Pretty much any ventricosa can grow in highland, intermediate, and lowland coditions - they're really not picky at all - although they are technically highlanders, as Capensis stated. And Nepenthes aren't really as picky as they are reputed to be when it comes to humidity, except for a few species. So really, don't worry about it, certain Neps can pitcher in as low as 15% humidity, possibly even lower!
And if the live sphagnum is doing Ok, then the humidity must be pretty decent.