Lots of good advice in the preceding posts. I have learned much since I became a member here.
I, too, use 40 gallons breeders for my two snakes. And your right, sometimes it is a pain getting temps or humidity balanced where they should be. I, also, live in an ancient, drafty house and use two heat lamps on each vivarium, both with CHEs (100w and 60w) on the warm side, during the winter months. I also use a large UTH (with a thermostat) under each tank. With this setup I am able to maintain reasonable (though not perfect) ambient temps for my snakes when the weather gets really cold outside.
As to humidity, I have a "split-screen" on the top of each vivarium that opens on the right or left side of the screen hinge. That way I can get into the vivarium for clean-up purposes, misting, changing the water, and feeding, without having to upset or move anything on the heating side. On the screen, I have a linen towel covering up about three-quarters of the screen (and away from the heat lamps) that I soak in water roughly every 24-36 hours. I use large cardboard slats, covered in aluminum foil, to cover the linen towel and minimize heat and humidity loss. With daily misting, I am able to to keep my snakes' humidity requirements met (though I'm going to have to start misting the larger snake's vivarium twice a day, now, inasmuch as she's going into shed and I want to keep her humidity a wee bit higher). I also cover the wrapped cardboard slats with more linen towels to keep the temps under control.
Is it a pain? Sometimes.
Do visitors to my home's living room find these vivariums to be fascinating? By and large, yes. Most find themselves wanting to know more about snakes and I am more than happy to oblige. :-)
<><Marc
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