No. 20°C for your CPU is quite low. That normally is a good thing, but 20°C = 68°F. The Laws of Physics make it impossible for the CPU to be cooler than the ambient (room) temperatures using standard fan and heatsink cooling - especially if powered up.
This means for 20°C to be correct, your room temperature must be several degrees cooler than 20°C/68°F. Is it?
As long as moisture is not condensing and running into or dripping on the live circuits, you have nothing to worry to about.
Note moisture typically collects when there is a fairly extreme change in temperature - such as might happen when the computer has been running and is warm, then turned off and allowed to cool in a cold room such as yours. Or when cold, powered on and allowed to warm up. So you need to ensure the case interior has plenty of dry air ventilation to ensure moisture does not collect in the first place.
While there is such a thing as too cold when it comes to electronics, your (indoor) temps are not near cold enough for those worries.
There are many who envy your cool CPU temps!
And yeah, when I think of AC, I think of cooling, not heating. Sorry about that.
Thanks. And no, there's no moisture inside the house or the pc's case. And yeah, I guess people should be a bit jealous In the summer while it's +40c outside, my GPU is near 65-70c and CPU around 50c, so I'm using additional 3 case fans to cool them off, while I turn them off in the winter.
GPUs run hotter than CPUs so 70°C for the GPU is nothing to worry about. And yeah, it is fine to turn off fans in the winter (I am sure it is much quieter that way) as long as there is still air flow through the case.