I tried to find a solution to the heating problem when the canteen is right next to the kitchen. At first I put large water pipes in between, thinking the cold water would stop the heat from spreading into the canteen, but that didn't work out. The solution is much easier: just dig a trench in your foundation around the kitchen and it will stop spreading too much heat to nearby locations. In the screenshots below you can see the heat being stopped:
Kitchen inactive: it's freezing cold in there...
Kitchen active for a while: the cold barrier in between the kitchen and canteen will stop the place from warming up too much
This should help managing the temperatures in your prison. The canteen now still requires a radiator, otherwise it gets too cold in there
