When the lights go off and everything cools down, the humidity goes up due to science and is normal. 70% humidity is not necessarily worrying, but it increases the chance of discovering bud rot or mouldy leaves when the humidity is high, especially late during flower. Keep an eye on it, and take reasonable steps to reduce the risk. Better airflow helps, either tweaking the ducting to optimise your extraction fan, or increasing the exchange of fresh air into your grow area, so that the humid air is actually replaced. Having a constant stream of air blowing on your plants also helps to move the air around and reduce the incidence of mould growing specially where leaves or buds are touching each other.
I use a Dehumidifier just to be on the safe side. Because I have 2 plants growing in a 4x4 and the one plant is really Busy! So I lollipop before I put in bloom and I keep the undergrowth clean. I have a fan just one the Bushy plant so air circulates through the plant plus, I have an oscillating fan that gives me good ventilation. But if you don't have a dehumidifier you may want to invest in one IMO. Because if you're growing in soil and you give them a feeding the RH is going to get higher when lights are out. If you don't have good ventilation and you go in there in the darkness or when the lights come on, you'll see some of the leaves will be wet from high RH. Just make sure you have fans blowing on your Plants or there's always a chance you could get Mold or budrot which I don't think will happen if you take the steps to get your tent well ventilated and then when you can pick up a Dehumidifier like on Craigslist should have them for cheap. Good Luck Sent from my iPhone using Grasscity Forum