If the floors of a building, whether concrete, suspended timber or brick on earth, are in really poor condition (dry rot, wet rot, furniture beetle, subsidence) you may decide to replace them. In this case the new floor will inevitably be of concrete and can be insulated when it is being laid. The techniques described below are appropriate if you want the building to have a high thermal mass. If you want a low thermal mass for intermittent heating you would be better advised to lay a floating chipboard floor as described above on top of your new concrete.
Assuming that you decide to benefit from the thermal mass of the concrete, the best way to insulate the floor is to excavate against the external walls to create a narrow trench down to the footings. Place expanded polystyrene 100mm thick next to the wall, resting on top of the foundations, and fill in behind it with earth to keep the insulation in place. Be sure that you do not dig below the bottom of the foundations or the building will start to move and may collapse. A normal concrete slab on hardcore can then be laid over the floor and over the edge of the insulation. If a polythene sheet dpm is used it can be laid under the concrete on a layer of sand to stop the hardcore from making holes in it. If a painted dpm is preferred it should be applied to the top of the slab with a 50mm screed laid on top of it to take the floor finish.
In some old buildings there are no proper foundations. You may dig down next to the wall and discover that about 150mm below ground level the wall widens out, and then stops. If you find this do not go on digging or you will undermine the building. The solution is to lay the concrete slab on hardcore, put a dpm (either painted or a sheet material) on the concrete, and then lay your insulation on top of this. The insulation can be made up of 50mm or 75mm thick expanded polystyrene standard duty type. On top of the insulation lay a 50mm thick screed of one to three or four parts by volume of cement to fine aggregate. The screed can incorporate some galvanised wire mesh reinforcement (chicken wire is suitable) laid near the bottom of the screed to guard against cracking, but this should not be necessary on a domestic scale floor.
We are assuming, perhaps wrongly, that if you are confident enough to lay your own concrete you know how to do it. If you have never laid concrete before an insulated floor is perhaps not the best place to start learning. If you do not feel that you can do the concreting yourself, and somehow the ‘wet trades’ like plastering, concreting and brick laying seem more difficult that the dry ones which involve only hammers, saws and nails, you can still use the ideas above for concrete floors and even carry out the unskilled labouring yourself, but have a builder to lay the concrete and screed.
The problem with work that involves some skill is that you need to do it a few times until you have the hang of it, but on most DIY jobs you only want to do it once so you do not get the practice. You could perhaps come together with a group of friends or neighbours to lay each other’s insulated concrete floors but you will have to draw lots for who is to have the first unskilled one done on their house!