Landlord's responsiblity concerning extreme utility bills due to lack of insulation

We have spent two winters in this house. The first winter we thought the bills were high due to the fact that we didn't put plastic on the windows (all brand new windows). The second winter we find out in February that the roof as no insulation in it. We even had ice build up on the roof so bad that it started to pour inside the house when the heat inside melted it. Our land lord says that if he has to put money into the house he'll just sell it which seems like a threat. He even had the nerve to raise our rent at the end of our old lease and still hasn't given us a new one. We feel stuck here because we don't have the best credit and no one in our area is renting. My last winter utility bill was 463.00 and that was in a mild month. Can anyone tell me what his responsibility would be concerning my huge bills?