Ventilation

It is essential to provide sufficient ventilation. But how to avoid cold draughts? Low level ventilation is best achieved through pipes from higher up.

Condensation

Good insulation avoids condensation but wherever cold steel is exposed drips will form. (If you use gas to heat or cook there will be huge amounts of moisture in the air.) It is possible to take advantage of this to make a DIY dehumidifier - condensing water out of the air on an area of cold metal connected to the hull, collecting and removing it.

Insulation 

There is some great informaiton about boat insulation materials on the website: http://justcanals.co.uk/articles_tech_insulation.htm

Their conculsion is that a spray foam is the best option as it has very good insulation properties, is easy to install, is vapour impermeable and bonds directly to the hull, so avoiding the risk of condensation and rusting.