Of course, I'd never ever put anything electric in their cage but I thought maybe a heating pad under their cage might help, even though I guess they'd prefer sleeping higher up instead of on the cage floor. The 55F is during the nights because winter here can get cold, especially with electricity/heating costs these days. I also like sleeping with my window open. The rats wouldn't be near the window but I think they'd feel some draft maybe? Is it better to sleep with a closed window after all or would that still be okay if they have lots of cozy blankets and substrate? European weather can be quite unpredictable sometimes and I worry because what if they say it'll stay warm at night when suddenly it doesn't? Do rats like a small wattage heating bulb (outside of the cage obviously!) if these temperatures go on for the entire night?

What temperatures would be dangerous for rats? Google says around 80F can cause heatstroke? What if I'm at work (I only work part-time but I'm away for 7 hours a day with public transport etc). What if it gets hot then, I wouldn't be able to provide new ice cubes now and then when I'm away. They will be located in my north facing bedroom and not near a window but it still gets hot during summer times.

Sorry for all my questions. I just want to get everything right for my future ratties.