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.
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.