Mix design (agstone)
50% hemp fiber for floor (2/3 for wall)
one bucket hemp (10 parts)
3 parts lime (15% of binder)
5 parts sand/clay (25% of binder)
2 parts portland cement (10% of binder)
Note: mix lime+sand/clay+cement+water first then add hemp.
Putting in a hempcrete floor is a great idea and perfect for making a floor without a DPM. We do loads of them here in France, usually with a drainage system on the exterior as you describe.
We usually put down a layer of pumice stones (min 20 cm) first which acts as a capillary negative/insulation layer, followed by a the first hempcrete slab about 15-20cm thick and then a final hempcrete screed which can incorporate the underfloor heating if its being installed. Some kind of geotextile layer is generally put down before the final screed goes down to allow expansion movement independent of the lower hempcrete slab. A small trench is dug around the footings, drainage pipe put in a and filled with gravel topped with a geotextile layer and a final covering of gravel.
The screed can be tiled or polished and protected with a wax finish. The screed can have a pigment added for colour or a coloured wax finish used. The wax finish can be made on the stove using a lump of bees wax dissolved into turpentine.