Last week I inspected a 12 year old townhouse. For twelve years the ducts from the two upper bathrooms have been venting into the attic----they were never terminated to the exterior. Considerable moisture can be vented into attic spaces----as is indicated by the staining of the attic floor near the ends of these ducts.
Under the right conditions, this moisture can condense on the roof sheathing and lead to mold/fungal growth----as can be seen in this next picture----in a different house.
All exhaust fans should be terminated to the exterior of the home, at a proper vent cap with a back draft damper, and the ducts should be insulated---like the ones in the first picture. These two vents could go to termination points at either the side wall or through the roof. On homes like this, that are 3 stories tall with very steep roofs, it is preferable (for maintenance purposes) to terminate at the side wall.
Charles Buell
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