Underfloor vents are really important for the health of your home. The airflow that is facilitated keeps any excess moisture from the ground away from your floor joists, thus helping to keep them damp and rot free.
It is really common to see vents blocked by: rising paths around houses; debris from rendering; deliberate blocking to stop draughts. When the vents stop working effectively the levels of moisture can build up in the wall structure as well as creating more humid air under the suspended floor. The ends of the joists are really in trouble when this happens.
Another common problem with maintaining good air flow under suspended floors is when one or more ground floor rooms have replacement solid floors installed. This blocks the flow of air under the house / remaining suspended floor unless vents / pipes are installed into the solid floor to maintain a through draught. This is very rare indeed to see. Just having one vent at the front or back of the house is not enough to give adequate ventilation for the remaining suspended floor as certain areas develop stagnant air pockets.
So we recommend ensuring that you have a good through flow of air under any suspended floor, even if this means creating vent channels through retrofitted solid floors.
Having a draught under your house might be a good thing for the health of the building, but not so attractive for keeping the inside of the house warm, so we recommend insulating your floor using breathable insulation so that you can have a healthy and warm home during the cold and wet seasons.