Calculating U Value of a room
I think the simplest way to evaluate this is with a spreadsheet, where you collect the area of each &quot;type&quot; and can plug in numbers for different insulation values for each type, and view the effect on that part and on the total.
i.e if all the windows are the same sort or will be when purchased, you can lump that area &amp; U value.
If there are different types, separate them into different rows in the spreadsheet.
The walls get a row, unless the walls differ, then they get as many rows as there are different areas of wall-types.
The ceiling gets a row.
The floor gets a row, but is often tricky to characterize since insulation value and exterior temperature is harder to know for sure on the floor.
...because that's the way I do it.
Other then &quot;metric R&quot; tacked on the end, this is all in &quot;English&quot; units mostly used only in the USA now, but it's easy to do in the units applicable to your area. A few numbers are input, most are calculated. It's easy to see the difference that changing one part makes to that part and the overall result. For instance, increasing my ceiling insulation can be done, but will have little impact on the overall use as it's already less than half what the walls are, and likewise less than half what the windows are.

When you get heavily insulated, you also need to pay attention to the heat lost (or exchanged) via ventilation, with or without heat recovery, as it becomes a significant part of the overall load.