Old extension - insulation options

[Stricko]

So we've got an old single storey corner extension, which seems to get cold very quickly. We are looking an options for insulation. The 2 outside wall are double skinned, with the cavity filled with something. It's the only part of the property that has a suspended floor, but it's just a chipboard floor and laminate over, and no insulation below. The roof was originally flat, so there is a void above the ceiling to the original boarding and felt. But a pitched tiled roof has been added later, to which there is easy access.

 

So, where am I likely to be losing the most heat, the uninsulated floor, or the ceiling? And is it worth insulating the roof void on top of the original felt?

 

And ideas gratefully received 😄

Edited February 8, 2023 by Stricko

[Mr Punter]

Loft insulation would probably be cheapest and easiest.  On top of the felt is fine.