40% RH is fine. The radiant heaters use up a hell of a lot of juice, though. I made an Electric Blanket Sandwich for my indoor girls, works GREAT. Then someone suggested water bed heating mats, d'oh! Waterproof!
nothing, 40% is a decent level. Living on the Wet Coast I have to pay big bucks to drive my humidity down to 60%. I know people who grow outdoors in the desert in buckets with no problems, you'll just have to water more often.
When humidity is too low growth slows down a little, but 40 is sitting pretty, you won't have bud rot or mold or anything, I would be dancing in joy if I had a RH of 40.