An interesting article from Drew Dawson of the Milwaukee Journal Sentinel, linked here for our members:
There are two laws that apply here:
Wis. Stat. 704.07(2)(a)2: "Keep in a reasonable state of repair all equipment under the landlord's control necessary to supply services that the landlord has expressly or impliedly agreed to furnish to the tenant, such as heat, water, elevator, or air conditioning."
ATCP 134.04(2)(b)2: "Heating facilities serving the dwelling unit are not in safe operating condition, or are not capable of maintaining a temperature, in all living areas of the dwelling unit, of at least 67° F (19° C) during all seasons of the year in which the dwelling unit may be occupied. Temperatures in living areas shall be measured at the approximate center of the room, midway between floor and ceiling."