I have a small room 5x6 made out of marble brick wall with two doors with a small radiator that is operating at around 97%, but it seems like it can't really get warm. It's -28°C outside, but the room can't seem to get up to more than 5°/6° degrees while other similar sized rooms made out of wood can easily get to 21°.

I know heat goes out when the doors are open when colonists go through, so I thought maybe they are just walking through it too much, but even when no one is going through the door it struggles to keep up the temperature.

Is there any special calculations you do about rooms leaking heat even if the doors are closed?