You can’t do it.
As you can see in the hint of sillHeight:
sillHeight_: An optional number in Rhino model units that sets the distance from the floor to the bottom of your windows on rectangular surfaces **when** the breakUpWindow_ input above is set to 'True'.
Notice the “WHEN”.
I can find the logic of your wish. You can add it in the github.
-A.
-A.