Sounds like a good idea to me! One thing though, I have found that my phals. do seem to require that temp. drop in order to spike. I have one phal. in my terrarium that has been there for an entire year, grown three new leaves, all nicely purple tinged (so I know it's getting enough light), and no sign of a spike anywhere! My other phals that are just in pots on my windowsill are blooming, and I think it's because I put them outside last spring (this year), and they got that temp. drop as I was hardening them off, and then they got enough light as I left them outside for the summer. So perhaps you could keep your terrarium near a window so that it gets colder at night. That is the only issue that I see. I would use glass for your shelf. That way some light can still get through from the top of your terr, and it's also just a nice unobtrusive shelf that would 'match' the rest of the terrarium. =) Are you converting an old aquarium, or building your own? You might want to add a fan in there if you want to really cram it full of orchids. Mount it inside though, or you will be taking away all your humidity.

we are planning on having 3 computer fans, two inside to circulate the air, and one leading to the outside to help control the humidity. The way I gave my plants their temp drop this fall was to leave the windows open in the room I grow in...the orchidarium we are building is going to have a large door in the front, when I wanted a night temp drop couldn't I just turn on the fans, open the door, and let the cool air in the room circulate through the tank? I have another question that I think you might be able to answer. I have read in the orchid forum that low humidity causes bud blast in phals, that is one of the main reasons that we are building the orchidarium. Does this mean that once they have bloomed/budded they cannot be taken out of the tank (for display in the house or to take to an OS meeting) because the buds will blast? It seems like the swing from optimum to less than ideal humidity would be stressful. Any thoughts? Thank you!

oops, forgot to answer one of your questions! We are building our own, when I say we, I mean my father who mentioned during a phonecall the other night that he was thinking about building me 'one of those indoor greenhouse things.' I have no doubt he can do it, since he built his own house, and MANY other things over they years (he's an engineer.) We are probably going to build something very simmilar to the last orchidaruim on this site:
http://www.dragonagro.com/terrarium/terrarium.htm

It's called the "Grand Cayman." Any advice would be much appreciated. The fan to the outside would be manually controlled so that it doesn't suck all the moisture out of the tank. Thanks again.
Amy

If you have humidity issues I would only use one fan if your tank will be 100 gallons or smaller, and 2 for a tank between 100 and 200 gallons. Forget the fan venting to the outside, because it will suck ALL of the humidity out VERY quickly, and the whole point of your orchidarium is to keep the humidity in! My 85 gal terr has no fans and it is almost completely sealed. It would be completely sealed if I could do that, but I have a power cord for my waterfall pump coming out the top, so that makes a crack in the glass top of course. I have trouble keeping my humidity above 60%! Phals prefer 80%, which I get it up to when I mist, but then over the next few hours it goes down again, and that's with just one little crack in the lid! I used to have two very tiny fans in there, and the humidity was always at 40% when they were running, which was lower than the humidity in the rest of my apartment! So I really don't think you want to do that, and that applies to your idea for cooling your tank. You will simply loose all of your humidity, which makes the whole orchidarium useless. You probably also want to put your fans on dimmer switches, so that you can slow them down if you need to (which you probably will). How big of a tank are you thinking of, BTW? The 'Grand Cayman' looks like a good set-up.

Unfortunately I don't have any brilliant ideas for cooling your tank without removing all the humidity. =( You probably want to consider an automatic misting system as well. If you do have one, then you could run more fans, and you could open it up at night for a cooling period (which of course doesn't need to be every night all year, but just for a few weeks, to initiate the spike), and then quickly raise the humidity again, or even mist while the glass is open at night once or twice. Hand-misting is an annoying chore. If you get a misting system, just go to a hydroponics store and they will tell you what you need. Make sure you either let them pick the pump for you, or buy a size HIGHER than you think you will need. If your pump is not strong enough, you will get a pathetic trickle, not a nice mist. I can't use the misting system in my terr. because I didn't get a strong enough pump (DH's fault), and haven't gotten enough money yet to buy a stronger one. They are not cheap, so get the right one the first time!

As for bud blast, I don't think it will be a problem. When you buy an orchid at Home Depot it has been moved from nice humid greenhouse conditions to a dry store, and they don't drop all of their flowers right away! Orchids do grow and even bloom in less than ideal conditions, as I'm sure you know, so I think that they are tougher than most people give them credit for.

Thanks so much, I'll make sure we modify the fans. I suppose if there was a humidity problem, I could simply open the door, anyway! I don't mind hand misting, I do it now and find it a nice theraputic activity. We are planning to make it almost the exact same size as the 'grand cayman' about 4 1/2' long 2' deep, 42" tall. We want the shelf to be all the way across the case instead of short-ish as it is in the sale model. I am hoping that they'll be able to stand losing humidity for a little while in the fall to get their temp drop. If they don't like it, I guess they'll just have to sulk, because it's the only thing I can think of. That dimmer switch idea is great! My dad will love it (he an electrician). Thanks so much for your help, I REALLY appreciate it!
Amy

sahoyaref: the reason the humidity is less than in the rest of your apartment is because humidity is exponentially decreased with increasing temperature. At 100C the air is 100% humid if all the gas is water, at 30C 100% humid means that 3% of the air is water! Wikipedia has a good bit on this. What this means is that the air in your house might be 40% relative humidity, but when you heat it up 5 degrees (due to the lighting) the same amount of water is only 25% humid.

Regarding cooling, the best approach (and the one I'm working on for my greenhouse for summer) is probably evaporative cooling. If you have a reasonably dry house (low humidity), then air that is humidified will cool (drop in temperature) relative to that in the room (and it increases your terr humidity). This humid air actually has the same heat - if you try to cool it down with a reverse cycle air conditioner it will take a little more energy to cool it to the same temp as the warmer dry air. Now, because this air is physically a lower temperature you can use this cooled air to heat exchange with some dry room air, cooling the dry air down first, then you evaporatively cool that cool dry air bringing it down to a considerably lower temperature again but now humid. Working out how to do this is left as an exercise. I think the limit with this approach is the dewpoint of the room air, which in summer in Melbourne is often only 10C, despite shade temps greater than 40C.

Having said all of that, I don't think I'll have a cooling problem in our non-airconditioned house in sweltering summer, because I think that most plants can survive high temperatures (

Amy, my husband is an electrician! Isn't it great to have one so close to wire all your lights and fans and stuff for free for you? =)

Nathan: always the logical, scientific approach. =) No, it's helpful, really. I do like to know why stuff happens the way it does, and now I understand! But while turning off the lights does cause the temp. to drop, it is not enough of a drop to initiate spiking in Phals.

sahoyaref: yeah, I don't think my plants need any special cooling (despite coming from cool temperate rainforest), but I can see your problem. Considering you have such wonderfully cold weather over there couldn't you run some polypipe from outside in through your terrarium and back outside? Then you put a tiny little blower on it and you'll suck heat out quite effectively. Make sure the pipe is insulated before it gets to your terr :) You could also use water cooling, but that is a lot messier (requires pumps, antifreeze etc).

Maybe the trick is to change the season on your phals so that the required cold session lines up with your local weather? I don't know what the requirement is - is it a long period of cold (3 months), or cycling every day?

no, just a 10-15 degree drop for about three weeks in October every year.
Thanks for all the posts!!
Amy

'Wonderfully cold' weather? Isn't that an oxymoron? =) I don't think anyone here actually enjoys minus 30! =) I don't want to coo my entire terr off though, because some of the plants wouldn't like it, and I'd loose my humidity (maybe), and I would need a lot of piping because my tank is not close to a window at all! I think I'll leave that phal in there for another year, and then if it still hasn't flowered, I'll just take it out. Too bad it's so firmly attached to it's mount! I won't be able to take it off, so I'll have to just stick the whole mount in a pot with sphag! =)