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TM41 Degree days: Theory and application (2006) (pdf)
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TM41 Degree days: Theory and application (2006) (pdf)

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Degree-days are a tool that can be used in the assessment and analysis of weather related energy consumption in buildings. They have their origins in agricultural research where knowledge of variation in outdoor air temperature is important, and the concept is readily transferable to building energy. Essentially degree-days are a summation of the differences between the outdoor temperature and some reference (or base) temperature over a specified time period.

A key issue in the application of degree-days is the definition of the base temperature, which, in buildings, relates to the energy balance of the building and system. This can apply to both heating and cooling systems, which leads to the dual concepts of heating and cooling degree-days.

This TM replaces previous guidance given in section 18 of the 1986 edition of CIBSE Guide B [CIBSE 1986] and Fuel Efficiency Booklet 7 [Energy Efficiency Office 1993]. It provides a detailed explanation of the concepts described above, and sets out the fundamental theory upon which building related degree-days are based. It demonstrates the ways in which degree-days can be applied, and provides some of the historical backdrop to these uses.

It read alongside Good Practice Guide 310: Degree days for energy management - a practical introduction (Carbon Trust 2006), which serves as an introduction to their use. The material in this document provides deeper insights into the degree-day concept, but can be used in conjunction with GPG 310 for advanced building energy analysis. This publication is designed to provide both the theory of degree-days and guidance on their use. It is of interest to two types of user: designers wishing to check the likely energy consumption of a particular design, and energy managers wishing to assess energy performance of existing buildings.

Available as pdf only.


Principal author: Prof. Tony Day (London South Bank University)

Task Group
Chair: Bryan Franklin
Members: Prof. Martin Fry (Carbon Trust); Prof. Michael Holmes (Arup); Tony Johnston (BRE); Jim Mansfield (formerly West Norfolk District Council); David Wood; Dr Hywel Davies (CIBSE Research Manager)