Calvin, to keep the humidity up the best thing is to add a water feature and also a fogger. You can rig an ultrasonic fogger to the terrariom. You have to add fans to keep the air circulation up otherwise everything rots. The tricky part is keeping the temperature down. I'm still trying to get there. I've managed to keep it between 20 and 28 degrees celcius but I supect that's too high for what you want.

This is what I've done

Cheers
Rodrigo

Its cheaper for you to lower the temp of your house and wear a sweater.

Brendan - I know eh! I've just had the ingenius idea of having the entire setup in the basement of my house, where it's naturally chilly...PERFECT! :D Since I'm supplementing lighting anyway, there is no problem. I'm going to have to look into misters though.

Rodrigo - beautiful setup. I saw your amazing pictures over on the orchid board :D Do you find that misters are the best for generating a 'fog' like high humidity environment? I find that the ones they have at grocery stores layer everything with a film of water and I'm afraid that will lead to rot but I suppose with good airmovement that is preventable.

Living in Florida, I have had to come up with a way to keep a terrarium cool for my pleurithallids of the genus lepanthes. I originally tried an orchidarium(very expensive)with a cool ballast light but this stayed way too warm for my needs. So I sort of made my own. I've posted this in gardenweb forums a couple times before. I attached a fishtank with the bottom removed on top of a 40 quart thermoelectirc cooler. It produces a stratified temperature environment that is about 55 degrees at the bottom and 65 degrees toward the top. This setup provides the right temperature, constant air circulation from the motor, and lighting at the top. I have been able to grow pleurothallids successfully with this for a few years now, but it is certainly not as attractive as the setup treefrog has.

Thanks for the compliments. the temperature does rise more than I'd like in my terrarium buut I haven't placed a glass top to separate the hood from the terrarium yet. I have an ultrasonic fogger that runs on a timer which raises humidity withought wetting everything. there is also a sprinkler and I hand spray every now and then if needed.
Cheers
Rodrigo

Terrariums sound cool. never have had one before and don't know if i will, but cool stuff.

ABSOLUTELY COOL!
I think they are better than greenhouses, because I like miniature stuff. They can be a challenge to set up and get balanced but are amazing when everything jibes right and maintenance is low.
Check the link below for more info.

Here is a link that might be useful: Dutch Vivariums