CIBSE AM11: Building Performance Modelling – what’s it all about?

The much anticipated update to the 1998 version of the AM11 CIBSE application manual on Building Performance Modelling was published in November 2015. The manual has been extensively updated reflecting the many changes in the building modelling landscape since the initial release, including advances in computing power and software, changes in legislation and increased industry focus on energy efficiency and comfort, as well as the many advances in available building technologies and built environment knowledge.
The document is certainly comprehensive and, although the tools and methods used are constantly evolving, it provides a useful reference document for those carrying out building simulation, or for those who would like to understand more about the possibilities and limitations of modelling. Inkling acted as an expert reviewer for this publication prior to its release.

This article summarises the main areas covered by the updated document.

 –    Quality assurance

Any modelling carried out needs to satisfy the client brief. It is essential that modellers understand the requirements of the modelling exercise i.e. what it is intended to explore and determine (which may be expressed explicitly or inferred from previous experience and knowledge – clients may require a modellers guidance when defining the brief ) so that they can contribute effectively to the project.

In any modelling, procedures must be adopted that reduce errors and judgement must be exercised to ensure that the modelling tools are producing realistic outputs. Therefore the modeller must have the required knowledge and skills and the tools chosen need to be suitable for the job. Effective communication of modelling inputs and results is a key part of the modeller’s job and should not be neglected.

Modelling is carried out for various purposes (such as to predict likely overheating risk, or to assess compliance with mandatory energy targets), therefore the document includes chapters on the main modelling types encountered. These modelling services may be carried out on the same model, may require a refinement of the initial model, or may require a totally different model to be created.

 –    Modelling for building energy performance regulations compliance and certification

Regulations and requirements will vary by country, in England and Wales regulations include Building Regulations Part L2A and Energy performance certificates. There may also be project specific requirements such as planning conditions and building environmental assessment schemes, such as BREEAM or LEED. AM11 gives detailed guidance on regulations in various countries and the main environmental assessment schemes as well as highlighting the difference between compliance modelling and design modelling.

 –    Energy modelling

There is a need to minimise energy use and CO2 emissions produced by buildings, therefore it is important to understand at the design stage how much energy the building is likely to use, ensuring that the most effective solutions are prioritised. Energy modelling is necessarily an iterative process, with the model being refined throughout the design process. Models also offer the opportunity to analyse and compare actual and modelled building performance at the post construction stage for example to assist with snagging, to inform ongoing energy monitoring or to compare alternative upgrade proposals.

 –    Thermal environment modelling

The focus here is on modelling to assess human thermal comfort. This can be carried out with varying levels of detail ranging from the use of simplified estimation methods or tools through to much smaller scale computational fluid dynamics (CFD) models. Factors influencing the choice of thermal comfort standard are covered, for example generally ‘temperature out-of-range’ standards are most suitable for buildings with undersized heating or cooling systems and ‘adaptive thermal comfort’ standards are most suitable for assessing performance in mixed-mode and naturally ventilated buildings.

–    Ventilation modelling

Ventilation modelling is commonly used to support the assessment of indoor air quality and thermal comfort. As for thermal modelling, ventilation modelling can range from simplified to highly detailed depending on the purpose of the analysis, timescale and budget. Commercial dynamic simulation tools use bulk air flow methods (otherwise known as zonal network methods) in order to model the movement of air around a building. Each zone is assumed to have a single air temperature throughout the space for each time step, this is a simplification of the actual conditions in a room, therefore suitable zoning of the building model is essential.

Commercial tools also allow natural ventilation to be modelled – the size, type and opening schedule (and/or conditions) for windows and doors are defined and the model performs calculations to determine the air flows in and out of the building. This is useful, for example, in the assessment of summertime overheating in naturally ventilated buildings – allowing both stack- and wind-driven ventilation to be considered.

–    Lighting modelling

Building users prefer natural daylight and views out (where the building function allows this). The distribution of daylight in a space will depend upon many factors including the building form, glazing layout and shading. In order to understand the effect of these factors at the design stage, spaces within the building can be modelled using daylighting software. This output of this assessment could be the distribution of daylight factor for the space or the more detailed Climate Based Daylight Modelling metrics. Daylight modelling also allows for artificial lighting controls such as daylight linking and dimming to be modelled to determine the likely effect on  energy consumption and heat gains.

 –    Modelling of plant and renewable energy systems

Commercial software is able to model plant performance in detail and couple this with the modelling of heat and ventilation flows throughout the building and to the external environment. This enables the performance of plant components to be included in more detail and for control algorithms to be defined.

Usefully AM11 also includes a chapter of varied Case Studies detailing the modelling objectives and approaches used for some real life projects.