It's been sitting on my pc for quite some time now, but I haven't got around to installing RAC2012. I'm curious to see if and how this works...

How it works "under the hood" no idea, but work it does !
Obviously you have to add a their shared parameter to your Room Tag and then in your schedule add the Field.
From there you can write your own Calculated Values, Ratios, percentages, etc.
After adding (or subtracting) windows hit their addin botton and it simply updates tags , schedules etc..
I tested last night both areas by "Height and width" option and by "Material" option (which I like) and both work very well.
The new Beta version has the Select by material option.
Now if he can sort out "Ventilation" portion of window, maybe by two material one glazing and one vented glazing, hes on a winner.