filling a void and underfloor insulation

nagsfold

We live in a 1960's bungalow which is on a slope so there is a large void beneath the property.
We also stripped the floor boards and just have rugs, which means a lot of cold draughts come up through the floor. I can, and will, seal these gaps as well as look into installing some kind of under floor insulation, but it also makes sense to try and fill this void with something to further cut down on the amount of cold air under the property.
Anyone any ideas, would like to use something environmentally friendly like straw, wool or paper but am concerned about this causing damp, encouraging rodents or would be a fire risk.......