Recommend that bathroom duct be extended to exhaust through soffit or approved roof vent. If venting to attic, treat ventilation requirement as though no vapor barrier exists. This requires 1-square foot of soffit and ridge vent for every 150 square feet of attic space. (.e. if attic is 1000 sf, you need 6.7 square feet of ridge vent and 6.7 of soffit vent. That is 960 square inches (144 in/sf). Bet you fall short.
By venting the exhaust fans and maintaining integrity of the vapor barrier, the vent requirement is 1/300. The same 1000 square foot attic area requires 3.3 sf of vent (480 in). Start measureing and see how close you are. You may need to add ventilation this spring.