Professional development
The CIBSE Journal CPD Programme
Members of the Chartered Institution of Building Services Engineers (CIBSE) and other professional bodies are required to maintain their professional competence throughout their careers.
Continuing professional development (CPD) means the systematic maintenance, improvement and broadening of your knowledge and skills, and is therefore a long-term commitment to enhancing your competence. CPD is a requirement of both CIBSE and the Register of the Engineering Council (UK).
CIBSE Journal is pleased to offer this module in its CPD programme. The programme is free and can be used by any reader. This module will help you to meet CIBSE’s requirement for CPD. It will equally assist members of other institutions, who should record CPD activities in accordance with their institution’s guidance.
Simply study the module and complete the questionnaire on the final page, following the instructions for its submission. Modules will be available online at
www.cibsejournal.com/cpd while the information they contain remains current.
You can also complete the questionnaire online, and receive your results by return email.
Simple thermal analysis for buildings
This module explores the parameters required to assess the relative thermal performance of the building fabric
There are several dynamic simulation packages in common use by CIBSE members to assess the heating and cooling loads in buildings. These enable the sophisticated application of computer algorithms to provide an interpretation of how a building will perform but, as with any complex tool, require a relatively high level of resource (information and time) to provide satisfactory results. However, the relative thermal performance of a particular building will depend on a few well-defined parameters. This CPD will explain these parameters (concentrating on opaque structures), and will set the scene for a follow-up article that will apply a simple, freely available tool to explore the need for active building heating and cooling. In May 2011, the CIBSE Journal CPD
article considered variations in thermal transmittance – U value (W·m-2
·K-1 ) – and,
subsequently, in June 2011 considered the effect of non-homogeneity on U values. The U value is determined by summing up the thermal resistances of a structure, with each resistance calculated by thickness, d (m),
divided by thermal conductivity, λ (W·m- 1
·K-1
value is characterised, it will only provide an interpretation of the steady state heat flow through a structure using the basic
70 CIBSE Journal January 2013 ). However, no matter how well the U
relationship Q = U x A x ∆θ, where Q is heat flow (W), A (m2
which the heat is flowing and ∆θ is the temperature difference (K) between the two sides of the structure. In the real world, there is rarely, if ever,
a time in a building’s life where there is true steady state. For even in winter, where outdoor conditions may be thought to be close to steady state, the building will be exposed to variations in the building occupancy, the radiant solar gains will continue cycling throughout the day and, critically, the building fabric will act to absorb and release heat both from the inside and outside. A reasonably accessible method for approximating the cycling flows of heat and assessing the resulting need for heating or cooling was developed by BRE, and adopted by CIBSE as the ‘admittance method’, more than 40 years ago1
. The output of the admittance method
is comparable with more sophisticated dynamic computer methods but is inherently limited, as it uses a simplified treatment of loads. For example, when assessing cooling loads, it treats the outdoor daily temperature profile as being constant over a repeating number of consecutive days and, for heating loads, it assumes a constant outside air temperature and no solar gain.
) is area of wall/roof through
Also, the basic implementation of the method maintains a constant infiltration/ ventilation rate (outdoor air passing directly into the room). However, despite its deficiencies, the benefits of the simplification from applying the admittance method still holds good even today, where a tablet computer (or a smartphone) can undertake sophisticated numerical analysis and calculate building loads. The thermal admittance method is particularly useful for those designers who are less experienced in building modelling, so that they can gain an understanding of the sensitivity of proposed designs to variations in the basic thermal properties of the construction.
Thermal properties that affect building performance The primary parameters that affect thermal performance of an opaque material are its density, ρ (kg·m-3 Cp, (J·kg-1 (W·m-1
); specific heat capacity, ·K-1
·K-1 ).
); and thermal conductivity, λ
The three terms may be combined as thermal diffusivity, α = λ / (ρ·Cp) m2
·s-1 ,
which is an indicator of how rapidly heat is conducted in a material. The depth that the daily changes in temperature reach within the material will depend on thermal
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