I have a 1600s or so scottish cottage, that's lacking in insulation.
It's some 20m*5m, with a suspended wooden floor, with no insulation
underneath. The temperature below the suspended floor is essentially the
average house temp.
During other works, the path (20*1m) along one long side of the house
has been dug up.
Is there any point at all in putting down 2-5cm of rigid insulation under
sand blinding, then membrane, and gravel, instead of just the gravel and
membrane?

How would I go about calculating the extra insulation effect?