Password and Zone Floor Area questions

Have you tried saving the spreadsheet to your desktop as another name? Floor area is the total space, or regularly occupied space of the room and Zone floor area is the daylighting zone established for that space or series of space. As far as note 5, what that is, is the area under the skylights (actual square footage) that is calculated by whichever one of the following three options produces the lowest amount of square footage. 1. the zonal floor area may be the ceiling height x 0.70%. 2. 1/2 the distance between the skylights. If you only have one, then you can;t use this method. 3. Is done just as noted. You never add in the floor area, its the floor area determined by the above methodology.

If your space is regular occupied and has no windows, then your floor area is the total regular occupied area of the space and your zonal area is 0. Your zonal area is the area of the daylighting zone calculated by either twice the window height in depth or using the 60 degree angle from the head of window, times, the length . So if no window no zone, but the regular occupied space is still included in the calculations. 171 sf of window in a room that size is not much, that is only 5% window to floor ratio general rule of thumb, depending on orientation, is at least 10%. So the spreadsheet is right, you have a little bit of window, with a very low VLT, in a big space, so it is not a very good daylighting scenario. How are calculating the depth of your daylighting zone?

What is the length and width of the room and what is the window head height? At this point are you only looking at changing the VLT soley based on earning the daylight credit via the prescriptive path? If you are, then i would say that you should not use the prescriptive calculations to design the daylighting performance of a building. It cannot be used to do it because it is only a formula that tells you if your inputs meet a certain parameter. It cannot tell you performance levels, it is not climate or orientated based. I just don;t recommend this option. Gabriela, to answer the question about changing the VLT would require alot more information of the space, climate, orientation, target illuminance levels, etc, etc. I would never say change it based on the prescriptive path.

Ok, so your zonal area is the area that is the length of windows and the depth which is established by either twice the window head height, which would be 20 feet into the space, or based on a depth by using the 60 degree angle.(See Reference Guide) What I am recommending is that you design the spaces/building based on set performance parameters and not based on earning points on a LEED scorecard. If you truly want to provide daylight to the occupants then design the space based on a set of performance parameters. Kind of like, "Form follows Function, or in this case "Form follows Performance". This will require at least simulation. It is also highly recommended that decisions made for daylighting are also coordinated with HVAC design. Does adding too much glazing and or a higher VLT increas the heat gain or does the introduction of daylightin reduce the lighting load. All parts work together, so they should be designed together. Right now, if you just want to see if you earn the credit, perform the prescriptive calculations. If you get it you get it, if not, maybe try the measurement option when completed.

Gabriela, I look at this a little differently. To me, you are given the option of a zone floor area that can be different from the (total) floor area if you want to break down the floor area into smaller zones. The way I do it for side lighting is in two steps. The first step is to create a new zone for every window condition. If you create a zone for the first window and your next window has the exact same size, head height and Tvis, you can lump the area associated with that window into the zone with the first window. If any one of these three things is different, you must create a new zone for that window. After you do this for all of your windows (no overlapping) you then do your VLTxWRF calculation. If your answer is between 0.150 and 0.180 you don't need to go to step 2 and you can enter your total zone area into the spreadsheet. If it is above 0.180, you are over-daylighting and you must enter 0 for your zone area unless you are providing automated shades, then you can enter the total zone area and you also don't need to go to step 2. If you are below 0.150, you need to go to step 2. In step 2, you calculate the area of your zone using this formula: ZFA = (WA xVLT)/0.150. So if, for example, you have a window area of 120 sf. and a VLT of 0.4, ZFA = (WA xVLT)/0.150 = (120 x 0.4)/0.150 = 320. There is one caveat and that is that your ZFA must be equal to or greater than your zone width. Otherwise you must enter 0 for this ZFA. If you have a room that is regularly occupied but no windows, you must still enter that room into the spreadsheet so that its area is included in the total regularly occupied space (ROS). You would just enter 0 for window area. I don't use the diagrams that shows the H and 2H measurements for anything but calculating a modified head height in the case that my ceilings dip below the 63deg line as shown in Figure 2. Sample Modified Window Head Height. (An addendum removed the words “and Daylight Zone” from the title of this Figure.) In that case, all glazing above this line is ignored. You only need to be concerned about note #5 if you have top lighting. If, after all of this, you are still not meeting the credit requirements, I would say that you should re-evaluate what your priorities are for the project, how much time and money you want to spend on getting this one LEED point, how committed you are to providing quality daylighting, how important daylighting is to your overall energy-efficiency goals, etc. If these things are a priority, you will want to devote a good deal more time and money on it. None of us can recommend you change your VLT based on the little information given here. If not, cut your losses and spend your time and money on other credits. You're not likely to meet the requirements via measurements if you don't qualify with the prescriptive method, but you can always keep that open as a last-ditch option if you need it after construction is done.

In reference to your post of December 14, 2012, if you don't used the H and 2H to get your Daylighting Floor Zone Area, then how do you make new zones for each window, or each window condition?

If you had an example, that might help to explain. The formula dictates (sometimes in a two-step process) the size of your daylight floor zone. The first step is to pick two points along your perimeter wall that are as far apart as possible (to reduce calculation effort), but only includes windows with similar conditions (sill height, head height and VLT). If any of the windows between your two points don't match these properties of the other windows, you need to put one of your two points between them and use this point as point 1 for your next zone. From these two points, you draw a line perpendicular to your perimeter wall back to the first wall it touches. Draw a perpendicular line from this point to the other line, forming a rectangle. (If your back wall is parallel to the perimeter wall as is the usual case, this will be the right along the wall.) The length of the lines that are perpendicular to the perimeter wall is the depth of your step 1 daylight zone. Use the formula. If your answer is greater than 0.180, none of this area qualifies as meeting the LEED criteria. You must include it in the total area but not the qualifying area, so this means that it counts against you. The only way to change this and still use the prescriptive method is to lower the window VLT. If your answer is between 0.150 and 0.180 all of this area qualifies as meeting the LEED criteria. You don't need Step 2, you can enter this area into the total sf that you report as qualifying. If your answer is below 0.150, use Step 2. In Step 2, you calculate the area of your daylight zone using this formula: ZFA = (WA xVLT)/0.150. So if, for example, you have a window area of 120 sf. and a VLT of 0.4, ZFA = (WA xVLT)/0.150 = (120 x 0.4)/0.150 = 320. As you can see this formula is just a manipulation of the original formula, working in reverse to figure the maximum floor area that qualifies. It is the same thing as reducing the depth of your zone until you get within the 0.150 to 0.180 range, it is just a more direct route. There is one caveat and that is that your ZFA must be equal to or greater than your zone width. Otherwise you must enter 0 for this ZFA. This caveat holds because in the prescriptive method, you can't use the windows between your two points on the perimeter to "light" an area that is more narrow than the width of your zone. Add this ZFA to your running total of qualified sf and move to your next two points along the perimeter. Does that make sense?

Thank you for your lengthy response. I think it helped. For example, we have a room that is a square measuring 28'-0" x 28'-0" deep for a total of 784 SF, and there are 4 windows, equally spaced, with a head height of 9'-6" and a sill height of 2'-6", at 3'-6" wide, for a window area of 98 SF (and a Tvis of .353). In this instance, the back wall is 28'-0" from the exterior wall, and there are no obstructions on the walls, ceiling or floors, so I count the whole 784 SF as my daylight zone, and get a VLTxWFR of .044. Is this correct? And, as my number doesn't qualify, I then look for a zone that will qualify, by using the second formula. I got ZFA= (9.8x.353)/.150 = 230 SF. So, as my exterior wall is 28'-0", I divide 230/28 and get 8.2'. So, my Daylight Zone is the length of the exterior wall and then 8.2' back from it (perpendicular). Yes? I greatly appreciate your time and help.

Yes. But as you can see, there is no need to do any more math after you get the ZFA number because that is the number that you add to your total qualifying area. For example, if your whole building consisted of just two rooms exactly like this one your qualifying area would be 230+230=460 and your total area would be 784+784=1568. You would not comply with the credit because 460 is only 29% of 1568 and you need 75%.

No, we don't meet the prescriptive parameters for this credit. Drat. Sincerest thanks for your time and help!

You're welcome!