I get about a 10 degree swing in temps just by having the lights on during the day (68-78 degrees) in an enclosed container. If you want automated humidity, hooking an ultrasonic humidifier up to a timer will do that quite well. You can disguise the humidifier by having it outside the cabinet and piping in the mist with PVC or flexible hose. Most use computer fans for air movment, but with a cabinet that big, you might be able to use a small 110 v fan to good effect.

Your biggest problem will be getting light to the bottom. Most light fixtures do not like the high humidity that comes with the territory in a setup like you are contemplating.

Ya, I was wondering how the fixture would be affected by the humidity.... On the humidity side I was planning on running the air through a damp filter medium, a sort of evaporative cooling that would humidify and cool the air. was going to use a water pump from a coffee table fountain to circulate water. and have the pump controlled by a humidistat/thermostat. Obviously haven't worked out all the kinks. thanks for your help

Assuming you are enclosing the shelves in some glass, there will be no problem with humidity, as the plants will breath lots of moisture and soon their will be little droplets on the glass in the corners. Control this by adjusting a small breathing space to the outside. Add some tropical houseplants if you don't fill it up with orchids. Put a light above each shelf. The lights will add heat during the day, as mentioned.

Start out simple and learn from experience. I expect my house will be too warm in summer and I will have to open the top. Fortunately, it's naturally humid enough here in Chicago in summer. I don't have air conditioning.

First of all, I'm sure your wife will really enjoy her new orchidarium when it's ready! 7' tall should be big enough, and don't underestimate the amount of light you will need. I assume you will light each level individually, since one fixture at the top would NOT be enough. You won't need to heat the unit at all, because normal room temps are enough, and the lights will add some heat. Don't buy 'grow lights' though. They are a waste of money. Instead, get 'cool whites' or 'Daylights' (the best). Better yet, get T5s. They are a slimmer bulb, with a much higher light output.

And Homer is lucky to get a natural 10 degree temp. drop in his tanks. I sure don't! So I would add some kind of cooling system, unless the unit will be placed in a window, where it will naturally get cooler at night in the winter (a temp. drop is not generally required year 'round).

For humidity, get a hydrofogger. Just a fountain won't do much. Either that, or hook up a misting system, though I have had to fiddle endlessly with mine, and it still doesn't work as well as I want it to, despite my having a much stronger pump (= finer mist) than the people at the store said I'd need. So I'd get a fogger. Someone here, I think it's Homer, hangs them from the top of the tank, so that the mist falls down over the plants. I think that's a good idea. Otherwise, it will tend to stay at the bottom of the tank. As for humidity control, if you want to get that complicated, I know nothing, but do a search on this forum, as that topic has been addressed before, with several good ideas. However, in an enclosed space, the humidity tends to stay at about 80%, which is perfect for orchids, so you could just have the humidifier on a timer, set to go off for five minutes every day or something like that.

And yes, an orchidarium is way better than humidity trays! There is some debate as to whether humidity trays are any good at all, whereas everyone knows how great greenhouses are, big or small.

Rather than one strong fan, I would put a few smaller fans throughout the unit. You'd probably need three computer fans (or their equivalent). Make sure the unit is pretty well sealed, or the fans will whisk all the humidity away. They don't even need to be vented to outside the tank, just let them circulate the air within the tank.

Make sure the unit has front sliding or hinged doors! =)

I asked my husband, an electrician, what you could do about the water getting on the light fixtures, and there is basically no fixture that will last forever without rusting that doesn't cost a fortune. (You would need to get vapour-proof or explosion-proof lights, which run at $800 per light. Ouch!). He suggested buying regular shop lights, the cheap fixtures that you can get at HD, or your T5 fixture if you go with those bulbs, and making the shelves out of wood that has plastic laminate on it. (I know most orchidariums use egg-crate shelves so that the water can drain away, but perhaps you can cut the shelves on a sligh angle, so that the water drains away to the sides and not directly onto the light fixture below it). Then mount your light fixture to that, sealing the screw holes with silicone or duct seal or similar product. Then you could build a sort of shield around the lights, so that water cannot directly spray onto them. Of course they would still eventually rust because of the high humidity levels, but it would be less dangerous. Take the diffusers off the light fixtures though, as they only block some light from reaching the plants. This will expose the bulbs underneath, but that shouldn't be too dangerous. Also, make sure everything is on a GFCI breaker or a GFCI plug. That way if anything shorts out, no one will get electrocuted, and no electrical fires will start.

BTW, what are you planning on making this out of? Glass? Plexi?

planning on making it wood frame enclosed w/ plexi. Glass would be nice i suppose. hoping to use a "swamp cooler" method of cooling. Blowing air through a moist filter medium and kept moist by a pump. Ya, GFCI is a definite. I think the swamp cooler should add enough humidity for sure. Thanks for the tip on the lights, already bought the fixtures, so no T5 lights, but I am using a total of 4 36 inch bulbs per shelf, and shelf height should be around 2.5 feet max. Hopefully it's enough. Better than where their at now for sure though. Thanks for all the help everyone!