ASHRAE Terminology

A Comprehensive Glossary of Terms for the Built Environment
ashrae.org/ashraeterms

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CDD
cooling degree days (CDD)

see degree day.

degree day

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.

degree day (Kelvin-day)

the difference in temperature between the outdoor mean temperature over a 24-hour period and a given base temperature, used in estimating heating and cooling energy use. For any one day, there are as many degree days (Kelvin-days) as there are degrees Fahrenheit (degrees Celsius) departure of the mean temperature for the day from the base temperature

HDD
heating degree day

see degree day.

heating degree day (HDD)

[i.e., HDD65 (HDD18)]. For any one day, when the mean temperature is less than the local or country-specific common temperature base. Annual HDDs are the sum of the HDDs over a calendar year.

infiltration degree days

a measure of the severity of the climate as it relates to infiltration (°F day [°C day]).