If thee is any way to keep it dry, "drywall" or gypsum sheets are very good insulators, especially if taped together to prevent drafts.
I thin k that, if your water barrels are not INSIDE your greenhouse, you wnat as much insulation as possible surrounding the barrels on the outside, so that they "thermally INside" the greenhouse as much as possible. Otherwise they will give up their heat to the atmposphere and not help your plants.
You do want lids, or water will evaporate and chill the barells, producing humidity instead of raised temperatures.
Allow as much air circulation as possible, to get the heyat out of the barrels and into the air.
If the greenhouse is v ery well insulated everywhere the sun doesn't shine on plants, it MAY hold heat well enough for you to run an elctric heater affordably on a thermostat to provide just 10-15 degrees rise, if that's enough.
