This question brings to mind an experience I had back in 2006 on a California Energy Commission project to translate their Certification Suite of 160+ building runs from DOE-2.1E to EnergyPlus that made me ponder the trade-off between technical rigor and practicality. Repeating what Neal said, blinds and drapes are almost always used, so that if one models a building without them, its cooling energy use will tend to be high. In DOE-2, drapes and blinds are accounted for by a SHADING-FRACTION that discounts the solar gain through the window, to which schedules of any complexity can be added to model their operation. Use of this keyword in DOE-2 is so prevalent that someone (not me) has even coined the name sill_sc.
So, on the Commission project, the person assigned the task of converting the window inputs asked me how to deal with the sill_sc, since shades in EnergyPlus are modeled more physically requiring 10 inputs for solar reflectance, visible transmittance, IR emissivity, conductivity, etc., none of which are knowable for these hypothetical models. Even if we were to venture guesses, there would still be the problem of getting functional correspondence to the DOE-2 sill_sc that were developed through trial-and-error.
In the end, my team decided to punt on this issue, modeling the buildings with no shade in both DOE-2 and E+, even though we knew that wasn't very realistic. This is why it left me pondering the trade-off between getting something right technically and getting something that is practically doable. I realize that it's now been 8 years, and maybe this situation has been ameliorated, but when I just checked the Energy+.idd V8.0, I still see the same inputs for WindowMaterial:Shade, and now an even more detailed WindowMaterial:ComplexShade. How about adding a WindowMaterial:SimpleShade for dummies like me?
BTW, I've seen sill_sc ranging from 0.60 in the commercial building prototypes I worked on the 90's to 0.80 in the Commission certification files.
To answer Neal's question on their operation, I've always modeled them as constant in commercial buildings, but in residential buildings, I've varied them between 0.80 in the winter (0.10 due to drapes + 0.10 due to dirt on the windows) and 0.70 in the summer (0.20 due to drapes + 0.10 due to dirt). Winter and summer are determined by keeping track of the number of cooling degree days over the past 4 days. For many years I had modeled residential drapes as 0.90 winter/0.63 summer, but the late Dariush Arasteh convinced me that people were not so diligent in their use of drapes.
