[Radiance-general] Current practice for LEED sky modelling?

Tue Jun 19 10:23:11 PDT 2012

Hi Lars, Rob,

>From what I understand, gensky produces accurate CIE sky descriptions
according to the older definitions of 3 different sky types (clear, partly
cloudy, and cloudy) - but does not include definitions for the newer CIE
standard that has 15 different sky types derived from 5 different
parameters (A-E).  For the older CIE sky types, I am not clear on what
gensky uses to predict the magnitude (zenith luminance, Lz) but from what I
have seen it does not match the methodology that the IESNA lays out for
predicting Lz.  With gensky alone I might get around 70,000 lux on a sunny
summer day where as the IESNA guidelines would predict around 100,000lux.
 This is the reason I wrote a IES_gensky.py python script, it produces the
same CIE sky distribution functions (adopted by IESNA) but also uses IESNA
guidelines to determine Lz based on some lookup tables.  I recall some
babbling from me on this topic around when I developed this:

http://www.radiance-online.org/pipermail/radiance-general/2003-October/001074.html

http://www.radiance-online.org/pipermail/radiance-general/2003-October/001090.html

IES_gensky.py and a corresponding IES_skybright.cal are part of all the
SPOT releases and I would be happy to send them along individually (not
sure I can attach to an e-mail to the group).  So I use this script to
define an IESNA standard clear sky with a sun (I agree that although likely
vague in the manual, a sun is supposed to be part of the clear sky calc)
and do my LEED calcs with that sky.  When I have looked at weather data and
very clear sky days, the illuminance seems to match this IESNA clear sky
model fairly well.

I have also never had this part of the calc questioned on LEED reviews.

Cheers,
Zack

On Tue, Jun 19, 2012 at 9:41 AM, Lars O. Grobe <grobe at gmx.net> wrote:

> Hi Chris, Rob, so what are you using as sun luminance? Some average from
> weather data? A theoretical value from a model? John's examples are a great
> demonstration for the problem. I am rather sure that an invalid assumption
> for the direct sun in a sky model would by far outnumber the effect of
> changing a sky distribution to whatever model. Is there common sense that
> if "clear sky" is referenced, I should use the CIE clear sky without any
> modification, just as what I get from gensky? Or should I feed in at least
> an illuminance reading either from my own measurement or from weather data?
>
> Cheers, Lars.
> --
> Dipl.-Ing. Architect Lars O. Grobe
>
> On Jun 19, 2012, at 17:59, "Guglielmetti, Robert" <
> Robert.Guglielmetti at nrel.gov> wrote:
>
> > Yeah, the LEED "clear sky" absolutely means a clear sky with a sun
> (unless
> > they've changed it recently in LEED). ALL of my clear sky simulations
> with
> > Lightscape, AGi32, and Radiance over the years have included the sun.
> > After all, aren't we trying to predict daylight performance??
> >
> > Hey Chris, when'd you move to New York? Congrats, and enjoy the humidity.
> > =8-)
> >
> >
> > Rob Guglielmetti  IESNA, LEED AP
> > Commercial Buildings Research Group
> > National Renewable Energy Laboratory
> > 15013 Denver West Parkway MS:RSF202
> > Golden, CO 80401
> > 303.275.4319
> > robert.guglielmetti at nrel.gov
> >
> >
> >
> >
> >
> > On 6/19/12 7:16 AM, "Chris Coulter" <Chris.Coulter at burohappold.com>
> wrote:
> >
> >> Lars,
> >>
> >> I have done several LEED compliance calculations in the past using clear
> >> sky with sun (+s as the gensky parameter). This allows for daylight
> >> redirecting devices to push light further into a space, and more
> >> recently the requirement for glare control above 500fc.
> >>
> >> My take is that the phrase "clear sky" does not mean "clear sky without
> >> sun" as might be mistaken in general radiance terms. I've always assumed
> >> the sun is included.
> >>
> >> None of our submitted calculations have been questioned to date, so
> >> assume that this is acceptable.
> >> Hope this helps!
> >>
> >> Cheers.
> >>
> >> Chris Coulter
> >> Senior Lighting Designer
> >> Buro Happold Consulting Engineers
> >> 100 Broadway, 23rd Floor
> >> New York, NY 10005
> >> Tel: 212.334.2025
> >> Direct: 212.616.0254
> >> Email: chris.coulter at burohappold.com
> >> Website: www.burohappold.com
> >>
> >> -----Original Message-----
> >> From: Lars O. Grobe [mailto:grobe at gmx.net]
> >> Sent: Tuesday, June 19, 2012 4:21 AM
> >> To: Radiance general discussion
> >> Subject: Re: [Radiance-general] Current practice for LEED sky modelling?
> >>
> >> Hi John,
> >>
> >> I am aware of that really nice overview. The critique of using an
> >> impossible sky model is hard to question. Excluding the sun also means
> >> that any technique  to make use of direct sunlight by e.g. redirecting
> >> it deeper from the perimeter into the building is not accounted for at
> >> all. So that is why I was wondering how folks doing their real-life jobs
> >> for LEED are handling the problem with the current standards in mind.
> >>
> >> Another question is whether direct sunlight, after it got redirected, is
> >> still direct sunlight in LEED terms.... or whether redirected means
> >> indirect here and I could again include it... leaving me with a more
> >> meaningful sky model (clear sky with sun, and only portions directly
> >> entering the used spaces being locked out assuming users would block
> >> them by sunshades) accounting for redirecting facades.
> >>
> >> I am not sure wether this is too much LEED-specific for the mailing
> >> list, but I was really curious how Radiance folks is working around the
> >> not-so-physically-based specifications of current LEED when setting up
> >> simulations.
> >>
> >> Cheers, Lars.
> >>
> >>> There's a critique of daylight modelling for LEED and other codes
> >> here:
> >>>
> >>> http://www.iesd.dmu.ac.uk/~jm/doku.php?id=academic:daylight-compliance
> >>>
> >>> It includes a discussion on sky models.
> >>>
> >>> Best
> >>> John Mardaljevic
> >>>
> >>> Reader in Daylight Modelling
> >>> Institute of Energy and Sustainable Development De Montfort
> >>> University, The Gateway, Leicester, LE1 9BH, UK
> >>> Tel: +44 (0) 116 257 7972
> >>>
> >>> jm at dmu.ac.uk
> >>> http://www.iesd.dmu.ac.uk/~jm
> >>> http://dmu.academia.edu/JohnMardaljevic
> >>
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> >>
> >>
> >>
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-- 
Zack Rogers, P.E., LEED AP BD+C
Daylighting Innovations, LLC
211 North Public Road, Suite 220
Lafayette, CO 80026
(303)946-2310
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