[Radiance-general] Help - Radiance / LEED

[Zack Rogers]

Hi John,

You make a very good point, reviewing the LEED docs again it does not
clarify the "clear" sky model to use.  I have always assumed it was the
IESNA recommended clear sky definition which does give consistent and
predictable horizontal illuminance.  In my practice, I have found measured
clear skies to match up very well with these guidelines. I live in sunny
Boulder, true, but last fall we measured 32 mostly clear skies in South
Carolina which all matched up very well with the IESNA predicted horizontal
illuminance.  The luminance distributions were not far off either.

>From what I understand, IESNA Handbook 9 adopted the same clear, partly
cloudy, and cloudy luminance distributions created by Kittler and also
adopted by the CIE at the time (I believe CIE has now adopted a broader set
of 15 skies or so  covering more intermediate conditions).  In addition to
this, the IESNA handbook 9 offers a series of tables and equations for
calculating the Zenith Luminance allowing one to predict the magnitude of
these skies under different conditions.  Does the CIE not have a similar
recommendation for calculating the zenith luminance?  From what I
understand, the CIE sky models rely on input of either zenith luminance or
horizontal illuminance from weather data.

BTW - the latest public review draft of LEED 2012 goes away from this
standard clear sky and requires the user to find the average hourly values
between the clearest and cloudiest days within 15days of Sept 21 and Mar
21st.  Someone must have been listening to your concerns!

Cheers,
Zack

PS.  I have a python "IES_gensky.py" script and a "IES_skybright.cal" that
generates the IESNA sky definitions.  I have been using this solely for
"design day" daylight illuminance calculations for the past 8 years in lieu
of gensky.  It is used and available in the SPOT program.  I'd be happy to
share with anyone, e-mail me directly and I will respond with it.

On Thu, Apr 14, 2011 at 2:53 PM, John Mardaljevic <jm at dmu.ac.uk> wrote:

> Jorge,
> This suggestion is not necessarily a help, and may even prove to be a
> hindrance, but if you're attempting the 'clear sky' option in either LEED or
> ASHRAE, you may wish to acquaint yourself with some of the er. peculiarities
> of the specification:
>
> http://www.iesd.dmu.ac.uk/~jm/doku.php?id=academic:daylight-compliance#various_guidelines
> Whilst one imagines that there must be some rationale behind these ‘clear
> sky’ formulations, it has never been laid bare and I have to confess
> bewilderment as to what the reasoning might be.
>
> Good luck!
>
> John
>
> -----------------------------------------------
> Dr. John Mardaljevic
> Reader in Daylight Modelling
> Institute of Energy and Sustainable Development
> De Montfort University
> The Gateway
> Leicester
> LE1 9BH, UK
> +44 (0) 116 257 7972
> +44 (0) 116 257 7981 (fax)
>
> jm at dmu.ac.uk
> http://www.iesd.dmu.ac.uk/~jm
>
>
>
>
>
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>



-- 
Zack Rogers, P.E., LEED AP
Daylighting Innovations, LLC
(303)946-2310
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