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The new ASHRAE 189.1 Standard for the Design of High-Performance, Green Buildings Except Low-Rise Residential Buildings also gives the options of performance, trade-off or prescriptive compliance. The prescriptive requirement for fenestration-to-wall ratio is 40 percent, so not quite as rigorous as the 2010 version of 90.1, but 189.1 also offers tough prescriptive requirements for the various windows metrics (U-factor, SHGC, VT) based on climate zone. The envelope trade-off and whole-building performance are options as well.
Already since the 189.1 standard was released in January, ASHRAE has issued two addenda that deal with how daylighting area is calculated in the performance compliance option and a minor change to the definition of daylighting to more closely align it with how 90.1 treats daylighting.
As a whole, according to ASHRAE officials, 189.1 is expected to result in buildings 30 percent more efficient than the 2007 version of ASHRAE 90.1. So facility managers who are already familiar with current 90.1 requirements are well-positioned to build in 189.1 compliance if that standard is adopted.
Window and glazing selection is no longer simply a matter of picking out the least expensive option. To get the best result — energy-efficient, code-compliant buildings — facility managers should work closely with engineers, daylighting consultants, product experts and architects to cover all of the many building systems affected by window and glazing selection. Benney sums this up succinctly: "We build buildings as a system now."