You say hairy Neps like truncata can tolerate low humidity, what other neps fit that discription?
My knowledge of Nep species and their characteristics is certainly not comprehensive. My best suggestion is to look up descriptions and pictures of various species. Off the top of my head, N. hirsuta is one... and I think that pilosa and the mistaken-for-pilosa chaniana both are. Some varieties of vietchii and hamata fit the bill as well. Actually, most of the toothy species - hamata, macrophylla, edwardsiana, etc. - do, but I don't recommend these to start because of their expensive status and demand for cool temperatures. Other "ultra-highlander" plants probably do as well; the hairs protect against changes in air conditions in general, including temperature, so I think it's likely to be found in any species that grows in more exposed or variable areas, though maybe not as a dominant trait.
There are some lowland/intermediate species that are hairy, too, but they also are probably not the best starting place. N. albomarginata is one (at least with my clone.) I think there's a form of bicalcurata that is fuzzy at least on the pitchers, but my own definitely has smooth, shiny leaves (for now, at least - it's still a baby.)
The hairs (trichomes in biology lingo) aren't the only adaptation to guard against drying. Thickened leaves are another good one, and in my experience most Neps will produce thicker leaves with a waxy cuticle if you give them plenty of light and normal household conditions. The thickness means that the leaf holds more water relative to its surface area, so desiccation removes less of the plant's total water than with thin leaves. The waxy coating also helps to limit the escape of water through pores in the leaf surface through chemical/molecular effects. It's important to note that, if you grow your plants in a terrarium or with poor light, they will think that they are growing under the shelter of trees, shrubs and grasses as found in their natural habitat. As such, they won't spend as much energy on toughening their leaves - through whatever combination of trichomes, cuticle and succulent leaves their genetics favor - because the surrounding vegetation should be protecting them from wind and temperature fluctuations.
Of course, a glass tank or window is not nearly as good an insulator as plant matter, nor does it gradually respire water into the air to help counteract drying factors, so a Nep's normal gameplan for dealing with sub-par conditions (to make cheaper, lower-quality leaves in hope of scrambling up above the shade of neighbors) will only make matters worse. Water stress limits a plant's ability to turn sunlight into sugar, since they need to break it apart for the hydrogen (er, at least, I'm pretty sure that's how it works.) Likewise, without light, a plant can't make use of what water and nutrients it has.
So at first, you're likely to see some pouting in the form of deformed or aborted pitchers, drying leaves, etc. Don't panic, and don't give into the pouting by sticking it in a closed container; sealing it up just means that the plant can't breath in addition to everything else. If you fear that water loss might kill it, put it in an open container (like a wide-mouthed jar or a large plastic bag) and give it a little less shelter every time it gets to opening a new leaf. The idea here is not to "hold in" humidity, but simply to keep away drying, drafty air and allow the plant to lose water in a steady, controlled manner. And remember that sunlight has a large infrared component, which turns into heat when it strikes an object. If sunlight is too hot/drying, put a bigger, healthier houseplant in the way to provide some filtering, but don't stick it in the shade altogether; the plant needs extra energy to adapt to harsh conditions, not less. It's preferable if the light is filtered, so that there are spots with some shade and spots with none, rather than uniformly dimmer conditions. This allows the leaves to experience the full intensity of the sunlight without forcing the entire plant to deal with it all at the same time. If you don't have a plant that you can use, a loose fabric, like cheesecloth, window screen or crocheted/knitted blankets will work as well.
If you want to take it out of the sunlight altogether, make sure to use very bright artificial light; if it's bright enough, it should look roughly as intense as sunlight that comes through your window when you compare them side-to-side. (Pro tip: don't look directly at the sun and your light bulbs; use a sheet of grey or white paper with half lit by window light and the other by your lamp; one side shouldn't be dramatically shadier than the other.) You'll be hard-pressed to find a light source as bright as the sun, but comparing like this will give you a more objective sense of your light than just using your intuitive sense of what's bright and what isn't. (Your eye is built to adapt to both bright and dim conditions, so you need to be looking at both light sources at the same time to eliminate the adjustments your eye makes.)
Finally, pick a big pot; a larger pot will dry out more slowly, which can help by providing a more stable source of moisture to the roots.
~Joe
