Raise the Roof: Comparing Cost Savings from Efficient Roof Upgrades

Due to global climate change, it is accepted that extreme heat events will become both more frequent and more severe. According to the Center for Disease Control and Prevention, if current emissions are held constant, heat-related deaths in the United States could increase from roughly 700 each year to between 3,000 and 5,000 each year by 2050. Outside of projected fatalities, excessive heat can lead to increased peak electricity use for cooling, which further exacerbates the problem by increasing air pollutants and greenhouse gas emissions. Urban areas are especially vulnerable with roofs accounting for 20-25 percent of land cover – the majority of which are dark and absorb about 80 percent of sunlight.

Displacing traditional black roofs with reflective white roofs or vegetative “green” roofs can reduce the risks and externalities associated with the urban heat island effect, but they have different maintenance and installation costs and confer different kinds of benefits over time. In “Economic comparison of white, green, and black flat roofs in the United States,” researchers from Lawrence Berkeley National Laboratory compare the economic returns for different roof choices using a 50-year life cycle cost analysis, breakdown the priced and un-priced benefits for each, and make a case for a black roof building code phase-out.

According to the study which integrated roof characteristics from previous research, white roofs reflect 55-80 percent of sunlight keeping the building cooler, reducing pollutants and emissions associated with excess electricity generation, and contributing to overall global cooling by increasing surface reflectivity. Green roofs reflect only 20 percent of sunlight, and therefore contribute less to global cooling, but the added vegetative insulation still helps to insulate the building and reduce associated heating and cooling emissions. Independent of solar reflectance, green roofs release moisture into the air through evapotranspiration, which can help to cool the local air temperature. In addition, green roofs reduce storm water runoff by retaining rainfall, which can reduce storm water management costs.

The analysis from Sproul et al. incorporates costs and savings from installation, maintenance, avoided energy use, avoided emissions, global cooling, and avoided storm water expenses. It does not incorporate factors that are less straightforward to monetize such as environmental and esthetic benefits or effects on public health. The 50-year life cycle cost analysis finds that white roofs are the most cost-effective option, providing a net savings of $96.3/m² over green roofs and $25.4/m² over black roofs. The net savings of white roofs are due to their relatively cheap installation and maintenance costs, which are comparable to black roofs. Green roofs have much higher installation and maintenance costs, which dominate over any additional green roof-specific savings from storm water management and energy savings. As a result, when compared to black roofs, green roofs lead to a net loss of $70.9/m².

The authors are upfront in addressing a number of limitations to this analysis. Many of the benefits from green roofs mentioned earlier are not accounted for in the life cycle cost analysis, so there are additional un-priced advantages omitted from the net savings. When annualized over 50 years, the cost premium for green roofs is $2-4/m²year, which may not be prohibitive for owners who care deeply about their added environmental and esthetic benefits. It should also be noted that the data are pulled from just 22 case studies where missing data points were filled in with average values. As more data points are collected over time, detailed cost savings for different roof options could be more accurately determined.

Despite these limitations, it is clear that regardless of costs, both white and green roof options confer benefits to society. White roofs are especially attractive given their comparable upfront costs to black roofs and added net savings over time. The authors recommend that all US cities and states as far north as Chicago, IL should implement mandatory no-cost programs for upgrading from black to white roofs, and policies like these are catching on. Effective January 2014, Los Angeles became the first major US city requiring that all new or refurbished buildings have a “cool” roof installed. According to the study, a program like this, phasing in white roofs on all US commercial flat roof buildings, could save up to $8 billion in avoided energy costs at little or no extra up-front cost.

In 2010, Energy Secretary Chu released a memorandum encouraging all federal buildings to install cool roofs in order to comply with President Obama’s executive order to reduce federal government greenhouse gas emissions by 28 percent by 2020. Based on the cost savings and positive externalities presented in this study, this approach should also be embraced by municipal governments in order to raise awareness and demonstrate the benefits of cool roof installation. By directly comparing roof saving options, this study delivers a convincing argument for phasing out black roofs, and provides a useful breakdown for owners choosing between the two replacement options.

Article Source: Economic comparison of white, green, and black flat roofs in the United States, Julian Sproul, Man Pun Wan, Benjamin H. Mandel, Arthur Rosenfeld. Energy and Buildings, March 2014

Feature Photo: cc/(phototouring)