"...3. If you're using artificial lighting then use ventilation on the lights! This is a must if you're using any high output lighting (i.e. metal halide or high-pressure sodium) and is reccommended even if you're using flourescents ... just direct a fan to blow under the lights on the top of the terrarium. This will make a dramatic difference in the temperature in the terrarium. If you want to get the most value for your money then also direct the hot air out of a window ... this will ultimately reduce the ambient temperature around the terrarium...."
My highland terrarium is about 120cm x 60cm x85cm. I use a 70W high pressure sodium light and I have to HEAT(!!![img]http://www.**********.com/iB_html/non-cgi/emoticons/wink.gif[/img] it during the days if no sun hits the terrarium. Temperature raises only about 2°C because of the 70W high pressure sodium light and this is placed directly upon the terrarium (with no glass betewenn the lamp and the plants).
Good high pressure lights do NOT (!!!!!![img]http://www.**********.com/iB_html/non-cgi/emoticons/wink.gif[/img] produce more heat than flourescent lights if they have the same wattage. (where should the heat come from [img]http://www.**********.com/iB_html/non-cgi/emoticons/confused.gif[/img])
Highland plants need cold nights but usually quite warm days and so heat is for me (here in Germany) absolutly no problem during _the day_
I have very big problems with cooling at night, because in summer ambient night time temperature in my room sometimes is more than 20°C. Evaporation cooling will drop the temperature not more than 1-2°C, an experiment with peltier elements was absolutly desastrous (temperature didn't drop more than 1°C but it consumes a lot of power).
My advice would be a cold room if possible. Heating during the day is _a lot_ easier than cooling during the night.
If you have to cool the terrarium I would highly recommend superb insulation.