But then I noticed moisture buildup on the inside of the roof. Screws sticking through the roof had water dripping from them. I turned the radiators back to full and the thermostat up again.
I'm restructuring the house, so not everybody will see it so clearly as I do, but we have to watch out. Our houses, particularly old ones are constructed to only stay nice with a good heat source inside during winter, in every room. Hot air will transport moisture out and keep it from depositing. If things cool down, fungi and moisture are a big risk. Hot humid air you breath out will find a cold spot to deposit it's moisture. It may take years before you find out where that is, and from experience I can tell you it can get quite disgusting when you finally rip out some plating.
The answer is: good, vapor tight (at the hot side, cold side should be open to vapor, or completely waterproof/tight!) insulation combined with good ventilation. That last one is very important, I also heard from people that closed their ventilation ducts... until water was dripping down the windows, or moist spots appeared in their wooden floors. Btw, a CO2 monitor can provide a good proxy measurement for your ventilation, imo.
Watch out people, you can save a lot on gas, but invite a lot of new problems. Don't go too crazy.