if experience high humidity outside the loft it will also be humid inside the loft , if theres enough fresh air entering the loft then the drying process would be quickened so air flow seems to be lacking , as you live in a very dry climate add some light via your roof also , a few photos can tell where you need to improve , if i was you i'd get a smoke bomb and smoke bomb your loft to check your air flow or in your case the lack of it not all people use lime in there lofts but it's cheap and will help to dry the dropping out quicker