Dear users I am really puzzled by Werner’s post hereunder. http://sel.me.wisc.edu/trnsys/mailinglist/archive2003/msg00297.html If I understand it well, characteristics for internal glazing systems should be specified regarding external glazing systems characteristics. For instance internal transmittance should be different than normalized value if spectrally selective transmission is performed by external glazing systems. The reason invoked being that the same wavelength cannot be absorbed twice. However when I think about it, I rather understand transmission coefficient (thermal or visible) as a probability of transmission averaged on a wavelength interval. Therefore I would intuitively conclude that each glazing system transmits radiation as expected regarding its normalized characteristics. When I simulate a fictive 2 * double glazing system in WINDOW with a very wide middle air gap I get the following results. SHGC Tvis Initial double glazing 0.54 0.78 2*double glazing simulated 0.41 0.62 Characteristics of initial glazing**2 0.29 0.61 So Tvis resulting from WINDOW calculation is close to the square of initial Tvis value but SHGC is in-between. I would need help to conclude. How should internal glazing systems be specified in Trnbuild? Also if Radiance users are amongst you I would be very interested to know how they handle this issue when specifying transmissivity values for internal glass materials. Best regards -- Antoine Gautier |