The Effect of Climate on Specifying Cool Roofs 

In the simplest terms, climate can be discussed with respect to Heating Degree-Days (HDD).  Cooling Degree-Days (CDD) are also used to help describe the climate for a given area, but we can generally describe the climate by only referring to heating degree-days.  Heating degree-days are physically measured and recorded on a daily basis for a given area.  The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 90.1 defines HDD65 as “…a notation that indicates that for any one day, when the mean temperature is less than 65oF, there are as many degree-days as degrees Fahrenheit temperature difference between the mean temperature for the day and 65oF.  Annual Heating degree-days (HDDs) are the sum of the degree-days over a calendar year.” To help simplify this concept, imagine a climate where, during a three-week period, the average temperature was 64oF for 10 days, 60oF for 5 days and 55oF for 6 days. Each of the 64oF days represents 1 HDD (a total of 10 HDD) because the average temperature was 1oF below 65oF. Each 60oF represents 5 HDD (a total of 25 HDD) and each 55oF day represents 10 HDD (a total of 60 HDD). The grand total for the three weeks is 95 HDD.  To help simplify this concept, imagine a climate where the average temperature is a constant 64oF for three weeks.  The climate for this area during this time frame can be described as having twenty-one (21) HDD.  However, more data collection would be necessary since HDD is the sum of all of the degree-days in a year.  A list of HDD values is contained in the latest edition of the ASHRAE 90.1 Normative Appendix “D” for most major cities in the U.S. 

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