Caulking between floorboards

[essex123456]

We have a first floor flat that I have been doing it up. We are above a shop and are about to fit carpet. There is a noticeable draught from below though gaps in the floor boards. Would it be wise to caulk the gaps? Or is it just as effective to put fibreboard over ( it has areas of this) and then just underlay and carpet?

Thanks for help.

 

[footprints]

Lay hardboard tape joins and mastic edges, cheap as chips and easy to cut. Pop a few hardboard pins in each sheet checking they will not go right through the boards into pipes cables etc