ASHRAE Terminology

the difference in temperature between the outdoor mean temperature over a 24-hour period and a given base temperature. For the purposes of determining building envelope requirements, the classifications are defined as follows: (a) cooling degree day base 50°F, CDD50 (10°C, CDD10): for any one day, when the mean temperature is more than 50°F (10°C), there are as many degree days as degrees Fahrenheit or Celsius temperature difference between the mean temperature for the day and 50°F (10°C). Annual cooling degree days (CDDs) are the sum of the degree days over a calendar year. (b) heating degree day base 65°F, HDD65 (18°C, HDD18): for any one day, when the mean temperature is less than 65°F (18°C), there are as many degree days as degrees Fahrenheit or Celsius temperature difference between the mean temperature for the day and 65°F (18°C). Annual heating degree days (HDDs) are the sum of the degree days over a calendar year.