What is the white spongey/chalky substance underneath my wooden floorboards?

is this upstairs or down, no its not normal you need ventilation under your floor to prevent rot in the timbers if its filled with something it prevent air flow with the potential to all sorts of dampness problems, I WOULD GET SOMEONE TO CHECK IT OUT good luck Alex

If the wooden floor that is missing is a laminate or engineered timber laid over the original floor then this will probably be an underlay or if a laminate/engineered floor laid on top of a concrete sub floor again will be an underlay or membrane,hope this helps.

Hi, as previously mentioned, if it is on ground floor level the suspended floor that you indeed have should be ventilated, this is by means of air bricks fitted in the walls to outside below your damp proof course. These can be blocked by raised flower beds etc thus not allowing free circulation of air under the raised floor, This will cause dry rot which maybe is what you have if it is in the woodwork. Check for the exterior air bricks, remove blockages and seek help from skilled carpenter to remove/ replace defective timbers.

If you live in a flat or have flooring quality chipboard in a relatively new house ( less than about 30 years old) it’s just the insulation, it’s about 120 mm thick under which you will find a plastic DPM. Don’t panic it’s meant to be there