Managing Occupant Comfort with Highly Transparent All-Glass Facades

Over the last two decades, energy codes have ratcheted down the allowable energy transmission through glazing systems. Architects, in many cases, had little choice but to specify glazing with reduced area or reduced visible light transmission (VLT) in order to comply with the changing codes. In the past few years the trend toward these reductions has reversed. The latest generations of solar-control glass coatings allow ever more visible light transmission, while maintaining solar heat gain coefficients (SHGC) that meet energy code requirements. At the same time, energy codes and green building guidelines have increasingly promoted daylighting as a means to reduce energy use by artificial lighting. The availability of these highly transparent products, combined with the greater emphasis on daylighting, has resulted in designers specifying large expanses of highly transparent glazing. Such a design strategy, while energy code-compliant, must be approached with caution. From a visual standpoint, occupant complaints of excessive visual brightness, veiling reflections, and glare can become a serious issue if an overabundance of daylight is not properly managed. Daylight management strategies include modifications to the glazing system, interior shading devices, room layout and material changes, and even modifications to the glass product itself. From a thermal comfort standpoint, as the percentage of glass increases and approaches 100 percent, the percentage of dissatisfied or uncomfortable occupants increases due to asymmetrical thermal radiation, drafts, vertical air difference, and hot or cold floors. Comfort strategies to offset the percentage of glass include the addition of perimeter heat, constant supply air flow and the careful placement and selection of supply diffusers.