Transparency and Shading Coefficient

Another issue is the ability to define both a transparency and shading coefficient value for a WINDOW material. Traditionally the shading coefficient is to solar radiation what the transparency value is to light. On difference is that the shading coefficient can also be used to account for the effects of external shading, but not usually in a dynamic way. However, to maintain consistency between WINDOW and other elements, both values affect the transmission of light through windows. In fact, both are completely inter-changeable and cumulative. Thus, if you specify both a transparency and shading coefficient of 0.5, the total transmission will actually be 25% (0.5 x 0.5 = 0.25).

There were a number of reasons for doing this. The first was that users were altering WINDOW transparency values and not seeing any change in the solar radiation - which caused much confusion to those new to the software. Second was the need to maintain consistency - if transparency affects solar absorption in a WALL or ROOF element, then it should also affect transmission in a WINDOW element. Thirdly, as they are cumulative, having the two values offers a way of applying dynamic shading to a window by using a blanket coefficients derived from a more complex shading mask study. Alternatively, the transparency of the WINDOW material could be manipulated with a script to simulate dynamic shading without losing the shading coefficient data for that particular glazing configuration.