Insulating joists

I've just moved into an Edwardian town house where the previous owner has stripped back the floorboards and waxed them. They look gorgeous but the draughts that run between the boards are really bad.

Could I insulate the joists using polystyrene? The ground floors are all suspended and have about a space of 20in which can be crawled into. There are also air bricks situated outside the house which circulate the air under the floors. Will insulating the joists cause the floors to rot?

A response from one of our very best tradespeople:

"Insulating the joists would be fine as long as you insulate them with the correct type of insulation; Celotex would be ideal as it conforms to building and fire regulations. A great way to stop the breeze is also to caulk the gaps in the floorboards, you can buy either coloured caulk (not great but not bad) or there is also sawdust or resin type filler you can get which will match the wood type of the flooring. With regard to the airbricks, you definitely shouldn't fill these; you must also be aware if the rooms concerned have any gas appliances in, that their operation will not be affected or become unsafe by limiting the airflow of the room. E.g. gas fires."

Lenny Bunn, Bunn & Sons Ltd.
Bunn & Sons Ltd. have grown over the last two years, starting out as just one plumbing and heating engineer then moving on to a family run business specializing in Kitchens and Bathrooms. They then went on to working with designers to enter the contemporary field, adapting what they'd learnt along the way to offer the public the same service you would get from a more expensive designer They now head up extensions and full interiors, working with only a trusted and professional crew, as well as continuing with all forms of property maintenance. Lenny and his team are based in Folkestone, Kent.