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.


Erik said...

The response you got back was confusing, but how it is first stated in the manual makes sense to me. If I were running the simulation, I would run one for 9AM and one for 3AM. As long as they were both above 75% (no matter how the areas are arranged respective to the two) then you are in compliance. That is how the manual reads, at least to me. And now that I've thrown my two cents in, I'll ask a question which will make you think "why did I read his two cents?" What daylighting program do you use?

Joel McKellar said...

You could be right... I could very easily be the one misinterpreting the response!

Daniel Overbey said...

What's always baffled me is why the LEED daylighting credit requires CLEAR sky conditions for the simulations. I've been doing illumination simulations for years and unless the project is located in a predominantly clear sky climate (e.g. Las Vegas, Phoenix), the default sky condition for modeling should always be OVERCAST. Typically, interior illuminance studies use the CIE Standard Overcast Sky condition as a default.

Unknown said...

The picture has 2 models....

the beastmaster said...

That picture has to do with Daylighting because it is a pic of Matt & Kim and their most popular song is Daylight

Chris S said...

For what its worth, just going by the plain meaning of the text, I'd agree with Erik. Break out the basic idea from elaboration. " 9 & 3." If they meant the subtractive reading they should have said at "both" 9 & 3. But hey, that's just my reading.

Anonymous said...

My reading also assumed the interpretation that Erik described. There is no "additive" or "subtractive," the two models are completely separate. I would make the phrasing clearer (and longer) by writing:

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.


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 3 p.m.

Joel McKellar said...

The beastmaster wins the photo contest... Let me know if there's a particular link you want highlighted and I'll mention it in my next post!

After thinking about it, I suspect Erik/ChirsS/Anon's interpretation. I'm going to try to get a more 'official' response and will update this post when appropriate.

Unknown said...

When I finally find someone who is willing to spend the amount of time to it takes to attempt this 1-point credit, hopefully this question will be solved. Until then, I maintain that the response I got from the Technical Customer Service supports Joel's subtractive compliance method. (Thanks for posting my comment and giving me credit and a link!) I think there is definitely still room for interpretation.

If it helps (which it probably doesn't), here is how I posed the question: "For the simulation method, (Option 1) what area qualifies, is it area that meets the 25 fc minimum for 9:00am, 3:00pm, both or either?"

I wish I had the subtractive and additive descriptions before I asked the question, because that is the best way I've seen to describe the question so far, short of drawing a picture. I will re-submit the question to the Technical Customer Service with all three interpretations and see if I can get a clearer answer.

Unfortunately, the burden of proof here is on the easiest compliance route because as long as there is doubt, no one is going to want to bank on getting the credit through the least restrictive method.

Allison Beer McKenzie said...

I was actually looking at the "Supplemental Daylight and Views Calculation Spreadsheet", which is required excel spreadsheet for this credit available through LEED Online just the other day to get a better feel for how the documentation for the new "combined" method works. I had been interpreting the two model method as Erik had indicated- as long as both models comply, you're good, but the spreadsheet doesn't seem to support that. In the calculator, you have to list each regularly occupied space and you simply get one box for how much floor area has daylight of 25 to 500 footcandles and one box for how much floor area has daylight above 500 fc. I don't know which model you take these values from! At first I thought the calculator maybe just hadn't been updated for LEED 2009, but since it gives the combination method, it clearly has... This makes it seem like the area must be compliant in both models, but who knows...

Anonymous said...

how am i supposed to calculate daylighting credit for a dynamic glazing which changes its visible light transmittance.