Careful consideration should be made to the client/employer's requirements for the building, its end use and overall budget for the envelope enclosure of the building. Early advice should be sought from the cost consultant to find out what is affordable to avoid over-specification and/or unnecessary redesigns.
2. Selection of materials
Selection of materials should be made to meet the specified performance criteria. Consider the following to determine what type of glass or metal panel types will satisfy the design intent:
- Strength (to determine glass/metal type and thickness): wind loads; live and dead loads; imposed loads by adjacent structures; thermal stress
- Health and safety, including Construction (Design and Management) Regulations requirements and risk assessments: human impact, to meet Building Regulations requirements; containment,such as barriers; post-failure behaviour, such as overhead glazing
- Environmental (to control thermal radiation and heat transfer by convection and conduction): control of heat loss (by insulated glass units and metal panels); control of solar heat gain (for example, using coated or tinted glass); control of UV (using laminated glass with polyvinyl butyral interlayers)
- Appearance: colour, reflectivity
- Quality requirements: control of impurities (such as nickel sulphide inclusions in glass); visual defects; edge damage; tolerances; flatness; durability; finishes (such as anodising/powder coatings).
3. Performance criteria
The specification should include the following minimum performance criteria to enable the contractor (and subcontractor) to prepare a compliant tender for the cladding:
- Structural movements and tolerances, including allowable deflections
- Design loads, including dead loads, live loads, imposed gravity loads and movements
- Wind/air pressure loads
- Thermal loads
- Moisture movement
- Thermal performance requirements to meet the new Part L of the Building Regulations
- Solar performance requirements of glazing including light transmittance, outdoor and indoor reflectance, UV transmittance, energy transmittance, outdoor reflectance and absorption, solar factor (G-value), shading co-efficient and U-value
- Allowable air permeability/infiltration
- Psychrometric data (climatic and internal design temperatures and humidity values)
- Acoustic performance
- Fire rating
- Durability requirements for primary and secondary components.
Materials and finishes should be specified to the appropriate standards and codes of practice. Specified elements should include fixings, adhesives, sealants, gaskets, metalwork, finishes and glass and coatings.
Installation of the proposed cladding system should be carried out by suitably trained and qualified personnel. Specify tolerances for manufacture and installation and insist that the contractor confirm that they are achievable or propose alternative tolerances.
For bespoke systems, a representative sample of the cladding should be tested for airtightness, water-penetration resistance and wind resistance, prior to final fabrication and installation.
7. Samples, prototypes and benchmarks