs painted black—which absorb sunlight and contribute to the urban heat island effect—white roofs can actually combat global warming by reflecting more sunlight back into the atmosphere. Based on an economic analysis of the costs and benefits of the three types of roofs, the researchers at the Lawrence Berkeley National Laboratory found that white roofs are three times more effective at fighting climate change.”

Carver + Schicketanz Architects

It’s s worth noting that although white roofs are effective at reflecting sunlight, reflected sunlight can also impact nearby buildings which in turn may increase their air conditioning loads.

As for heat island effect, both roof types are successful at reducing urban heat differentials. Green roofs do this by converting sunlight into photosynthesis by plants, while white roofs do so by allowing so little of the sunlight to be absorbed by its reflective surface.

Green roofs reduce the impact of each new building on the municipal storm drainage system and surrounding watershed. They reduce flooding, erosion and artificial heating of water which helps preserve fisheries and other aquatic life. 
The soil in green roof systems acts like a sponge and absorbs excess rain water. Research has shown that extensive green roof systems can reduce runoff by up to 90+% annually (varies with climate, soil and pitch of roof).

Aesthetics and Function

Green roofs allow architects to create aesthetically pleasing “ eco- friendly “ roofs that can contribute to many of LEED points. However, Green roofs do not offset climate change.

For instance, these 10 beautiful green roofs are an example of what is possible. Just look at Villa Bio ( by Enric Ruiz- Geli), or Mill Valley Residence by McGlashan Architecture, The 8 house by BIG architects, and Paraty House by Marcio Kogan Architects and you will realize that it is near impossible to match that type of beauty with a white roof.

Beyond aesthetics, Having functional usable space is an additional bonus that gives green roofs the edge in this category.

Green roofs are becoming so popular that even buses are picking up on the trend.

Yes, you can build and design a white roof to be usable for recreational purposes, but will it be to the same effect as a garden type setting? Highly unlikely.

Denver Green Roof

Conclusion

In my opinion, when it comes to environmental benefits of green vs. white roofs, we are comparing apples and oranges.

If you’re comparing metrics, white roofs seem to perform better based on their ability to deflect sunlight back into the atmosphere. However, green roofs seem to me to have more positive indirect effects to the environment that are difficult to measure.

I believe that good marketing plays a large role in the emergence of white roofs on the eco-friendly roofing scene. I am not negating the positive effects that white roofs have on the environment, but I personally prefer the aesthetics and impact of green roofs. Aside from the fact that my architectural background draws me to a beautiful roof that resembles a garden, the well roundedness of its environmental benefits appeal to me the most.

Both roof types have economics benefits over the long run (although the white roof gets a firm upper hand in this category), both roofs can are good for the environment, and both can be aesthetically pleasing.

Ultimately, IMHO, the green roof trumps the white roof in the majority of the credits , which is why if I were a building owner, I would go green

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