Although we support the committee’s good intentions to promote daylighting, the committee has unfortunately chosen a flawed measure in VT/SHGC. The proposed solution does not achieve the desired objectives of increased VT and reduced solar heat gain. A better alternative would be to tie visible light to the amount of lumens at the workplace, as opposed to tying VT to SHGC with no standard as to the amount of lumens reaching the workspace.
The VT/SHGC requirement should be removed for the reasons described below. Suggested solutions are described at the end.
VT/SHGC does not account for glazing area.
A fixed VT/SHGC ratio does not account for the simple fact that a large darker piece of glazing actually admits more light than a small clearer piece of glazing, yet this requirement penalizes the glazing that is providing more total light. Also, the large darker piece of glazing provides more uniform illumination and less glare on the workspace. Glare and nonuniform lighting (e.g. small bright areas adjacent to darker areas) can lead occupants to close blinds and/or turn on more task lighting, increasing lighting usage.
VT/SHGC for a whole product is not the same as VT/SHGC for just the glazing.
It is common to see many solar selective products listed with a center-of-glazing VT/SHGC over 1.5. However, it is not correct to assume that the center-of-glazing VT/SHGC is the same as the whole product VT/SHGC, just because it is a ratio. The proposed requirement is for the whole product, based on the whole product VT and SHGC ratings, and includes the effect of the frame.
VT/SHGC ≥ 1.5 precludes practical glare control.
Although we strongly support daylighting and lighting controls, daylighting design must balance high VT with glare control. If the VT is too high, excess glare leads to lower productivity, closing blinds, and/or increased task lighting. The market has been experimenting with “the clear look” for several years, and has responded by introducing several new low-e products to address this balance, with low SHGC and medium VT. However, the VT/SHGC ≥ 1.5 goes too far (in part because the whole product VT/SHGC may have been confused with center-of-glass VTg/SHGCg), and would preclude the use of products specifically designed to balance daylighting and glare.
There are better options for promoting daylighting.
While there are a number of possible better options, the GICC recommends the requirement be based on DOE’s definition of spectrally selective glazing (LSG >= 1.25).
The U.S. Department of Energy defines spectrally selective glazing as having a light-to-solar gain (LSG) ratio >= 1.25, where LSG is simply the center-of-glass VT/SHGC. The committee could choose to simply establish this as a requirement in all zones. The term LSG is widely used in the industry and would help avoid confusion between whole product ratings and this center-of-glass value. Although U and SHGC should be whole-product numbers including glass and frame effects, daylight design is concerned with only the light that is actually admitted through the glazing. As a result, architects and designers widely use the LSG (light-to-solar gain) ratio for daylighting purposes, whereas whole product numbers only lead to confusion. The definition of LSG can be added to ASHRAE 90.1, and based upon NFRC 200 technical procedures.