One other possible cause?.... In the winter we do not use the vents and plug up the vents from the bathroom ceiling with reflective insulation. COULD THIS BE CAUSING THE FROST WHEN MY 2 KIDS TAKE LONG HOT SHOWERS IN THE WINTER? I thought the new exhaust location in the soffits would just go up in the attic also (because hot air rises and the wind blows fierce). Should I block the soffits eaves in those 2 rafter cavities? The attic gets plenty of air.

Originally the water in the exhaust duct to the roof was the problem. Did I cause the frosting by blocking the vents in the winter. We have a bathroom window that gets opened when they take a shower but that probably goes up into the attic bt the soffit too.

Here is a link that might be useful: pictures of problem