A practical guide to drafting a clear, measurable performance specification for facade systems before tendering.
The Difference Between a Performance Spec and a Prescriptive Spec
A prescriptive specification names the specific profile type, thickness, and manufacturer, while a performance specification defines the required outcome (such as a maximum thermal transmittance or a minimum air infiltration resistance) and leaves the fabricator free to choose the engineering solution that achieves it. The latter approach opens the door to competition between different solutions that may be more economical or higher-performing, but it requires precise drafting to avoid any ambiguity in interpreting the requirements.
Setting Thermal and Structural Performance Criteria With Measurable Figures
The specification must include specific figures that leave no room for interpretation: the maximum U-value, the minimum air and water infiltration resistance classification per an approved standard such as EN 12207 and EN 12208, the design wind load in kilopascals, and the maximum allowable panel deflection under that load. Vague phrases like "good thermal insulation" or "adequate wind resistance" aren't measurable and don't protect the project owner in a later dispute over whether execution matched the specification.
Mock-up Testing Requirements Before Approval
The specification must explicitly state the requirement to fabricate an actual performance mock-up matching the project specification, and subject it to approved laboratory testing for air and water infiltration and wind load before full-scale fabrication is approved. Defining who bears this test's cost, which accredited lab it will be conducted at, and how binding the results are should be part of the specification text, not a detail left for later discussion.
Warranty Clauses and Long-Term Performance Liability
The specification must distinguish between material warranty (covering fabrication defects), performance warranty (covering the system's continued achievement of insulation and sealing criteria over a defined period), and workmanship warranty (covering installation quality). These warranties' durations and scope must be clearly stated, since some fabricators offer a general "warranty" with undefined duration or scope that's difficult to invoke later.
How to Evaluate Different Fabricator Bids Against the Same Specification
When tendering based on a unified performance specification, each fabricator should be required to submit a documented technical calculation proving the proposed solution achieves every performance figure stated in the specification, not just a verbal claim of compliance. Bid comparison should include a compliance matrix comparing every specification clause against what each fabricator has committed to, making it easy to spot any deviation or exception before the contract is signed rather than after execution has begun.
