To me, all bathroom exhaust fans need to vent "outside". Either to the side of the house or out through the roof. NO bathroom fans should vent into the attic.
In my area, building code clearly stats that exhaust fans must NOT vent into the attic. They need to vent outside. Not too sure what the old building code was but if the new building code states to vent bathroom fans outside, it has its reasons behind it. re: Moisture, frost, mold, etc. etc.
If this was my home, I'd bring it up to "new" building code - before costly damages start appearing.
BTW: Before adding insulated vent piping to your existing fan, do confirm the CFM (vent power) of your existing fan. If previous home owner installed it, they probably went for the lowest cost exhaust fan (instead of proper CFM size fan). Measure your bathroom square feet and confirm the existing fan is of proper "CFM size". If needed, purchase a proper (or slightly larger) bathroom size exhaust fan. Most often, the fan's exhaust pipe size determines what size of "insulated piping" is needed in the attic. For good CFM chart, surf: http://www.naturalhandyman.com/iip/i...n/infexha.html
Hope this helps as well....