Some lemurs are housed strictly indoors. For example, some of the small, strictly nocturnal species would be impossible to deal with outdoors.
Other species are housed in enclosures that are part of the main building of the DLC. These runs used to be technically outdoors, but individuals can be moved indoors.
Other species are housed in these enclosures during part of the year, and then live in the forested enclosures. Over the years, these enclosures have been encircled by fences, and have some electricity protecting them, keeping the lemurs in, if they get too close. The size of these enclosures varies, but they essentially have enclosures that have been fairly large (several acres). In one of the enclosures, there's a classroom. I'd call it the best classroom at Duke.
NOTE: There was an earlier mention of "free range" lemurs. Technically, any time they are free to roam and establish a home range (the area that they use throughout the course of a year), they are said to be "free-ranging." This is obviously the case in nature. At Duke, because they are not quite in that sort of situation, I have seen papers in which these primates are said to be "semi-free-ranging."
Temperatures in North Carolina can be much cooler than in Madagascar, but likewise can get warmer than in Madagascar. There used to be heat lamps in covered shelters for the lemurs; it's been a few years since I've studied there, but I would imagine the heat lamps are still around.
If I were setting up a tour, I'd try to go during the mid-late Spring, Summer, and early Fall, when lemurs are in the natural habitat enclosures. Early summer means that if there are kids, they should be old enough to start taking their first steps. ALWAYS cool! Check to make sure that the lemurs are in the natural habitat enclosures.
The DLC is unlike any other primate center I have experienced, and I'd probably not have become a primatologist had I gone anywhere else for my bachelor's.
The facility has an important role in research, conservation, education at Duke, and in public education (as per alteran's earlier post). It is not a zoo. It does a lot of things. So try to schedule well in advance. There's also a good place to buy all sorts of lemur things (my favorite being the bobbleheaded lemur).
Of course, if you have a few thousand dollars, I know a few sites in Madagascar that are priceless!
Cheers,
Lavabe