I am working on the daylighting prescriptive method for a veterinary hospital. We have a large lobby and open area near the front on the building with a large amount of glazing to provide plenty of natural daylight. The site is located in a very dense forested area plus the window area is protected by a large roof overhang ranging from 5-9 feet in depth.
After calculating the VLT x WFR, it ends up being greater than 0.18. The floor area is 1,471 SF with a window area of 670 sf of window area above 30 inches. I understand that the point is to prevent glare, but with the overhang and the existing site conditions I would think the glare would not be an issue. Does anyone have any suggestions what I can do? Is there a way to incorporate the large roof overhang to help reduce the VLT x WFR down below .18?
Thanks so much for your help
Seema Pandya
Sustainability ConsultantSLP
150 thumbs up
April 5, 2010 - 12:05 pm
Hi Stephen, in addressing the first question of lowering your VLT x WFR below 0.18, unfortunately there are limitations to using the prescriptive method. The prescriptive method doesn't address added daylight features, such as light shelves, exterior vegetative shading, or overhangs. While your calculations may be indicating that you are above 0.18, the reality of the on sight conditions may be different. I would suggest either doing a computer simulation or hand held light measurements. However, the deep exterior overhangs and shading from exterior vegetation may lower the amount of daylight that enters in the space.
I am interpreting your second question as asking whether exterior overhangs and or vegetation can count as glare control. It sounds like you have a good case. The Reference Guide already recognizes exterior overhangs as a "fixed exterior shading device". You might want to write a CIR for your project if you want to also include the exterior vegetation.