[Radiance-general] modeling glazings

[Jack de Valpine]

Hi Christoph,

I think that there are a lot of things that need to be considered here. 
Based on some work that Greg has consulted with us on, the basic 
radiance material definitions output from Optics 5 are perhaps too 
simplistic to fully account for a variety of glazing related behaviors. 
Nevertheless let me make a couple of points that may be obvious to many:

   1. There are many types of glazing that will have front and back side
      reflectances that are different and the "glass" material type is
      probably not suitable for these materials.
   2. If you want, it is possible to combine the front and back side
      definitions to one material definition and thus surface geometry
      (one polygon) with uniform normal orientation into our out of the
      space as follows

        void BRTDfunc TestSystem_front_and_back
        10
            if(Rdot, 0.230, 0.289) if(Rdot, 0.263, 0.336) if(Rdot,
        0.326, 0.371)
            0.585 0.594 0.521
            0   0   0
            .
        0
        9 0 0 0 0 0 0 0 0 0

    The "if(Rdot, ReflFront, ReflBack)" will apply the relevant
    reflectance depend upon which side of the polygon is being
    evaluated. This may be suitable depending on the needs (and
    information available) at hand. However this material does not
    suitably account for angular dependencies that may impact
    reflectance and transmittance.

    Note also that if you were to use two pieces of geometry, then you
    would need to be very careful of what you think you are applying
    material wise. For example if you developed an insulating glass in
    Optics 5 made up of 2 glass layers and export this then this needs
    to be applied to "one" polygon not "two."

There is a shell script called "glaze" that is in the current release of 
Radiance (probably need to get the latest HEAD release for some 
updates). This script works by enabling the user to specify the makeup 
of the glazing (currently allowing for single and double pane glazing as 
well as frit coverage). The makeup is based on a small database that 
includes front and back reflectances as well as transmittances for a few 
glass materials. Some of these have been taken directly from the 
Radiance output provided by Optics 5. We have also added a feature 
recently that allows a separate file to be specified with additional 
glazing types (eg build your own). The material definitions (BRTDfunc) 
that the script outputs have a more robust model for single and double 
pane glazing systems. Probably, the best thing to do is try it out to 
see how it works.

One last note/thought. I think that as far as things go, Optics 5, is an 
excellent resource. The issue is really how to effectively use the data 
in a useful Radiance material definition.

Regards,

-Jack de Valpine

Reinhart, Christoph wrote:

> Dear colleagues,
>  
> I know, I know. The topic of modeling glazings in Radiance using 
> Optics 5 data has been extensively discussed in January 2004. I just 
> reread the emails with great interest in the archives. BUT I am still 
> undecided whether to go with Optics 5 is worth the effort.
>  
> in January, everybody agreed that creating a specific glazing in 
> Optics 5 creates an output as follows:
>  
> #
> # header info
> void  glass       TestSystem_glass
> 0
> 0
> 3     0.638     0.647     0.568
>  
> void  BRTDfunc    TestSystem_front
> 10
>       0.230     0.263     0.326
>       0.585     0.594     0.521
>       0 0 0
>       .
> 0
> 9 0 0 0 0 0 0 0 0 0
>  
> void  BRTDfunc    TestSystem_back
> 10
>       0.289     0.336     0.371
>       0.585     0.594     0.521
>       0 0 0
>       .
> 0
> 9 0 0 0 0 0 0 0 0 0
>  
> Philip mentioned that to use this material description, one would need 
> "two glass polygons for each window, one for the front and one for the 
> back". I assume this means:
>  
> TestSystem_front polygon FrontOfWindow
> 0 0
> 12     0     0     0
>          1     0     0
>         1     1    0
>         0    1    0 
>  
> TestSystem_back polygon BackOfWindow
> 0 0
> 12     0     0     -0.01
>         0    1    -0.01
>         1     1    -0.01
>         1     0     -0.01
>          
> Is this correct? How does the glazing know its transmittance, i.e. how 
> does TestSystem_glass come into play? Also, does anybody have any 
> references or experience how much more accurate a simulation gets if 
> one uses the full optics 5 file instead of a stripped down version 
> such as:
> void  glass       TestSystem_glass
> 0
> 0
> 3     0.638     0.647     0.568
>  
> Any thoughts are elcome,
>  
> Christoph
> Christoph Reinhart, Ph.D.                        tel: (613) 993-9703
> Research Officer                                      fax: (613) 954-3733
> Institute for Research in Construction      e-mail: 
> christoph.reinhart at nrc-cnrc.gc.ca
> National Research Council Canada        1200 Montreal Road M-24, Ottawa
> Ontario K1A 0R6 Canada
>
> Daylighting Analysis Software:
> The Lightswitch Wizard (initial design)   www.buildwiz.com 
> <http://www.buildwiz.com/lightswitch/index.cfm>
> DAYSIM (expert software)                       www.daysim.com 
> <outbind://26/www.daysim.com>
>
> IEA Task 31                                            
> http://www.iea-shc.org/task31/
>
>  
>
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>
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-- 
#	John E. de Valpine
#	president
#
#	visarc incorporated
#	http://www.visarc.com
#
#	channeling technology for superior design and construction

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