Question about heat island

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    Michael Ermann

    Roof Albedo. Roof Emissivity. Heat Island Effect.

    Benjamin Franklin was the first to document the link between surface color and temperature. On a sunny Philadelphia day, he put swatches of fabric on the snow and watched as the snow melted first around the blacker fabric. The darker fabric let more of the sun’s heat energy in.

    Because one roof can be 70 degrees warmer than another one next door, because low-sloped roofs are associated with larger buildings that need cooling in their cores year-round, and because we are doing so much construction in the sunbelt, it is often in our interest to specify cool roofs. They not only reduce cooling loads, but also mitigate the urban heat island effect.

    Cool roofs have two dimensions, albedo and emissivity. The first one, albedo, is relatively easy to visualize, but the second, emissivity, is not. High albedo roofs feature white or shiny surfaces, and therefore absorb less of the sun’s radiant heat. Albedo is measured in reflectivity, with a reflectivity of 0.0 corresponding to a condition where all incident solar heat striking the roof is absorbed, to 1.0, where all solar heat striking the roof is reflected. High-performing (low-energy) roofs reflect at least two-thirds of the sun’s radiant heat, and therefore have a solar reflectance of at least 0.65.

    Think of a room with two doors, an entrance door to let heat in and an exit door to let heat out. If albedo or reflectivity is a measure of the entrance door width, emissivity is then a measure of the exit door width. Keeping a cool building then means we want a small entrance door (high reflectivity) and a large exit door (high emissivity). To own this idea, you have to believe—I mean really buy in—to the concept that warm buildings radiate heat to the night sky, thereby cooling the buildings. Any two objects that “see” one another exchange heat, provided that one is hotter than the other. The night sky is large (as viewed from the roof) and cold (compared to the roof), and the roof itself is large (relative to the building) so the exchange of radiant energy between the roof and sky can be efficient under the right circumstances. Higher-emissivity roofs, ones with specially formulated membrane coatings or proprietary ballast, do a better job of radiating heat away from the building to the cold sky. Emissivity is also one of those metrics that range from 0.0 to 1.0, where a value of 0.0 theoretically corresponds to no heat radiated to the night sky and a value of 1.0 means that all heat is radiated to the night sky. And again, a value of above two-thirds, or 0.66, is considered “high-emissivity.” So to be labeled a cool roof, the roof’s surface must have a reflectivity greater than 0.65 so it absorbs less heat on sunny days . . . .and an emissivity of greater than 0.66 so that it radiates the absorbed heat back to the night sky on clear nights.

    Someone developed a single-number metric, the solar reflectivity index (SRI) to combine these two ideas, albedo and emissivity. SRI falls somewhere between 0 and 100, with a higher SRI corresponding to a higher-performing roof—one with a small entrance door for heat to be let in and a large exit door for heat, once it has entered, to leave. If you are designing a large building or a building in a warm climate, you’ll want to spec a roof with an SRI value of at least 78. SRI is not important for this exam, but I thought I'd throw it in.

    Heat Island Effect

    Urban heat island effect: the local urban microclimate is warmer than surrounding hinterlands, especially after sunset on sunny, still-air days when the thermal mass of the city slowly releases the solar energy it absorbed all day

    Causes: reduced natural landscapes, fewer trees, absence of vegetation, fewer bodies of water (evaporation and transpiration cool the microclimate), more hardscapes, more roofs, dark-colored pavements and roofs, building and road geometries that block breezes that would have otherwise flushed the hot air from the city

    Solutions: just what you'd think. . . plant urban trees and reduce asphalt paving! Also disturb less greenfield, design light-colored pavements and light-colored or green roofs, wide boulevards to allow uninterrupted summer breezes to flush cities of built-up heat.

    So to answer your original question, you'll want HIGH (not low) emissivity to combat urban heat island effect.

    —Michael Ermann, Amber Book creator.

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    Gang Chen

    Yes, a number of other factors also affect heat island effect. See link below:

    https://en.wikipedia.org/wiki/Urban_heat_island

    Gang Chen, Author, Architect, LEED AP BD+C (GreenExamEducation.com)

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    Wenqi Huang

    Thanks Gang!

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    Wenqi Huang

    Thanks Michael! It's pretty clear and detailed!

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    Aleksandar Stojkovic

    Mike, Interesting trivia there. Apparently, in addition to the white and black cloth, Franklin compared the results to a piece of glass. The glass and the black cloth sunk a similar amount. Any idea of what albedo glass has? Going along the logic that reflective surfaces have high albedo, one might think that glass has a high albedo. Although Franklin's test here clear shows that it doesn't...

    On the other hand, low-e glass (low emissivity glass) is considered higher performing because of the lower SHGC. I guess the emissivity for glass does not have the same connotation it does for roofs.

     

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