Performance requirements

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. This is as opposed to prescriptive specifications which set out in precise detail how something should be done.

Performance requirements might be set out in briefing documentation, performance-based specifications, output-based specifications, legislation and so on.

The design of a building can be divided into precise performance requirements which might include:

• Cost.
• Capacity.
• Appearance.
• Durability.
• Strength
• Stability.
• Thermal performance.
• Comfort.
• Weather tightness.
• Fire protection.
• Lighting
• Ventilation.
• Security.
• Safety.
• Privacy.
• Energy efficiency.

No specific advice need necessarily be given to indicate how the specified standards should be achieved as long as they are achieved.

For example, the UK building regulations are regarded as being performance based, even though the Approved Documents to the Building Regulations provide suggestions, which, if followed will achieve the required performance. .

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 are ‘prescriptive requirements’ where, in the case of a wall 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 and the input of the contractor and other suppliers, but it can also make their task easier as they do not have to research different approaches. Performance requirements allow numerous interpretative designs which can bring time and cost savings as well as exploiting the available skills and materials.

[edit] Related articles on Designing Buildings Wiki

• BREEAM performance targets.
• Brief.
• Key performance indicators.
• Milestones.
• Output-based specification.
• Performance bond.
• Performance gap.
• Performance in use of completed buildings.
• Performance specification.
• Performance specified work.
• Specification.
• Suspension of performance.