What part of the country are you in? Is it warm right now?
I live in an old building, too, with steam radiators and window unit air conditioners. In the winter we have no control over the heat, and it's often too hot in here.
What I've done to try to regulate temps is just keep an eye on the thermometer. If it starts inching up, I open a window in the room where the fermenter (or carboy) is. (It's actually in a closet in that room, which seems to stay at a more consistent temperature as the rest of our condo fluctuates - being a smaller, closed off space).
Anyways, the main thing is to try to actively adjust for changing temps. Use your windows and/or air conditioning and keep an eye on the thermometer.
Hope this helps!