Do I need to include a ventilation system in the basement and why?

Not necessarily, however it is important that heating and ventilation are balanced in order to prevent problems associated with condensation. Also, under Ciria 139 Levels of Dryness document, where a Grade 3 level of dryness is required, no dampess is acceptable and therefore active measures to control humidity may be necessary. This may take the form of ventilation, dehumidification or air conditioning.