Daylight analysis help!

[Scott Dombrowski]

Ok, so I've been asked to assist with some lighting calculations to satisfy the LEED 8.1 requirement for a building we've designed. I am using Max Design 2010, made sure my model is airtight, and followed the suggestions outlined in both Autodesk white papers ("Getting Started" and "Advanced Concepts") on daylight simulation.

 

I have a few questions.

 

1) What's the best source for inputting values for Diffuse Horiz and Direct Normal Illuminance in the CIE Parameters box? The value suggested for Diffuse Horiz by the Autodesk Ecotect website for my site's location is 687fc. However, if I use the Perez All Weather data, I get values ranging from 1600fc to 2300fc, depending on which equinox day I pick. I have no data source for Direct Normal Illuminance.

 

2) Another variable that seems to make a big difference is the number of final gather bounces. The Autodesk white paper recommends values between 4 and 7 bounces. 7 bounces produces higher footcandle measurements at my 30" work surface height than 4 bounces, which makes sense. But then, 10 bounces increases the measurements over 7, and 20 has higher readings than 10. So what's the most accurate?

 

3) What's the deal with sky portals? Does using them invalidate results? They sure help that outside light get inside.

 

Using the CIE clear sky model with the Diffuse Horiz value suggested by the Ecotect website and no sky portals, a lot of my rooms are very dark. I have values of 15fc just two feet in front of a 2.5' x 4.5' north facing window. The corner of this small room is getting a whopping 3fc. That seems pretty dark in my inexpert opinion. Using the Perez weather data, 30 final gather bounces and a sky portal gets me above the minimum 25fc required all over that room (including the dark corner), which is fine, but I don't want to cook the numbers. I want it to be accurate.

 

So, any help or guidance would be greatly appreciated. The architect has been standing over my shoulder the past few days fretting at the numbers coming out of the calculations. The day for making changes to the building has passed, so this is purely for verification for the LEED credit at this point. Thanks!

[pfbreton]

What's the best source for inputting values for Diffuse Horiz and Direct Normal Illuminance in the CIE Parameters box?

 

The LEED Credit 8.1 relies on daylight factors simulated with the CIE sky model excluding the direct contributions for the Sun. The data is expressed in a percentage of light inside vs outside. In other words, the absolute values have not impact.

 

10 bounces increases the measurements over 7, and 20 has higher readings than 10. So what's the most accurate?

There is either something fishy with your model or you are simulating a very complex light path. A light bulb in a labyrinth will take a lot of bounces to converge to the proper solution while an open atrium will take less. Our tests indicated that the precision is good enough with 7 bounces for a typical offuce interior, but you do not give us indication about your model. Maybe one material has a 100% diffuse reflectance?

 

What's the deal with sky portals? Does using them invalidate results? They sure help that outside light get inside.

We have not validated how accurate they are photometric measurement wise

 

I have values of 15fc just two feet in front of a 2.5' x 4.5' north facing window.

 

Don`t look at the absolute numbers. Look at daylight factors for the 8.1 credit... The ratio is what is important according to its definition.

 

Hope this helps, but as you see its a bit like structural simulations, while the software let you simulate things, it does not mean that the building will stand straight - you need to be able to interpret the results based on your own experience.

 

You can reach me privately if you need further assistance.

 

[raterry]

I'm now in a similar boat to Nauticus27. Just when I think I know what to do, I get confused. We are working on a hospital and need to achieve LEED 8.1. I tested of a portion of the building to see if it worked and I'm getting some interesting numbers but I'm not sure if they are accurate as I'm not sure I am measuring the light properly to satisfy the credit. The LEED requirements state to demonstrate through computer similation, 75% of the spaces need to achieve "daylight illuminance" levels of a minimum of 25fc and a maximum of 500fc. The Light meter helper objects give me the option of total, direct, and indirect illuminance levels which measures in fc's, but the "daylight factor" is a percentage? Which option do I need to use?

 

Also, why does the LEED 8.1 requirement say "in a clear sky condition" but the MAX help file states "A Daylight Factor" measurement is required to obtain credit 8.1...and is assumed to be that of the standard overcast CIE sky model?

 

If I need fc's I can't use "daylight factor" as it's numbers are a ratio or percentage? Is that correct?

 

Lastly, say our floor plan is 50,000 sf, can I just create a light meter helper object that is the same size of the floor plan and make the roof "invisible" to camera and calculate that way or do I need many individual light meters in the various spaces? Thanks so much for your assistance.

-=Rob

[raterry]

The LEED Credit 8.1 relies on daylight factors simulated with the CIE sky model excluding the direct contributions for the Sun. The data is expressed in a percentage of light inside vs outside. In other words, the absolute values have not impact.

 

 

 

Don`t look at the absolute numbers. Look at daylight factors for the 8.1 credit... The ratio is what is important according to its definition.

 

 

You can reach me privately if you need further assistance.

 

 

According to LEED 2009, 8.1 doesn't mention "daylight factors" just daylight illuminance levels. It seems impossible for me to achieve the min 25 to max 500 fc's in a room. the corners have slightly less than 25 fc's and there are bright areas right beyond the window that have measurements in the 1000s+

 

If I display Indirect illuminance, the bright spots go away but the corners are still dark and I can't imagine "daylight illuminance levels with a clear sky" should ignore direct sunlight anyway...Am I missing something?

[raterry]

More interesting info. According to the white papers that Pierre FB wrote, he talks about how to properly model the glazing using either Pro MAterials or Arch&Design mats. I did a test using both with the equivilent settings and the difference in FC data was significant. Anybody know what could be causing this?

 

Is it safe to assume that nobody on this site uses the Daylight Analysis tools because this thread seems to be between me and me. Is it because most memebers are freelancers? Can anyone offer assistance?

[Scott Dombrowski]

Hi Rob-

 

Sorry for the hugely delayed reply. I haven't had many lighting analysis projects for a while and other deadlines have left me no spare time to explore it. Some more projects are coming up, though, so I'm back into trying to figure out how this all works.

 

Anyways, I'm still somewhat confused. According to the LEED documentation, the 8.1 credit can be computed three ways: hand calculations, computer modeling, and on site metering. The hand calculations are the easiest since the end result is expressed as the daylight factor that Pierre mentioned. If you don't already have a 3D model of your building and your light paths are uncomplicated, this is the way to go. If you already have a 3D model of your buildings (the architects are designing it in Revit, for example), then it's probably as much time to do either the hand calculations or doing a lighting calculation in Max. For tricky light paths, however, doing the calculation in Max is required. In my previous project, I had a room within a room that I needed to calculate light levels in. The daylight factor hand calculation does not provide a case for a situation like that. Also, once the architects see the results from a lighting calculation, it seems like everyone wants to do it. Its a lot easier to interpret visually, and it means they don't have to do the work. I do.

 

So, for the "Simulation" option (as it's called in LEED 3.0, "Daylight Simulation Model" is what it's called in LEED 2.2), the results are required to be in footcandles. For a space to count, it must be between 25fc and 500fc, though you can ignore that upper limit if the architect "incorporates view-preserving automated shades for glare control...."

 

Here's my setup, but I've no idea how accurate it is:

 

For the project location, get the .epw weather file from the Energy Plus Weather Data site. Load that into your Daylight Parameters section for your Daylight object. LEED requires the model to be simulated with a CIE clear sky, so the closest I've found to that is under the mr Sky Parameters, changing the Sky Model to CIE and then under the mr Sky: CIE Parameters, selecting Clear Sky. The part that confuses me is the Diffuse Horiz: and Direct Normal Illuminance values in that same section. When you use a weather data file, those values are greyed out because they automatically come from the weather data file. But are those values average values for that location, or are they what was specifically measured on that day? If they're not average values, then it seems that one could pick whichever equinox gave them better results.

 

Next, the materials. I'm using Arch&Design materials for everything... they are what were referenced in the white paper describing the validation of the lighting analysis tools. For each interior surface, I have a generic Arch&Design matte material with the Diffuse Color set depending on the reflectivity of the surface I'm applying it to. Walls get a 75% diffuse reflective material (color .750 .750 .750). The ceiling gets 85%, the floor gets 20%. Are those standard LEED accepted values? Nope, but they seem to make for an okay generic setting. If you know what paints and carpeting you'll be using, you can try to figure out the actual reflectance value, but I think it is important that you just use shades of grey. If you start putting actual texture maps in your materials, you really start messing with the results unless the texture maps are calibrated and probably all sorts of other problems.

 

For glazing, I try to figure out what the transmittance value the architect wants to use. If a manufacturer and product has been specified, then I go through the process Pierre outlined here. If nothing has been specified, I've been going with the thin glass preset of the Arch&Design material. I make sure the geometry for the glazing is a single plane, which usually means deleting a bunch of faces from all the glazing geometry that comes in from Revit. The project I'm on now is using Kalwall, so I used the procedure Pierre outlined here to come up with the right material for that.

 

Next, I place a light meter in each room, 30" off the floor. I increase the length and width segments until there's a sample every 2 feet, or thereabouts. LEED 3.0 asks for 5 foot intervals, but LEED 2.2 said 2 foot intervals, so I stuck with that.

 

Now, I run the Lighting Analysis Assistant and make sure that there are 0 invalid settings. If there's an invalid setting, it tells you what it is so that you can go fix it.

 

Finally, render settings. The white paper lists the settings they used for Final Gather on page 14 (table 4). Basically, it's a medium Initial FG Point Density (1.0) with lots of Rays per FG Point (2500), little Interpolation Over Num. FG Points (5), and 6 diffuse bounces.

 

Now you can calculate your light meters. It'll take a while, depending on how many light meters you have and how dense the sample grid is. You don't have to do an actual render... you can just select one of your light meters and click the "Calculate All Light Meters" button in the light meter's Display parameters section (or in the Lighting Analysis Assistant). Once the calculation is done, you'll end up with a nice colored light meter with values at each grid point.

 

If you're just checking light levels as part of the design phase, this is the point when you call the architect over and they say "Ooooo" as you fly around the room looking at the light levels. If you're doing this for a LEED submission, I'm not sure what officially comes next. I exported the light meters to CSV files and made a little spreadsheet that calculated what percentage of each light meter grid points was above 25fc and then what percentage of all light meter grid points was above 25fc. That's as far as I've gotten because we haven't done an official submission yet.

 

So the parts I'm a bit fuzzy on:

* What's a good source for the Diffuse Horiz. and Direct Normal Illuminance values for the mr Sky: CIE Parameters? This number drives the entire simulation, so I want to make sure it's right.

* Are there accepted generic reflectance values for ceilings, walls, floors? Not every manufacturer supplies reflectance values for their products, so other than getting a meter and testing an actual carpet tile, I'm not sure what ballpark values to enter here. Reflectance will also have a big impact on light levels, so it's importance to get this figure close too.

* Is the Autodesk glazing material accurate for lighting analysis? Seems easier to set up than an Arch&Design material.

* What's expected to be included as part of the LEED submission? A perspective of each room with light meters overlaid? Plan views of the light meters (render from top view with ceilings and roofs hidden to camera)? Light meters overlaid on the CADD floor plans? I have a sample of the spreadsheet from the LEED book, so I think I'm straight on that, but I don't know what else to include.

 

Sorry for the epic-long response. This is a topic that there seems to be little information on, but I anticipate having to do a lot of this work in the future and I want to make sure I'm doing it right. Aand if by some stroke of luck I am doing it right, other people would probably benefit from not having to scour the intarwebs for hours and hours to come up with the same information.

[Scott Dombrowski]

Also, I'd like to put in a request for more intuitive light meters. Right now, you can only use a rectangle. What I'd like to be able to do is use a perimeter of a room as a shape and fill it with a light meter grid at whatever spacing I'd like. Now, for irregularly shaped rooms I have to either add a bunch of light meters to fill in the space or use one big one and in my spreadsheet manually remove each sample that falls outside a wall or inside a column, which is a bit of a drag.

[vikoum]

Hello to everyone.

I have seen that Max Design can give you the lighting analysis.

I want to get something similar with pseudocolors for Max, using Vray.

 

Does anyone know if there is something i can use?

[Karl Larsen]

It's hard to believe that architectural projects used to be designed with just a pencil and paper - I wonder how all those great architects from history managed to get through their projects...