Here is a link specific to Wisconsin:
https://www.dhs.wisconsin.gov/mold/tenantlandlord.htm
Local health authority and/or building inspector can be called on to confirm the problem and recommend a remedy. This is no cost to the tenant, and really necessary where the landlord disagrees about the cause of the excess moisture. If experts agree it is the attic vent issue, then he will be ordered to fix it. If it is moisture and spores that accumulated because of the leaking roof, they may be ordered to correct that too, and how to go about it.
Advice is to consult a lawyer before withholding rent.