Costs: Curtain wallings

The science and art of curtain walling is a highly specialised area. A whole-life cost and performance assessment provides a rational framework within which to assess the many component options.

Curtain walling systems

• Stick systems are site assembled and carry a higher risk of workmanship non–conformities.
• Unitised systems are factory manufactured units comprising framework, glazing and infill panels. This reduces time on site and can cut workmanship defects.

Framing

Framing materials include: aluminium, low-carbon steel, stainless steel, plastics or timber as well as composites.

Aluminium extrusions are the most common option, usually based on alloy 6063 to BS EN 755–1. Aluminium would have a service life of more than 60 years. The type of finish dictates life to first maintenance or replacement:

• Anodising to BS 3987, minimum 25 microns thick, 30 years or more
• Polyester powder coating to BS 6496, minimum 40 microns thick, 15–25 years
• Polyvinylidene–fluoride (PVDF or PVF2) minimum 25 microns thick, 20 years or more.

Fixings should be stainless steel or other strong non–corrosive material, fitted to avoid bimetallic corrosion.

Glazing

Glass has an expected service life well in excess of 60 years. Insulated glass has shorter expectancies: edge seal failures and infiltration of water vapour into the space between the panes are the main causes of failure. Insulating glass units to BE EN 1279 should be specified to give an expected service life of 25 years. Safety glass should comply with BS 6206.

The configuration of the glazing system influences the thermal characteristics of the facade, which is calculated to ISO 10077–1. The less heat is lost through the glazing, the lower the U–value. Measures to lower U–values include:

• Metal frames with thermal breaks to cut linear thermal transmittance
• Spacing frames further apart reduces U–values by increasing the insulating cavity. But frame cross–sections, glazing thickness and hardware strength may have to be increased at higher capital cost.

Glazing U–values are calculated to EN 673, values lower than 1 W/m²k are achievable. Factors that affect glazing U-values include double or triple glazing, low-emissivity coatings, or uncoated glass.

Choices for the gas to fill the gap between panes include air, argon or sulfur hexafluoride and krypton.

Gaskets and sealants

Gaskets and sealants are often the shortest-life components. Non–cellular EPDM gaskets to BS 4255 have an expected service life of 15-30 years or more.

Sealants should comply with ISO 11600, with evidence from manufacturers for durability. Neutral cure type silicone sealant has an expected service life of 5-20 years.

Energy

A whole building approach, started at outline design process, is most likely to provide an energy-efficient solution to energy consumption. Issues to take into account include building orientation, form, aspect, lighting, internal layout and materials used internally.

A suite of European standards offers assurance of curtain wall performance in different locations:

• Watertightness BS EN 12154 defines five classes based on variable air pressures and a water spray rate of 2 l/min/m².
• Air permeability BS EN 12152 tests confirm air permeability for overall area is no more than 1.5 m³/m²/hour for five air pressure classes from 150 to 600 Pa or more.
• Resistance to wind load BS EN 13116 confirms performance of deflection and fixings.
• Site test of watertightness BS EN 13051 may be used to check for water leakage for installed curtain walling by water spray with or without applied air pressure.

Table notes

Curtain walling framing based on nominal 100 mm x 50 mm sections at 1000 horizontal centres, 3 m storey height, double-glazing plain, toughened, 6-16-6 mm, unless otherwise stated. Window and doors included. Costs include cleaning, maintenance, recoating replacements and an allowance for repairs. Cost and work frequencies are generic, based on a simple 1000 m2 of glazed cladding. The model assumes the framing cladding is not replaced during the 60 years. Energy costs excluded. Major refurbishment period is 25-35 years and includes replacement of insulating glass units, gaskets and capping to frames as necessary. A discount rate of 3.5% is used to calculate net present values.