Figure 1. ASHRAE Climate Zone Map. Cool roofs are currently required in Zones 1-3 only.
The overwhelming consensus is that cool roofs are a clear top choice in warm climates, but what about cooler ones?
Studies and decades of real-world experience clearly show that cool roofs are net energy savers and improve thermal comfort in Climate Zones 1-3. The model codes (ASHRAE and the I-codes) already include requirements for some new and replacement roofs to be highly reflective in these areas.
But what about “cool, northern” climates like Climate Zone 4? Shown in yellow on the ASHRAE Climate Zone Map in Figure 1, Zone 4 stretches from the Mid-Atlantic across the southern Appalachian states to the southern Midwest.
There are a number of myths that have led to a notion that the dividing line between “warm” and “cool” lies between Climate Zone 3 and Zone 4. In “cool” climates where heating degree days outnumber cooling degree days, the traditional thinking goes, the cost of extra heating demand caused by cool roofs in winter would offset the cooling energy cost savings in summer. Despite decades of market experience and a vast body of research supporting the net benefits of cool roofs in Climate Zone 4, this line of thinking has been an obstacle to cool roof policy in the United States. Let’s dispel some of those myths by looking at a few facts.
Winter heating penalties associated with cool roofs in cool climates are vastly overstated. Higher insulation levels in Climate Zone 4 do not offset the benefits of cool roofs. Research over the last couple of year (field and modeling), some of which I’ve cited in this article, show that the so-called “winter heating penalty” is much smaller than many
Figure 2. Peak demand is remarkably similar across climates. Source: Dr. Jim Hoff. “Reducing Peak Energy Demand: A Hidden Benefit of Cool Roofs.”
thought. Specifically, a field and modeling study done at Princeton University’s campus (in Climate Zone 4) compared cool and black membranes over roofs with insulation levels up to R-48. The studies show that cool roofs reduce heat inflow in summer but have the same heat loss in winter as black surfaced roofs over the same level of insulation.
Another study evaluated the impact of reflective roofs on new and older vintage commercial buildings in cold locations including Anchorage, Milwaukee, Montreal, and Toronto. All cities in the study are located in climates zones north of Climate Zone 4 and experience longer, colder winters than cities in Climate Zone 4. The study finds that “Cool roofs for the simulated buildings resulted in annual energy expenditure savings in all cold climates.” The study also identified peak energy savings in addition to the base energy efficiency gains.
Figure 3. Projected temperature change for mid-century (left) and end-of-century (right) in the United States under higher (top) and lower (bottom) emissions scenarios. The brackets on the thermometers represent the likely range of model projections, though lower or higher outcomes are possible. Source: USGCRP (2009).
Heating and cooling degree days are not a good way to determine the appropriateness of cool roofs. Heating/cooling degree days indicate the intensity of the annual heating/cooling demand in a location, as a function of how far the outdoor air temperature is below/above a “comfortable” temperature and how much of the year is spent below/above that threshold. These metrics paint a misleading picture because they are based on outdoor air temperature and do not account for the sun’s ability to heat buildings or on the heat generated by human activity in the building. To illustrate this point, consider a cool sunny day during which the outdoor temperature approaches, but never exceeds, the comfort threshold (meaning zero cooling degree days). The sun may nevertheless heat the building enough throughout the day to require air conditioning by late afternoon, and cooling degree days would then underestimate actual cooling energy use.
Conversely, the sun’s heat on a cold sunny day may cause heating degree days
Figure 4. Energy cost increases and total damages from rising heat. Source: Solomon Hsiang et al. “Estimating economic damage from Climate Change in the U.S.” Science, June 2017.
to overstate the true demand for heating energy. This suggests that reflective roofs can save energy over the course of a year even if heating degree days exceed cooling degree days. Or take heat from building occupancy and activity — many commercial buildings run space cooling year-round, thus negating the concept of a heating penalty altogether. The effect of occupancy will only increase as building standards require more insulation and fewer air gaps. The comparison of heating and cooling degree days, though simple and logical-sounding, is actually a very unreliable rule of thumb for the assessing the suitability of reflective roofs.
Peak energy demand reduction is a huge, but often overlooked, benefit of cool roofs in all climate zones. Reflective roofs save the most energy during peak energy demand periods, like hot summer afternoons. Field studies indicate a peak demand savings of 15 percent to 30 percent resulting from reflective roofs (see http://www.coolrooftoolkit.org/wp-content/uploads/2012/07/CEE_FL-Cool-Roof.pdf).
Unfortunately, most energy savings calculators exclude peak demand, thus painting only a partial picture of the energy savings opportunity of cool roofs. Peak reductions are more than just an energy saver. Most utilities charge a peak demand fee to non-residential customers based on their maximum demand in a given period of time. This fee can be more than half the bill for some customers. Peak
Figure 5. Summers in New England could soon feel like summers in South Carolina. Source: Union of Concerned Scientists. “The Changing Northeast Climate,” 2006.
demand is also different from “base” cooling demand because it is not driven by climate. The graph in Figure 3 compares base and peak cooling demand for all U.S. climate zones and finds that peak demand requirements in Minneapolis are the same as they are in Phoenix.
“Cool” climates in the United States are starting to feel a lot hotter. Scientists predict an average increase in temperatures of 4-6 degrees Fahrenheit in the United States over the next 30 years or so. But as the maps in Figures 4 and 5 show, the amount of warming and its economic impact will be most acutely experienced in parts of the United States covered by Climate Zones 1 through 4.
It won’t just be hot areas getting hotter. An analysis by Union of Concerned Scientists forecast that, under a high but realistic emissions scenario, summers in New York City (the northernmost city in Climate Zone 4) could feel like South Carolina. Recently, the school district in Eau Claire, Wisconsin committed to replacing its black membrane roofs with white ones to help reduce temperatures during their increasingly hot summers. So, even if one still believes that Climate Zone 4 is too cool for cool roofs now, it certainly won’t be for long.
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