How to slow down evaporation when cooking? Does water evaporate faster when rolling boil with a lid or simmer without a lid? If I use a certain stove plate with a certain sauce pan set to its lowest setting it will cause the water to roll boil with lid on and just simmer without a lid.

My question is, will the lid compensate for that water that evaporates faster (I assume?) when it is rolling boiling compared to when it just simmer?

The context is brewing tea. I have found a new tea that should be cooked for as long as possible (an hour is not too much) and I want to leave it unattended. 

How to avoid too much evaporation?
Covering (using a lid) will greatly reduce evaporation, whether at the full boil or just simmering...or sitting on the countertop.