NFRC has announced they are exploring the possibility of rating windows and curtain walls for daylighting. The article goes on to explain that there are a lot of variables, a lot of factors going into this. Some points for them to consider:
- They mentioned building orientation as a key factor. Yes, but that’s only one part of the equation. The other is latitude – how far north/south of the equator the building is located. Another factor they mentioned was solar angle, which not only changes hour by hour, but minute by minute, if not second by second. The incident angle of the sun on the glass changes how much actual light moves through the glazing. How will that be modeled and standardized?
- What about surrounding buildings? Daylighting out in the middle of the prairie might be a little different than downtown NYC and surrounding conditions, topography, etc.
- And what floor are we talking about? The 5th floor in NYC’s going to see a different amount of sun than on the 35th floor on a high-rise building or on the 1st floor on a building that’s sitting out in the middle of an open field.
- What about the window framing and the shadows it casts as sunlight comes through the windows? As the sun moves across the sky, isn’t that going to change, too? How will that affect the daylight rating?
- What about weather? Clear and sunny isn’t the same as cloudy and rain (come to Seattle, I know). What about snow? Or better yet, the day after a snowstorm, or reflectivity off ground conditions?
Given more time, more points could be added, but hopefully you get the drift.
But what I don’t get is don’t architects already do this, especially through LEED, which provides daylighting credits? Why is there a need for a separate daylighting rating system? Are there daylighting aspects in LEED that fall between the cracks?
The glass manufacturers have for as long as I can remember listed visible light transmittance in their Sweet’s catalogs and online technical data pages, along with solar heat gain coefficients, shading coefficients, etc. That works for the present system; it’s what’s used to develop the LEED points.
All we can ask of the NFRC at this point is to consider what, if anything, this will add to the building owners who eventually will have to pay for any testing or certifications that may come out of this. And that any rating system reflect what the industry as a whole believes will make life better for all concerned, from the subcontractor up the line to the building owner.
That may have been the case with the residential window rating system. It’s not been the case with the commercial/site built certification process, which is still not where it should be to benefit the industry (my opinion).
Your thoughts? I’d love to hear what others think about this issue.