I have a living room window with a stupidly large window sill. It's kind of a reverse bay window or a window "booth" where the window. Basically the underside of the window sill is exposed to the outside and if there is any insulation, then it doesn't work.
The underside is about 135cm wide by 78cm deep (yep, this is the window "sill" or maybe "platform" would be a better term). It's a huge heat sink in winter.
I was looking for any advise on insulating this. It would have to be something strapped to the bottom of the sill.