You can try adding a small pan of water and see if that helps. Or you could use the 3-2-1 method: 3 hours naked, 2 hours wrapped in foil, 1 hour naked. Just adjust the time as appropriate for the ribs. Baby backs need less smoking/cooking time than spares and beef ribs. Personally, I tried this method once and while the results were good, just too much fooling around. I get good results just going full on naked for 4 to 5 hours, depending upon the meatiness and and fat content. Also, I smoke at 225 throughout the cook.