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.

Defining Regularly Occupied Space: Outliers

While looking into a question about whether or not a particular fixture qualifies for task lighting (it did... I think), I stumbled upon a CIR that explained the differences between what is considered regularly occupied spaces (ROS) in a residential space vs. non-residential spaces.  That has since blossomed into me going through each applicable credit and seeing what pops up...

I decided it would be useful to summarize the ROS rulings for weird spaces here... Understand that I'm rather scant on details here, so if you have a space that sounds close to one of these below it's probably worth reading the full summary before making a final decision. Those with access can reach the CIRs here.

Get it?

In each case below, the rating system, credit, and ruling date for the applicable CIR are included for further research on your own.  In many instances, the rulings do not expressly state that some of these areas are or are not regularly occupied, but the inferences are fairly clear.  In some instances, a space may be listed both as ROS and non ROS based on very slight changes of use. I strongly recommend reading the original rulings if you think it may apply to your project!

Regularly Occupied

• Shipping and recieving warehouses in an industrial facility (LEED-NCv2.2 - EQc6.1 - 01.18.08)
• Manufacturing floor in an industrial facility (LEED-NCv2.2 - EQc6.1 - 01.18.08)
• Prison cells in a prison(LEED-NCv2.2 - EQc6.1 - 08.13.08)
• Reading/work stations for patrons at a library (LEED-NCv2.2 - EQc6.1 - 10.22.07)
• All spaces except closets, utility rooms, other storage areas, and bathrooms in residences (LEED-NCv2.2 - EQc6.1 - 05.12.07 and revised 10.13.07)
• Nurses stations in hospitals  (LEED-NCv2.2 - EQc6.2 - 07.14.08)
• Rarely occupied assembly halls in army facilities (LEED-NCv2.2 - EQc8.1 - 05.12.09)
• Courtrooms, holding cells, and jury deliberation rooms if no security requirements impede the ability for views in a courthouse (LEED-NCv2.2 - EQc8.2 - 03.22.07)
• A greenhouse in a school where classes or groups occupy the space as part of classes (LEED-NCv2.0 and 2.1 - EQc6.1 - 01.24.06) 
• Gymnasium, cafeterias, conference rooms, libraries, and staff lounges for faculty work in elementary schools (LEED-NCv2.0 and 2.1 - EQc6.1 - 10.05.04)
• Circulation paths within open office environments in offices (LEED-NCv2.0 and 2.1 - EQc6.1 - 06.26.01)
• "Roving scientist" desk areas used intermittantly in the California Academy of Sciences (LEED-NCv2.0 and 2.1 - EQc8.2 - 12.21.04)
• Industrial shop in a metal piping and duct fabrication facility (LEED-NCv2.0 and 2.1 - EQc8.2 - 08.29.03)
• Portions of warehouse space where truck receiving and unloading occurs and occupants work all day in an industrial wherehouse (LEED-CSv2.0 - EQc8.2 - 10.21.08)

Non-Regularly Occupied

• Lobbies, circulation areas, and book shelf stacks in libraries (LEED-NCv2.2 - EQc6.1 - 10.22.07)
• Closets, utility rooms, other storage areas, and bathrooms in residences (LEED-NCv2.2 - EQc6.1 - 05.12.07 and revised 10.13.07)
• Exam rooms and break rooms in hospitals  (LEED-NCv2.2 - EQc6.2 - 07.14.08)
• Rarely occupied warehouse area when employees are assigned office space elsewhere in a warehouse (LEED-NCv2.2 - EQc8.1 - 01.29.08)
• (maybe) Simulation labs in a college facility (LEED-NCv2.2 - EQc8.1 - 04.20.07)
• Secure teaching labs (owner previously had issues with patent infringement) in a corporate training facility (LEED-NCv2.2 - EQc8.2 - 10.21.08)
• Courtrooms, holding cells, and jury deliberation rooms if security requirements impede the ability to allow views in a courthouse (LEED-NCv2.2 - EQc8.2 - 03.22.07)
• Pick-up area/lobby for a food bank (LEED-NCv2.0 and 2.1 - EQc6.1 - 08.09.06)
• Recieving/Processing area where no employees are 'stationed' in a household materials collection facilty (LEED-NCv2.0 and 2.1 - EQc6.1 - 09.19.06)
• A greenhouse in a school where classes or groups do not occupy the space as part of classes in a school (LEED-NCv2.0 and 2.1 - EQc6.1 - 01.24.06)
• Staff lounge if only used for short breaks and not for faculty work in an elementary school (LEED-NCv2.0 and 2.1 - EQc6.1 - 10.05.04)
• Computer training and multi-media conference rooms in a multi-use building (LEED-NCv2.0 and 2.1 - EQc8.2 - 05.24.04)
• Field house and competition gym in a... field house and competition gym? (LEED-NCv2.0 and 2.1 - EQc8.2 - 03.04.03)
• Sales stations where only final sales are completed (otherwise employees are on the floor with customers) in a retail store (LEED-CI - EQc6.1 - 02.24.09)

Visible Transmittance: Rules of Thumb

There comes a time in every EQc8.1 Daylight & Views, Daylight 75% of Spaces calculation where you need to fill in value for the "visible transmittance" (Tvis) of the glazing, normally before you've actually specified the glazing. At least this is true for option one - glazing factor calculation, or option two - daylight simulation model. Of course if you're lazy you can pawn the responsibility off on some schmo who now has to measure the actual light levels after construction (option three - sounds expensive, but I wouldn't really know because I'm the schmo filling out option one all the time)...

EQc8 Supporting Calculator

You can wait until you've actually installed the windows to run the numbers, but unless you have copius amounts of glazing you're probably setting yourself up for an unhappy result. So what is a reasonable value for Tvis?

Your first assumption might be to notice that the "minimum Tvis" values for daylight glazing is 0.7, while the minimums all other glazing is 0.4. To be clear, these are not required minimums! You could potentially have a giant room made of nothing but Kalwall with a Tvis of something like .15 and still get the daylighting point. The "minimums" are really poorly named, as they are more like benchmarks of light transmission performance. The higher visible transmittance you have the better off your daylighting factor becomes.

Another thing that will help is to know that visible transmittance described by LEED as "Tvis" is frequently listed in different ways. I've seen VLT, VT, and other similar configurations. It's listed as a percentage or a decimal with about equal frequency. Pella uses a "VLT%" and lists whole numbers. A "53" rating for their products yields a 0.53 rating for LEED.

The Values

You probably won't see any values much higher than .7, and that's for clear glass. As a general rule, the better the window is as an insulator, the lower the visible transmittance. If I was running through a daylight calculation without having already selected the windows, I would assume the 0.4 or .45 Tvis for ALL windows, as that tends to be the lower end of what's available. A more accurate average would probably be closer to 0.5-0.6, but don't hold me to those numbers! I would stick on the conservative side until I knew otherwise, but then again you will need far greater area of window to make up for a lower Tvis value. The bottom line is that if you assume too low of a Tvis value then you're oversizing the windows, but too high and then you risk losing your points (and good daylighting) if you undersize the glazing. The Efficient Windows Collaborative has a very helpful tool for showing the tradeoff between a lot of light coming into the building and heat gain. Note that the highest Tvis for "EnergyStar" qualified windows in Charleston is 0.55

Common Manufacturer Tvis Ratings

• Pella - Put in your zip code and then go to the list that says "Document Types" and select "U-factor, Solar Heat Gain Coefficient..."
• Anderson - Anderson lists their VT ratings under the "Performance" tab of the basic product description page.
• If you have a site to add covering Tvis please let us know in the comments section!

LEED CS folks look out!

For some reason the USGBC has left off a valuable tool from the LEED-CS credit pages on LEED-Online for EQc8.1 and EQc8.2! On the LEED-NC projects there is a VERY helpful EQc8 Credit Calculator for you to use that can be found on those pages. The CS and NC requirements are the same for these points with the exception of developing a feasible tenant layout. It's an interactive PDF that adds up all your square footage room by room and then runs the calculations for both EQc8 credits. All you have to do is input the glazing square footage, the Tvis, and where it's located (vision sidelight, daylight sidelight, skylight, etc.) and it will tell you where you stand. The EQc8 Supporting Calculator can be found here!!! The link will direct you to the LEED-NC sample credit templates for Environmental Quality. You will need to extract all of them and toss the rest. The file you want is EQc8-SupportingCalculator.