Last edited 04 Aug 2014

Cooling degree days

Cooling degree days (CDD) are used to give an indication of the effect of outside air temperature on building energy consumption during a specified period of time. They represent the number of degrees and number of days that the outside air temperature at a specific location is higher than a specified base temperature (or balance point). This gives an indication of how much cooling will be required in the building.

The base temperature of a building is the temperature above which it needs cooling, which will depend on the building type and use.

So for example, if the base temperature is 21 degrees, and the outside air temperature is 30 degrees for eight hours (one third of a day), then that represents 3 cooling degree days ((30-21) x 1/3 = 3).

Adding up the cooling degree days can give a weekly, monthly or annual figure.

Cooling degree days can be found for locations with publicly available meteorological data and so it is necessary to locate an accurate weather station, with good historical data close to the site being investigated, or in circumstances similar to the site being investigated. Weather station data such as that available from airports is often considered to be the highest quality.

Websites such as Degree Days Net can be used to calculate cooling degree days for a given location and base temperature.

Data is also available to allow calculation of ‘heating degree days’ representing the number of degrees and days the outside temperature is below the base temperature and ‘growing degree days’, based on the temperature range within which plants will grow.

Heating and cooling degree days can be used to help assess or compare different potential sites for development. They can also be used as a way of normalising weather between different sites, allowing comparison of the performance of different buildings, or for normalising weather between different years to assess the change in performance of a building.

However, despite being a very simple concept, they are difficult to use effectively in practice, and do not always provide an effective way of influencing design decisions. There can be difficulty separating out heating and cooling energy consumption from overall energy consumption, and heating and cooling requirements are influenced by other factors such as; other heat sources (including people, lighting, equipment and solar gain), thermal mass, thermal insulation and so on. In addition, heating and cooling inputs tend to be intermittent rather than continuous, and buildings are not necessarily always occupied.

It is possible to take account of these matters in calculations and through the use of sub-metering, but this can become a very complicated process, based on a deceptively simple indicator. The use of degree days should be treated with caution as part of a broader process of analysis, providing a general indicator for order of magnitude assessments rather than for accurate, detailed comparisons.

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