SW simulation professional (2D thermal analysis) will be needed for such calculations.
But you will be able to compute heat transfer only (and you will have to create solids for the air inside the window).
If you only need U, then you will have to have (and to know) how to calculate U from temperatures and heat transfer.
Clearly sw simulation is very usefull if you want to study how heat transfers in your model and how you can optimize it.
But if you want to know U, then there are many free or very cheap softwares that are perfect for this.