Hi, glad to see you have temporarily solved the dripping in your workshop. I know I'm a bit late to the thread but I might be able to explain the points that others have mentioned so you realise the problems. Its not intended as critisisim of what you have done and I mean no offence just thought it might help you or others in the future.

Starting at the beginning...........
Air can hold water, as moisture, but the amount is dependant on its temperature. Warm air can hold more water than cold air. As its temperature drops there becomes a point when it can’t hold any water at all and it is separated out on to whatever surface it can (dew point). This has nothing to do with the temperature of the surface or type of material. So as Jonathan said below 5 deg condensation will be present, therefore the temp needs to be kept above this temp.

This then poses the next problem that the warmer air can now hold more water and as it rises and touches the cold steel roof it is cooled and releases the water, as its the roof it then drops back down. Heating a room with cold surfaces will not fix the problem, in fact the more you heat the room the more moisture it can hold and the worse the condensation on the steel roof will be.

so there are 2 types of condensation:
1. a lower limit dew point that affects unheated spaces
2. cold surfaces in heated rooms
The two solutions are to remove all the cold surfaces or remove the water
Removing the cold surfaces will only stop the excessive build-up of water on them (in the case of the roof, stop the dripping). The water is still present and will find its way to the next cold surface. For this option to work you either need to use a breather membrane to allow the water through but not get back and then provide a means of escape , or stop the water getting through ie, vapour barrier. Using a vapour barrier keeps all the moisture in the room so ventilation will be required. Removing cold surfaces doesn’t solve the problem it just moves it elsewhere
Prevention is always better than cure so removing the water has to be the best and far easier solution, heat the room to just above the and get a dehumidifier. Problem solved and it’s transferable to your next workshop.

This is a basic and simplified description and there are a few extra points if dealing with a domestic house but in the context of a garage/workshop its fine.

I do understand your need for a quick fix and if you are not there long then you might not have any problems with moisture build up between the polystyrene. I’m not going to dwell on the fire risk but it might be worth setting fire to one of the off cuts. The stuff I have seen gives off thick black smoke and goes molten, not good if it on the ceiling!

All the best