My team opted to use the measurement approach to document this credit for a recent 48,000 sq ft interior project. The space has an amazing amount of natural light due to 2 story perimeter windows on every exposure and 3 large light wells with clerestory windows at the building interior. In the southwest corner we got foot candle readings between 1475 and 1102 fc!! I checked it with different light meters and there was a swing of +/- 40 fc. This seems really high butrelatively consistent no matter what light meter I use. I haven't used this method before so maybe this is typical but was looking for some guidance as to what the norm fc levels might be for interior spaces on a window line. Basically, I'm looking for a way to justify the fc levels to what I'm guessing will be a skeptical review team. Any thoughts?. . .

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Jill Perry, PE

Consultant

Jill Perry, LLC

LEEDuser Expert

298 thumbs up

Wed, 11/09/2011 - 18:27

Robin, What time of day did you take the measurements? I'm assuming they were done recently, in Oct or Nov? What is the transmission of your glazing? Do you have any shades or other glare protection and are they in place when you are doing the measurements? You need to have your glare mechanisms in place when you do your measurements. Are they automated? If you're shades are automated, you are excluded from having to be under the maximum. Are you taking the measurements in direct sun? Your measurements are not outside of the normal range of direct sunlight. The swing is normal too. For my own interest, do you feel there is too much daylighting and/or do you feel there is glare? I say "feel" because these things can be somewhat subjective. Your eyes might give you a different feeling from your light meter and your neighbor. This is a good example of why the credit requires daylight to have glare control and be within a range of footcandles (min AND max.) Glare and over-lighting are the two main enemies of good daylighting. As you are over the max. fc limit, it is unlikely you'll be able to swing the review team. What type of activities occur in the building? The only thing I can see that might convince them is if the activities require the amount of light you're getting per the IESNA Handbook. Which, in my recollection, is detailed surgery or hand drafting! Close the blinds and retake the measurements early or late in the day (since this version of LEED doesn't prescribe a date or time to take measurements). Let us know how it goes!

Robin Bass

(No longer at Huntsman Group)

LEEDuser Basic Member

19 thumbs up

Wed, 11/09/2011 - 19:21

Hi Jill, Thanks so much for your reply. The reference guide doesn't say that the glare control needs to be down when the measurements are taken so I had originally taken them will all of the blinds (manual and mechanized) up. I took the readings on September 7th from 11:30 - 1:30. There are blinds on every window, manual in open office areas for occupant control and mechanized on windows above head height and at the clerestories. Not sure what the Tvis is for the existing glazing- it's definitely not clear. I will take the readings again with blinds down - ugh!! The users are computer programmers and they keep the blinds down all the time. Amazingly, the space doesn't feel overlit even with the shades up. It feels really great. It was bewildering taking the readings and having them be so high because it feels nice in the space - not like your standing on the surface of the sun! :) We have had zero complaints from what is a very sensitive user group with regard to light levels and glare. This tenant is very attuned to their employees comfort and have already sent out surveys soliciting this kind of feedback from them. Everyone LOVES the daylighting. No complaints about glare or light levels. Our lighting power reduction, including the task lights is 46% below ASHRAE. I know they are saving more than that becasue the lights are only on for an hour in the morning and a couple of hours at the end of the day (especially this time of year). Great success story for daylighting - hopefully the second round of measurements will reflect this! Any thoughts on an acceptable fc range I should look for with the blinds down? Thanks again for your help!!! Robin

Jill Perry, PE

Consultant

Jill Perry, LLC

LEEDuser Expert

298 thumbs up

Wed, 11/09/2011 - 21:28

You're right, it doesn't say if the blinds should be down or up. This to me means it is your choice. Glad to hear this level is acceptable and there are no glare problems. The only other thing I would double check is that your meter is on fc and not lux. I can't say what to expect for the blinds down because I don't know the transmittance of the blinds - hopefully below the 500 fc. Great job on the lighting power density. Good luck!

TODD REED

Energy Program Specialist

PA DMVA

LEEDuser Expert

626 thumbs up

Thu, 11/10/2011 - 14:53

People will be more acceptable to higher daylight levels then electric lighting levels. So 50fc of electric lighting on a surface may seem glary or even too much, but daylight levels of 50 or even 100 are perceived differently and are more accepted. We are animals, therefore natural occurrences are accepted by us. Two stories worth of glazing along with a bilateral lighting condition is the reason for such high illuminance levels. Typically one would not have that much glazing for computer use. I would look at levels with the blinds half down to be similar to single story glazing. The reality with LEED is that not every project can earn every credit nor can every credit be tailored to meet every type of project or condition.

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