When it comes to energy efficiency, all agree that in the summer, white roof membranes reflect the sun’s rays, lowering air conditioning costs — while dark roofs absorb the sun’s rays, raising air conditioning costs. Despite summertime savings, some studies seem to indicate that black roofs provide a year-round net savings by helping to heat buildings during the winter. The truth is, those heating bill savings don’t outweigh the costs of air conditioning in the warmer months, even in the most northern states.
Heating degree days (HDD) and cooling degree days (CDD) are measurements used to identify how cold or hot the temperature was on a given day or period of days. Working from the neutral temperature of 65 degrees Fahrenheit (no heat, no air conditioning needed) a HDD measures degrees lower than 65 and a CDD measures degrees higher than 65. For example, a day with a mean temperature of 40 degrees has 25 HDDs. Two 40-degree days in a row have a total of 50 HDDs, and so on. Conversely, a day with a mean temperature of 80 degrees has 15 CDDs. If the next day has a mean temperature of 83 degrees, it has 18 CDDs. The total for the two days is 33 CDDs.