Humidity issues--a dry room?

[dkspftw]

Hey all,

This is my first post as I'm about to own my first ball. I've kept a number of exotic pets before, most of which are much more demanding in their care than pythons, so I figured I can handle it.

Don't like to do anything half way, so my first BP will be a gorgeous little 2012 female mimosa.

I'm not a huge fan of the sweatbox aesthetic, as I'd like the snake to be in a visually appealing environment and be somewhat visible when not totally up in a hide.

Anyway, I've got a pretty standard exoterra screen-topped terrarium, 36x18x24'' I believe, with about 1'' of cypress mulch for substrate. Hides are made of driftwood and lace rock arranged so as to make caves and look all cool. There's an extra large standard issue exoterra water dish, and I heat the enclosure with a 100w ceramic heat emitter and a 30w under the tank heating pad. Both are controlled by an on/off controller with a temperature probe, and my other thermometers show the numbers are where I want them.

But I can't for the life of me get the average humidity over 15% or so. I've got 2 digital probes in the tank, and the one that's a few inches from the water dish ranges from 20-30, but the one near the heat source and most of the hides usually sits at about 12-15%.

I've taped up about 85% of the screen top with thick, nonporous tape, the only openings being a strip where I put some LEDs to light the tank during the day and a few cracks around where the heat emitter dome sits. The dome is surrounded by taped-down tinfoil.

I've added an auto-sprayer and I have it set to go for about 20 seconds every 8 hours. This will briefly up the humidity, but in doing so the temperature drops 5-8 degrees. And it seems to take less time for the humidity to drop than for the tank to heat back up.

I think the room is unusually dry due to it being winter and the heat being on in my building.

The vendor who is selling me the snake says I shouldn't worry about it too much, and I should just manually give the snake a good spritzing when it shows sign of approaching a shed. He's a successful, well reviewed breeder, and I don't think he's trying to trick me. I've already paid for the thing, after all, so he has nothing to gain.

Should I increase the frequency and/or duration of the spraying?

Should I seal off the strip under the LED with something clear? If so, there will only be 1'' or so of exposed screen on the top, although there's obvious openings in the front of the terrarium, where the doors open and close. Would that be enough air for an adult BP?

In theory, the snake comes in Wednesday, so I'd like to get this stabilized ASAP.

Thanks in advance for your expertise, and I'm sure this boils down to frequently asked n00b questions.