Curtain wall requirements imply teamwork

In the design of metal-framed curtain wall, there are generally five key concerns: structural integrity, provision for movement, weather-tightness, energy efficiency, and sound control. Photos courtesy Vitrum

Air Infiltration

Industry recommendations limit air infiltration through the wall shall not exceed 0.3 L/s•m2 (0.06 cfm/sf) of fixed wall area, plus the permissible allowance specified for operable windows or doors when tested in accordance with ASTM E283, Standard Test Method for Determining Rate of Air Leakage Through Exterior Windows, Skylights, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen, at a static air pressure difference of 300 Pa (~6.26 psf).

Canadian specifications typically require a maximum allowable rate of air leakage not to exceed 0.10 m3/h/m2 (0.06 ft3/min/sf) of the sample curtain wall area when tested at an air pressure difference of 75 Pa (1.57 lbf/sf). These values are per the Glazing Contractors Association of British Columbia (GCABC) Glazing Systems Specifications Manual.

NBC Part 5, Section, states a requirement of 0.2L/s•m2 for fixed portions, including opaque portions and 1.5 L/s•m2 for operable portions. Any jurisdiction that has adopted the 2015 NBC would have these requirements. Section lists exceptions to these requirements. Specifiers should be sure to verify the specific requirements of the local jurisdiction.

4. Energy efficiency

Although metal and glass are materials with inherently low resistance to heat flow, improving thermal performance can be accomplished by minimizing the proportion of metal framing members exposed to the outdoors, eliminating thermal short circuits by means of ‘thermal breaks,’ using high-performance insulating glass, and providing adequate insulation in large spandrel areas. In addition, the large glazed areas allow natural light to penetrate deeper within the building to supplant electric lighting and save energy, while optimizing solar heat gain using low-e coatings.

In Canada, minimum national energy performance is determined according to the Model National Energy Code for Buildings (MNECB) for Part 5 Buildings, or ASHRAE 90.1 Energy Standard for Buildings Except Low-Rise Residential Buildings criteria, however, some provinces and territories may have their own provincial energy codes.

Thermal transmittance

According to the Public Works and Government Services Canada’s and the Canada Mortgage and Housing Corporation’s (CMHC’s) Glass and Metal Curtain Wall Best Practice Guide, the fixed light area of the curtain wall must have an overall thermal transmittance (U-factor) [W/(m2•K)] not exceeding that stipulated by the specifier. Note, the U-factor so selected will be defined by applicable codes based on project location and may be guided by other program criteria, such as Leadership in Energy and Environmental Design (LEED).

The overall coefficient of heat transfer (U-value) is an important property of a curtain wall system. When compared with the solar heat gain coefficient (SHGC) of the glazing, an overall energy performance level can be determined. A test for thermal transmittance means heat flow due to conduction, radiation, and convection.

U-factors are tested per AAMA 1503, Voluntary Test Method for Thermal Transmittance and Condensation Resistance of Windows, Doors and Glazed Wall Sections, or simulated per AAMA 507, Standard Practice for Determining the Thermal, Performance Characteristics of Fenestration Systems in Commercial Buildings, or (optionally) applicable U.S. National Fenestration Ratings Council (NFRC) testing, modelling, and validation protocols.

In Canada, using the methods described in SCC’s CSA-A440.2, Energy Performance of Windows and Other Fenestration Systems, allows U-factor to be determined either by computer simulation or through testing per referenced ASTM methods, per the GCABC Glazing Systems Specifications Manual.

Condensation resistance

The resistance of highly conductive curtain wall framing to condensation under winter conditions is important in a cold climate. Testing or analysis to assess the condensation potential of a curtain wall system is carried out by one of three different means, each with its own limitations. These means include a simple, large chamber test, a formal thermal chamber test, and computer simulation.

The fixed light area of the curtain wall, including glass and metal framing, should have a condensation resistance factor (CRF), not less than that selected by the specifier based on climate zone when tested in accordance with AAMA 1503, which yields the CRF. In Canada, this is done using the test method specified in CSA-A440.2. Although not a mandatory part of this standard, the results are used to calculate the Temperature Index (I). Note, CRF and I cannot be mathematically reconciled to each other.

5. Sound Control

Insulating and/or laminated glass generally improves sound attenuation. Where a high degree of sound insulation is required, such as near airports and in metro areas, air leakage through the wall, and resonance of rigidly supported glass lites, also should be minimized. For more details, refer to AAMA TIR-A1-15, Sound Control for Fenestration Products, or 1801-13, Voluntary Specification for the Acoustical Rating of Exterior Windows, Doors, Skylights and Glazed Wall Sections.

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