Daylighting Simulation in LEED 2009: 2 Models = Unanswered Questions

Despite the fact that I almost live on the interwebs and have a penchant for butchering images in photoshop, I’m not the most technologically advanced person when it comes to Revit and the bevy of associated environmental modeling programs available. I got an email from one of my colleagues asking how to interpret the following requirement for compliance with EQc8.1, Daylight and Views, Daylight credit using Option 1, Simulation, and I’d like your help in determining an appropriate response:

“Demonstrate through computer simulations that 75% or more of all regularly occupied spaces areas achieve daylight illuminance levels of a minimum of 25 footcandles (fc) and a maximum of 500 fc in a clear sky condition on September 21 at 9 a.m. and 3 p.m…”

The issue here is that they ask you to generate two models (one at 9am and the other at 3pm), but no guidance is provided about how to combine these models to show that 75% of the space meets the footcandle requirements…

What the hell does this image have to do with a post about daylighting compliance?*

Previous editions of LEED forced you to simulate the daylighting levels at noon, and I suspect that by using solar positions that are ‘lower’ in the sky more projects should be able to claim credit since daylight should penetrate more deeply into the building. The reference guide goes on to discuss other issues surrounding the simulation, but none resolve the problem brought to my attention earlier today. My reading of this leads me to two possible interpretations:

‘Subtractive’ Compliance

In the ‘subtractive’ compliance scenario, you would take the two floor plans and run the daylight intensity model for both and mark the compliant square footage on each. You would then combine the two models, and only include the spaces that are compliant on BOTH models in your calculations to determine if 75% of the overall regularly occupied space and mark your compliant square footage. The result is a combined space that is smaller than either model individually.

‘Additive’ Compliance

This interpretation would require you to generate the two models, but instead of cutting out the spaces that are not compliant at both times, you combine the two for a larger footprint than was created by either model.

The final word?

After I wrote 95% of this post, it occurred to me I should scan the forums for an answer, and I found a reasonably definitive answer on the LEEDuser forums**, courtesy of Jill Dalglish at Dalglish Daylighting:

“I received this statement in a response from USGBC Technical Customer Service:

‘The simulation needs to document compliance at both 9:00 a.m. and 3:00 p.m.. Only areas that meet the requirements at both of these times are considered complaint.’

To me, this means that you cannot take an average and you cannot evaluate the two times separately, ie. you cannot include the space in the compliant area if 75% of it meets the footcandle requirement at 9am and then also meets it at 75% at 3pm (unless that 75% overlaps.)”

This clearly supports the ‘subtractive’ method of compliance, but I’d love to hear from someone who has performed and submitted and had this strategy approved in a LEED review. Please share your experience by leaving a comment!

Forgive my Rant

Another sidenote to anyone at the USGBC responsible for developing the reference manuals: This is ridiculous! It’s one thing that this slipped through the cracks in the first edition of the reference guide, but to also miss it in the addenda makes me want to scream… It would be one thing if it was a minor issue, but people cannot complete their documentation without this critical piece of knowledge! How is this overlooked (at least) twice?

* Free kudos and (if desired) a link to the site of your choice to the first commenter who points out what, if anything, this picture has to do with this post… Liz, you can’t enter!

** FULL DISCLOSURE: LEEDuser is a sponsor of this site… I know there’s been a bunch of of ‘full disclosure’ posts lately, and I can only offer my word that it’s not by design! I really do use my sponsor’s sites on a regular basis.

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