I have t & g ceilings throughout my house. Cold upper Michigan climate. Humid in summer. The boards were applied directly to trusses that have faced fiberglass insulation stapled to the bottom of the truss. (home is over 25 yrs old log construction. R 48 total. Vented attic. Ventilation baffles. Only one small area has a scissor type truss over a large window so does not have the total r48. Otherwise flat ceilings. No air barrier. How would I install an air barrier in the attic and could that cause mold problems as vapor from inside the house escapes into the many nook and crannies through the t&g? What should I use for the air barrier? 

thanks
pat