The value is based on the Analytical Construction assigned to the Window type.

You could edit the right Constructions.xml file (for example: c:\Program Files\Autodesk\Revit 2017\en-US\Constructions.xml).
Reality check: I am not a scientist so I have to ask what kind of material gives you 1.1 transmittance value? Isn't 1 the maximum you can get, which is basically no material.