Ok, so I've been asked to assist with some lighting calculations to satisfy the LEED 8.1 requirement for a building we've designed. I am using Max Design 2010, made sure my model is airtight, and followed the suggestions outlined in both Autodesk white papers ("Getting Started" and "Advanced Concepts") on daylight simulation.
I have a few questions.
1) What's the best source for inputting values for Diffuse Horiz and Direct Normal Illuminance in the CIE Parameters box? The value suggested for Diffuse Horiz by the Autodesk Ecotect website for my site's location is 687fc. However, if I use the Perez All Weather data, I get values ranging from 1600fc to 2300fc, depending on which equinox day I pick. I have no data source for Direct Normal Illuminance.
2) Another variable that seems to make a big difference is the number of final gather bounces. The Autodesk white paper recommends values between 4 and 7 bounces. 7 bounces produces higher footcandle measurements at my 30" work surface height than 4 bounces, which makes sense. But then, 10 bounces increases the measurements over 7, and 20 has higher readings than 10. So what's the most accurate?
3) What's the deal with sky portals? Does using them invalidate results? They sure help that outside light get inside.
Using the CIE clear sky model with the Diffuse Horiz value suggested by the Ecotect website and no sky portals, a lot of my rooms are very dark. I have values of 15fc just two feet in front of a 2.5' x 4.5' north facing window. The corner of this small room is getting a whopping 3fc. That seems pretty dark in my inexpert opinion. Using the Perez weather data, 30 final gather bounces and a sky portal gets me above the minimum 25fc required all over that room (including the dark corner), which is fine, but I don't want to cook the numbers. I want it to be accurate.
So, any help or guidance would be greatly appreciated. The architect has been standing over my shoulder the past few days fretting at the numbers coming out of the calculations. The day for making changes to the building has passed, so this is purely for verification for the LEED credit at this point. Thanks!
