I now live in Hungary but also own a small rural cottage in north east Scotland, which is rented out. I lived there myself from 2002 to 2011 and there was always a problem with cold floors. I think the reason is the position on the side of a hill and the construction of the cottage. There is a large field which drains onto my garden and then this run-off must find its way under the cottage which is again at a lower level than the garden (which is held in place by a wall). The cottage is very old and would originally have been a low thatched building. Most of the walls are made from boulders gathered from the fields, bound together with clay and straw. External rendering has been applied to this stonework but only down to ground level. There are some airbricks just below ground level, each with a little brick surround.
I assume the soil under the floorboards is always wet and the evaporation of this dampness causes the coldness. There is central heating plumbing and wiring under the floorboards so I'm not keen to put in concrete floors. I don't really want the upheaval of ripping up the floors to put in insulation either ( I had wondered about suspending slabs of expanded polystyrene between the joists just under the boards).
What I am now wondering about is the polystyrene beads used for cavity wall insulation. If these were blown into the void under the floorboards would they serve as insulation? For cavity wall applications they are meant to be bonded together with an adhesive but I'm not sure if that would be a good idea in this case. I don't know if the void actually gets partially filled with water but if it did the beads would float on top of it if they were loose. In such a case I would guess they would still reduce evaporation in two ways:
1) by reducing the surface area of water exposed to the air
2) by insulating the colder water from the warmer heated spaces within the house.
Any comments much appreciated as my tenants have cited the cold floors as one reason in a notice to quit.