WFR question

You use the area in your daylight zone which is either determined by 2x the window height for the depth, or if there is an obstruction, i believe it is then a 63 degree angle. This is the area that is used in the calculations.

Cecillia,

See this guide by LEEDuser. http://www.leeduser.com/credit/NC-2009/IEQc8.1#doc-tab

And page 400 of this "Addendum Details". https://www.usgbc.org/ShowFile.aspx?DocumentID=9377

You use the floor area that makes the result come out between 0.150 and 0.180. If your initial result using the 140 sf comes out above 0.180, none of the area complies. If it comes out below 0.150, adjust the depth of your floor area down until the result is above 0.150. This is the area of the room that complies that you enter into the spreadsheet. You can break up the space how you like as long as each one is connected to a perimeter zone with a window.

Jill Dalglish wrote: "You use the floor area that makes the result come out between 0.150 and 0.180. If your initial result using the 140 sf comes out above 0.180, none of the area complies. If it comes out below 0.150, adjust the depth of your floor area down until the result is above 0.150."

Jill, just to clarify, are you saying that we are allowed to adjust 'bay depth to core' as much as we want until we get a value between 0.15 and 0.18? Can you please clarify a bit if you don't mind, as this has not been my interpretation in the past - rather it has been that the depth is 2 x the wall height, or less based on a 63 deg angle if there is a dropped ceiling or other obstruction. This has always produced wildly different values for each zone, often out of the 0.15-0.18 range. As a result we have basically given up on the prescriptive path as it is far too rigid. If we have no control over the bay area or window area, we would basically need a different Tvis for each zone, which is unrealistic. We have instead tried to learn Ecotect / Radiance to show compliance (which I think is more accurate / fun, but much more time consuming, and a big disappointment if you put in several hours of work just to see that you don't meet). In fact I have considered the rigidness of the prescriptive path is the US/CAGBCs way of saying "sure you can do it this way..but don't waste your time, it wont work unless you have a square box). If you are saying otherwise this may change a lot of our approach!

FYI, I am in Canada and using the LEED Canada Reference Guide, which is even more vague than the US version on this credit, proving to be very frustrating. As far as I can tell, our Ref Guide gives no indication to being able to adjust the bay area.

As much as I would rather be encouraging use of simulation instead because it is much more informative, I do understand that it is more costly than the prescriptive method.

So, the answer is Yes, and I finally found the link to the document again from LEED where I had originally seen it. Click here: https://www.usgbc.org/ShowFile.aspx?DocumentID=9377 then go to page 400. A little more than half way down the page, it says "If the result is between 0.150 and 0.180 the space counts as a daylight zone. If the result is between 0.000 and 0.150, a portion of the space is [compliant]. Calculate the [compliant] floor area as follows: Divide the calculated result by 0.150 and multiply by the floor area of the space. This fractional result represents the floor area to be counted as qualifying daylight area." This math is basically shrinking the area of the bay to the point where it just meets 0.150.

There is no calculation solution for if you are over 0.180. You will need to modify your design to get it under the 0.180. And, I would say that if you are over the 0.180, you are *definitely* over-lighting with daylight and the project would benefit greatly from a re-design.

You still need to do the 63 degree calculation to determine your eligible head height.

What is your backing for saying that a space is definitely over-lighted when only using the prescriptive path? This is not to start any issues, (just a professional discussion) but since this method considers nothing in relation to climate, location, orientation, etc. and you have found a study or have yourself some examples that would make the number 0.180 a daylighting benchmark, i would like to know.

Jill,

Thanks a lot for the clarification ,that really hopes. Now to determine whether or not the CAGBC will accept this method.

We re-ran our prescriptive calcs for a MURB building using this method and are finding that the Tvis which gives us the most compliant area is 0.31. This seems really low to me. In fact, I rarely see below 0.65 or so with a reasonable SC we like to see. Is this is a Tvis you would expect?

It seems to me that by allowing this method when you are below 0.15, and not when you are above 0.18, they are placing a real emphasis on glare reduction. I can't understand why glare is such an issue in these calculations, especially when they mandate glare control anyways. Anyone have any insight into this?

Todd, I am basing it on Cecilia's original question regarding a 140sf office with 0.7 Tvis. It is my professional opinion based on experience that in most climates, locations and orientations, this office does not NEED 36 sf of 0.7 VLT glazing for adequate daylight. In fact, I'm surprised that the original design with a 25 sf window doesn't comply. I can see an argument based on certain climates, locations, orientations, window and ceiling layouts where it might not be overlit. Again, the prescriptive method is a very poor method for evaluating daylighting, which I know you agree with. But, I do think that, just based on experience, if you achieve the 0.180 in the calculation, in most situations, you can do with reducing window size or VLT. Going the other direction is not as easy as I can see a lot of cases where a good daylighting design will come under the 0.150. What is your experience?

Ciaran, all of this information can be dug up from the usgbc.org web site. The link actually refers to the 5 May 11 addendum, not the full reference guide.

Adam, the glare control they require is for direct glare. I think what they are trying to accomplish with the restriction on going over the recommended VLT*WFR is more of a "quantity" glare avoidance meaning that glare can come from high contrast but also purely high quantity of light. Here in Colorado, I only recommend VLT above about 40% for north facades, but I can see in Canada why you might want a higher corresponding SC value. I have seen VLT installed here at 18% facing south that almost looked too bright. It is all relative once you get inside the building and your eyes have adjusted to ambient levels (to a point.)

Thanks Jill,
I have found and have data from our own projects, both simulations and actual measurements, that many of the spaces would not earn the credit using the prescriptive path, yet are exemplary in simulations. We have gone back and done measurements in a few of our projects to verify our simulation methods and these spaces are meeting the requirements in overcast skies (measured) for about 50% of the space. Which was the design strategy in which we implemented.

Ciaran,
The prescriptive method is really meant for projects that are not truly integrating daylight into the design. You are correct that to meet it, you have to have these very low VLTs. Understand this prescriptive path is somewhat based on ASHRAEs' method of establishing the percentage window to wall ratio to reduce solar heat. (I believe it was ASHRAE) So a project not really considering the multiple factors of the impact of the sun on the building are somewhat forced into these low VLT type glazings.

Jill,
18% VLT and it was too bright? I know the air is thinner there, but that is like wearing sunglasses.

I did say "almost" too bright. Can you say that you've never had your sunglasses on and still had more than enough light? The view windows on this EnergySmart high school have a VLT of 18%. http://www.psd.k12.co.us/school/fossil-ridge-high-school

The windows on this fairly new high rise alternate between 20% and 8% (eight percent) VLT. When I was there and before I knew what the VLT's were, I guessed that the 8% glazing was 30%. http://www.xcelenergy.com/staticfiles/xe/Marketing/Files/CO-Bus-EDA-1800...

IT ALL DEPENDS ON HOW YOUR EYES ARE ADJUSTED. On an average day, if there are 5,000 fc outside, 8% of that is 400 fc, right? Now, that is right inside the glass, but to look through it, it doesn't look dark.

Don't be afraid to lower your VLT and correspondingly your SHGC and save on cooling load. Prove it to yourself first by visiting buildings with low VLT and low SHGC.

Todd, what I was asking was, in your experience have you seen a space that was over the 0.180 by a prescriptive calc, and would be hurt lighting level-wise by cutting back on window area or lowering VLT?

Jill I have yet to use the prescriptive method in any of our projects so i cannot say that a space that is over 0.18 would be hurt by lowering the VLTs. I know many of our spaces would not meet the 0.15, yet are well daylit spaces from many aspects. So based on that i can see where a space over 0.18 would not be effected in providing adequate daylight by reducing one or both factors. Its worse to be over daylit then under daylit.

I;m not convinced on those very low VLT glazings. So you get a SHGC of below 0.20 with such low VLT. I can use a VLT of between 60 and 70 and get a SHGC of between 0.25 and 0.30. In our experience and how we do things is that we bounce between the daylight sims and energy models on this matter to find the correct glazing that meets both the energy and daylight goals. What we have found is that once you get below a certain SHGC, the difference is negligible (weighing in other factors). The big difference is the savings from daylighting, and those low VLTs will not get you them. Everyone forgets about the frame and when you have a high performance glass in a cheap conductive frame you are not helping yourseld at all. There are so many variables and that is why cookie cutter buildings don;t work.

I have been in many buildings with these super low VLT glazings and i have yet to be in one where the lights where not on when the sun was out. Secondly the actual quality of view is dramatically decreased. Occupants in those spaces experience a dramatic difference between interior and exterior lighting levels. There have been studies on this and the next version of LEED is proposing a VLT lower limit to even earn the views credit.

I will say that is really dependent on the location and what the target daylight levels are. So if you are meeting what you targeted and have provided a good daylit environment for your occupants, then you have done the right thing.

A lower SHGC is only one reason for a using a lower VLT. The other is to not overpower the electric lighting design. It is very difficult to daylight most spaces 100% of the occupied hours. Therefore, you will be using some electric lighting to augment the daylighting. However, if you have 400 fc on the wall next to the window and 100 fc on the wall away from the windows, the wall with 100 fc will appear dark and most occupants will want to turn on the lights in this situation. For this reason, lower VLT can result in higher daylight harvesting.

I'm not saying that low VLT's are appropriate in all cases. I'm just saying that they should be considered.

I'm not sure why you brought the frame into the conversation. Are you saying this affects the VLT somehow?

I have been in these buildings in many cases where the lights are off and the daylighting is nicely balanced across the space. I invite you to come out and experience them. Come up to the 20th floor of the Xcel Energy building and I will show you the beautiful view of the Rocky Mountains through the 8% and 20% VLT glazing. The view is not diminished.

I would like to read the studies you are referring to. Can you tell me where I can find them? Of course there is a dramatic difference between interior and exterior lighting, there usually is. What is the harm in this?

Thanks for the discussion! This is fun!

Jill I will glady take you up on coming out to Colorado to experience the scenery and the buildings so when i get the chance i shall.

I brought in the frames because it seems that when the discussion of energy and daylight is brought about they are not discussed as much as they should. It is all part of the system . So it was added topic in this great discussion.

I see, and agree about the contrast levels with the lower VLT used. An interesting concept in balancing out the daylight in spaces where daylight levels cannot be reached at the back of the space. So rather than overpower the occupant within 10 feet of the window wall just to meet 25 fcs near the back of the space. Use the electric light to make up the difference and provide all occupants a comfortable environment.

The Excel building has floor to ceiling glass, so the VLT is going to have to be lower, agree and see where you went this. Again, its about balancing all aspects together to achieve the goals set. I would not have recommended anything in high VLT range. My reference of 60-70 VLT would be for a project with an established sill height with a window to floor ratio of between 10-20% depending on the orientation. This would be to start.

Completely agree that you cannot daylight a space 100% of the time and meet all the other requirements without the aid of electric light. In fact daylight and electric light should work in harmony to achieve the required daylight and contrast levels.

I will dig up those reports that i have read on the quality of views and post them. I have also had discussion with others about the clarity of glass and the effects of visual quality. Its not that the view is obscured or blurred, it is the idea about the amount of light received by the eye and the reduction in the subtleties that occur with daylight as the sky changes and the sun moves. The basic idea is to provide a view to the exterior that is as close as it could be if you were outside. This relates to biophilia.

To be clear, I am not responsible for the Xcel Energy building but I have been in it with the glazing supplier.

Jill,
Two reports and a book,
A room wit a view: A review of the effects of windows on work and well being."

Heschong and Mahone" windows in office 2.6.10, page 120 in the results. However, you have to read alot of the report to come to the their conclusion on this.

Biophilic Design, Chapter 15, and the sections on windows.

There is one i read and of course i did not bookmark it or save it to our library, that really had some specifics on the effects of lower VLT glass and view.

One question with the Excel building, how is the view at night from the interior to the exterior? Sometimes, depending on the actually property of the glass, the reflections at night for one looking from the inside is terrible, its like looking in a mirror.

Thanks for the info.

A side note: "Xcel" is their misspelling, not mine.

I have not been in there at night. There is that potential.