RH is the measure of how much water is in the air, relative to how much water the air can hold.
This is all related to temperature. The higher the temperature, the higher the amount of water can be suspended in the air.
So, at night, when your temps drop, the amount of moisture in the air is the same, but the amount the air can actually suspend decreassed, meaning your RH goes up.
The guys were right in saying you can resolve this with a dehumidifier but it can also be controlled by raising your lights out temps.
I had a similar problem a while ago and solved it by splitting my grow room in half and running one half 6am-6pm and the other 6pm-6am, this gave me a much more even room temp and therefore a much more even humidity.
A knock on effect was that yield increased due to the even temps as well as a more stable humidity.
Hope this helps.
W
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