Last edited 28 Dec 2019

Performance requirements

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Performance requirements typically comprise a set of criteria which stipulate how things should perform or the standards that they must achieve in a specific set of circumstances. These might be set out in briefing documentation, specifications, legislation and so on.

The design of a building can be divided into precise performance requirements which must be established under the various functions that a building must typically achieve. These might include:

No specific advice need be given to indicate how these functions can be achieved as long as they are achieved. (In the case of the Approved Document of the Building Regulations, some suggestions are given which, if followed will achieve the required values). Nevertheless, UK building regulations are regarded as being performance based.

An example would be:

‘The walls should be constructed to achieve a U-value of xW/m2/°K’ where ‘x’ is the value to be attained. This is a performance-based requirement as it stipulates that the wall must perform to the stated U-value (in terms of thermal efficiency) but it does not specify how it might be achieved.

The opposite of performance requirements is ‘prescriptive requirements’ where, in the case of a wall (as given above) the requirement may specify (ie ‘prescribe’) a specific size, materials and a set of construction techniques to achieve the required value.

Typically, a prescriptive requirement might be as follows:

‘To achieve the required U-value, external walls should be constructed with 102mm facing brick, 75mm of insulation, 50mm clear cavity, 100mm internal blockwork and 25mm internal plaster.’ This prescribes a method of building, giving precise instructions as to how the end result should be achieved.

It is usually assumed that in a prescriptive scenario, what is prescribed has been shown to work. Although it limits the creativity of designers, it can also make their task easier as they do not have to research different ways of doing things.

With prescriptive requirements, there is usually little or no choice as to how things can be done. In contrast, performance requirements allow numerous interpretative designs which can bring time and cost savings as well as exploiting the available skills and materials.

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