I have never had any problems with my lenses steaming up inside a tropical butterfly house.
Having said that on one of the occasions I was shooting in the house there was a chap who was so upset with his lens because it kept on steaming (according to him) and I noticed him and when asked he said that the heat was intense and his zoom kept on giving problems as it kept on hunting. Now I am really not sure if the hunting he mentioned was really due to that. In my view the hunting was purely due to the lack of light inside there as I was shooting and needed extremely high iso as my image below shows. And he must have assumed its because it was steaming, hence the hunting.
After a few minutes he went into the car and came back with a macro lens and then said that one wasn't steaming and it was fine. In other words what he tried to imply was that it wasn't hunting. Go figure

, that just gave more weight to what I was thinking, that it was just poor light

.
I have used the 70-200 f/2.8 zoom, the 300mm prime, the 180mm macro, the Sigma 24-70 and at times I have spent as long as 4 hours in a row inside the tropical house shooting butterflies in flight and never had my lenses steam.
But please note the temperatures may vary from place to place. I must admit that the ones I have been to the temperatures are relatively mild and not much humidity.
Once I went to Woburn safari park and they have a tropical birds house and when I went in there it was so humid that I decided to come out. Didn't even want to risk my kit getting steamed.
Hope this helps.
The following image was taken on one of the butterfly houses in St Albans and I had spent there a good few hours, note the high iso required
butterfly world-6210 by
ZuluArt, on Flickr
Camera Canon EOS 7D
Exposure 0.002 sec (1/500)
Aperture f/8.0
Focal Length 300 mm
ISO Speed 2000
Exposure Bias 0 EV
Flash Off, Did not fire