Millipedes in general require moist substrate; that is, moist to the touch, but not so wet that one can squeeze water droplets out of the substrate. The amount of moisture required depends on the species, but this is the general rule of thumb. Specific humidity numbers, while often touted as being very important, do not matter so long as the soil moisture is appropriate and ventilation is not excessive.
While some species can be kept without ventilation for some time, this is not recommended, and depending on the enclosure and the species in question specimens will oftentimes suffocate or poison themselves with fumes from their toxic secretions (a problem with many polydesmids). Therefore, some ventilation, whether a few holes in the lid or a well-placed screen vent, is recommended; if you are having to moisten the substrate more frequently than once or twice a week, it is likely best to reduce ventilation. If the substrate does not start to dry out within a few months, or the substrate becomes and remains muddy for long stretches of time, or water pools in the bottom of the enclosure, an increase in ventilation is strongly recommended. Cross ventilation is not required typically, but can be useful in some situations, especially if stackable enclosures are preferred.
In general, room temperature (70°-78°F/21°-25.5°C) is suitable for most round millipedes, many flat millipedes, and the majority of glomerids, though it is important to always research a given species prior to ensure you can provide the proper temperature. Many species found at high-altitudes or northerly latitudes require cooler temperatures (some Spirobolus, many North American Paeromopidae), while those found in areas with very little temperature fluctuation or in very consistent microhabitats may do poorly outside a temperature range of a few degrees in either direction. The upper limit for temperature is usually considered to be 82°F/27.7°C for most species, while below 60°F is often a problem for non-temperate species.
It should be noted that for many European and more northerly American/Canadian enthusiasts, the winters are severe enough that an additional source of warmth is often necessary to keep temperatures in the proper range. Heat pads and ceramic heat emitters are recommended for this purpose, though they should never create hotspots in the enclosure that are greater than the overall ambient temperature, lest the specimens burn themselves while burrowing or walking through that portion of the enclosure. Rheostats connected to the heating components are therefore recommended if the enthusiast is having issues with maintaining a proper temperature. Heat lamps often dry out the enclosure excessively and/or cause specimens to hide if the light is not a red bulb, and are therefore not recommended for use with millipedes. Heat rocks should never be used for any invertebrate.