The following answer to a very vague question is, by necessity, general in nature.
In our relatively mild climate I would likely pump the empty cavity (assuming it is not a timber framed house), find and address the low hanging fruit with regards to air leakage (actually do an airtightness test first to find the issues), seriously consider a simple whole house ventilation system but would likely not add iwi to the mix for various reasons.
Contrary to general perception, quite often the actual windows & doors are NOT the main cause of air leakage in houses and this surprises people when demonstrated why (Anecdotally, window replacement is recommended in less than 1 in 10 existing houses I test/survey for air tightness)