Floor insulation is one of those things people skip because it's the most disruptive to install, but in my experience it makes a disproportionate difference — particularly in stone buildings where the ground acts as a massive heat sink year-round.
With a bothy specifically, you're dealing with dense masonry that will absorb any heat you put in before you feel a scrap of benefit. Skipping the floor means you're essentially heating the earth beneath you. I've seen setups where someone had decent wall and roof insulation but bare concrete over rubble, and the heating system was working twice as hard as it should have been.
That said, the method matters enormously. A cold stone floor usually means ground moisture is a concern too, so a proper build-up — DPM, rigid PIR board (something like Celotex or Kingspan), then either a floating timber deck or screed — is worth doing properly or not at all. A thin layer of insulation installed badly can create a condensation trap that rots the structure from underneath.
For a static caravan or emergency-use building I'd possibly accept just walls and roof as a compromise, but for anything you're occupying regularly in a Scottish or northern winter, the floor becomes non-negotiable in my view.
Curious whether others have retrofitted floor insulation into an existing bothy without taking the whole floor out — whether a floating deck over existing flags is viable or if it just creates a damp problem.
