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Responding to ‘Ballast Cooler Than You Think’

The recent article by Annie McCarren called ‘Ballast Cooler Than You Think’ published in Environmental Design and Construction provides an overview of the “cool” properties of thermally massive ballasted roofs.  We largely agree that ballasted roofs can be a good roofing option under the right circumstances.  There are, however, a few inaccuracies in the article that are worth dispelling because they recur frequently in discussions of cool roofing concepts.

The extra thermal mass of a ballasted roof can make a lot of sense, particularly in climates that are hot in the daytime and cool at night.  Some codes and standards do allow ballasted roofs to count as exceptions to cool requirements, though the threshold weight required for the tradeoff is pretty high and will probably not be a cost-effective substitute for reflective roofs. For example, California’s Title 24 building code allows stone ballasted roofs weighing 17 pounds per square foot or a roof with pavers weighing 25 pounds per square foot to qualify for the exemption, which is heavier than most ballast rock on the market. Ballasted roofs may also increase nighttime cooling energy demand in areas with hot days and warm nights.

There are a few inaccuracies to address, however.

First, reflecting sunlight is not the same thing as reducing heat flow into the building (with insulation) or delaying heat flow into the building (with thermal mass). Increasing roof reflectance rejects solar energy as light, cooling the building and the outside air. Adding insulation to the roof reduces heat flow into the building, while raising the roof surface temperature and warming the outside air.  Adding thermal mass to the roof makes it slower to warm during the day and slower to cool at night. Ideally, a designer would opt for a high-performance roof system that optimizes the solar reflectance, thermal resistance, and thermal mass of the roof to cost-effectively gain the benefits of all three approaches.

The next few issues arise when the author says:  “A cool roof can be a great choice for buildings in warm, southern climates where the number of cooling degree days exceeds the number of heating degree days. The term ‘cooling degree days’ refers to the number of days that a building would need to run an air conditioning unit versus a heating unit to stay at a set temperature.”

One problem here is that the author neglects to mention that cool roofs are a viable roofing option in northern climates.  While cool roofs certainly make a great choice in warm southern climates, they can also provide net energy benefits in places where energy usage for heating exceeds cooling energy consumption. There are plenty of peer-reviewed studies to support the notion that cool roofs can provide net energy savings in northern climates. Perhaps more importantly, the well-established and rapidly-growing marketplace for cool roofing in northern regions (covering billions of square feet of roof space) demonstrates that the author is presenting an overly narrow view of the energy benefits that cool roofs are providing to building owners across the country.  The case for cool roofs is even stronger when the health benefits of cooler cities (such as cleaner air and lowered heat health risks) are factored in.

A second problem with the author’s statement is the misuse of heating and cooling degree days as a means to determine if reflective roofing makes sense from an energy savings perspective.  The article incorrectly defines heating and cooling degree days. Heating/cooling degree days gauge the need for heating or cooling that is driven by the difference between outdoor and indoor air temperature. For example, annual cooling degrees days (base, or threshold temperature of 65°F) would be determined by calculating for every hour of the year the outside air temperature and subtracting the base temperature (in this case, 65°F), summing all positive values, and dividing this total by 24 (hours in a day). But because cooling degree days and heating degree days are based on outdoor air temperature and do not account for the sun’s ability to heat buildings, they paint a misleading picture. To illustrate this point, consider a cool sunny day during which the outdoor temperature never exceeds 65°F (meaning zero cooling degree days). The building may still require air conditioning to remove its solar heat gain. In fact, cool roof savings/penalties are not proportional to cooling/heating degree days. Rather, they scale with the solar energy that strikes the roof during hours when the building is cooled/heated.

It’s Unanimous – Los Angeles is a Cool City

The Los Angeles City Council has unanimously approved an update to the city building code, making LA the first major city to require the installation of cool roofs on all new and refurbished homes.  LA already requires cool roofs on commercial properties, but this latest update to the code puts the city out ahead of all other major cities when it comes to cool roofs.

You can read more about it here: Climate Resolve press release

Megacity Mayors Double Climate Action

The C40 Cities Climate Leadership Group (C40), a partner of the Global Cool Cities Alliance, has just released a report on the efforts of member cities to reduce greenhouse gas emissions.  The report updates a 2011 report, and shows that the mayors of 63 of the world’s largest cities have taken nearly 8,000 actions to address climate change over the past 2 years.

You can read more about it in C40′s report here:
Climate Action in Megacities, Volume 2.0

The release of this latest report coincided with the C40 Cities Climate Leadership Group Mayors Summit, which was held earlier this week in Johannesburg, South Africa.

Sustain Magazine Features Cool Roofs GCCA Article

GCCA recently submitted a short article to the University of Louisville’s SUSTAIN Magazine summarizing the building, city, and global benefits of reflective surfaces. While this won’t be news to many of you, I think it is always helpful to try to raise awareness amongst new audiences.

GCCA Discusses Cool Roofs on NPR

GCCA’s Executive Director, Kurt Shickman participated in the Midday with Dan Rodricks show on NPR station WYPR in Baltimore. I was joined Joan Jacobson, author of the newly released Cool Roofs for a Cooler Baltimore report for the Abell Foundation. The show aired at noon on Thursday, October 31st. Here is a link to the podcast of the show.

Cities are Adding Cool Roofs to Their Codes

Over the last few years, cool roofs have begun to make their way into the building codes throughout the country.

Cool roofs made its first appearance in building codes in California’s Title 24 in 2005, requiring initial solar reflectance (SR) of 0.70 and initial thermal emittance (TE) of 0.75 on low-sloped commercial buildings throughout the state. Since then, Title 24 has slowly cranked up the reflectivity requirements. The latest round of revisions, for the 2013 code, require an aged SR of 0.63 and an aged TE of 0.75, or an aged solar reflectance index (SRI) of 75. California’s Title 24 now also requires cool roofs on low-sloped residential buildings and on steep-sloped residential and commercial buildings throughout most of the state (SR of 0.2 and TE of 0.75 or SRI of 16).

Both IECC 2012 and ASHRAE 90.1 require cool roofs (aged solar reflectance of 0.55 and aged thermal emittance of 0.75) on low-sloped commercial buildings in climate zones 1 through 3. ASHRAE 90.2 has required cool roofs (initial solar reflectance of 0.65 and initial thermal emittance of 0.70) on low-sloped residential buildings in climate zones 1 through 3 since 2007.

The national above-code standards, or stretch-codes, all also have cool roof requirements.

– ASHRAE 189.1 requires an initial SRI of 78 on low-sloped commercial roofs and an initial SRI of 29 on steep-sloped commercial roofs in climate zones 1 through 3. ASHRAE 189.1 also allows compliance through ENERGY STAR.

– ENERGY STAR defines cool roofs as having an aged SR of 0.50. ENERGY STAR currently does not require a TE rating for its products.

– In order to receive a credit for cool roofs within the LEED system, a building must have an SRI of 78 covering 75% of its roof area, if it has a low-sloped roof, and have an SRI of 29 covering 75% of its roof area, if it is a steep-sloped roof. LEED also has a sustainable sites credit which encourages the use of reflective pavements and hardscapes, as well as the use of vegetation.

– IgCC – The International Green Construction Code has a Heat Island Mitigation section in Chapter 4 (Site Development and Land Use). This section requires that at least 50% of the hard scape be either reflective, shaded or have permeable pavement. It also requires that at least 75% of both steep-sloped and low-sloped roofs meet minimum solar reflectance standards or employ vegetative roofs. The reflectivity requirements are as follows: a minimum aged SRI of 60 for low sloped roofs and a minimum aged SRI of 25 for low-sloped roofs.

Several cities have developed their own cool roof requirements.

– As of January 2012, New York City has required cool roofs on all new and replacement low-sloped roofs. Roofs must have an initial SR of 0.7, an initial TE of 0.75 or an SRI of 78.

– In 2010, Philadelphia passed a law requiring new roofs, or additions on existing buildings to comply with ENERGY STAR’s cool roof ratings.

– In 2013, Washington DC adopted cool roofs into its building code, using the requirements from IECC 2012 (which cover climate zones 1 through 3).

– New York City, Philadelphia, and Washington DC are all in climate zone 4.

Cool roofs certainly have a strong foothold in the national model codes, the national stretch codes and volunteer standards, and even in a growing number of city codes. At GCCA, we are excited it help expand their scope, as appropriate, and expect that the next few years will be exciting ones for cool roofs.

GCCA Releases Cool Roofs and Global Cooling Issue Brief

There are a number of papers that support the idea that reflecting solar energy back out into space would offset the warming effect of atmospheric CO2. For a high-level overview of how it works and the state of the research, check out this GCCA Issue Brief. GCCA regularly publishes Issue Briefs on hot topics covering cool roofs and urban heat islands. This recent article in the San Francisco Chronicle also does a nice job summarizing the opportunity.

If you really want to dig into the details, check out the following papers in our Knowledge Base:

Global Cooling Updates

The Long Term Impact of Increasing Albedo in Urban Areas

Global and Local Effect of Increasing Land Albedo

Regional Climate Consequences of Large-Scale Cool Roof and PV Deployment

Effects of Urban Surfaces and White Roofs on Global and Regional Climate

LBNL Response to Jacobsen and Ten Hoeve (2011)