So, I did this, actually, and here's what I found:

Section 602.4 of the International Property Maintenance Code is hereby revised by amending the following: 602.4 Indoor occupiable work spaces shall be supplied with heat during the period from October 1 to March 31 to maintain a minimum temperature of 65° F. (18° C.) during the period the spaces are occupied

So it seems like it's the landlords problem.