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Where does your bathroom ceiling fan actually vent to?
It seems straight forward enough that we have an exhaust fan in a bathroom to help remove the hot humid air and moisture that is generated from the shower and sink right? Right. But, most people won't ever wonder where that fan is blowing all of that moisture. It should be blowing (venting) it to the exterior of the home to help reduce moisture build up in the bathroom, keep our mirrors from fogging up, and keeping that moisture out of the rest of the home.
However, an extremely common defect that I find all the time is bathroom fans that are either improperly vented, or the vent duct work has come loose or fallen off and it is no longer venting to the exterior. Most of the time it means that that bathroom fan is now venting into the attic. In some cases, in a very well ventilated attic, it doesn't cause too much harm but its still not doing anything good and is incorrect.
In other cases it can lead to costly repairs and health hazards (mold and mildew). These images below were from a bathroom fan that had disconnected from the vent termination going out through the roof. I don't know how long it had been disconnected but it had delivered enough moisture for a long enough period of time to cause some significant problems.
Problem 1)
The bathroom fan duct came loose from the roof vent termination.
Problem 2)
Moisture had been building up for a long enough time to saturate the OSB roof sheathing causing it to delaminate and lose its structural integrity. It developed a serious mold and mildew problem on the roof sheathing throughout the attic. It was more severe in areas close to the vent, but was prevalent in the entire attic. There were also moisture stains running down the drywall fire barrier on the attic wall and some of the structural roof trusses.
Problem 3)
There was enough consistent moisture build up and condensation in the attic that it was rusting the galvanized steel connectors (gang plates) on the structural roof trusses. Reducing their structural integrity and causing some of them to become loose.
Long and Short
Bathroom vent fans (all vent fans for that matter) should always vent to the exterior of the building to prevent moisture and condensation related problems. If you can get into your attic and access the vents and ducting it is typically a fairly simple repair. So if you own a home, just go check to see if they're attached properly and working as they should. If you're buying a home and get an inspection, make note if there is mention of a bathroom or kitchen fan that vents into an attic and get it taken care of asap. It doesn't seem like it would be a big deal, but it could either be a big problem down the road or a simple fix if caught in time.
Vent Fan Requirements
Fan should be a minimum of 50cfm (cubic feet per minute)
Ducting should be 4" diameter
Vent/ducting should terminate to the exterior, through an appropriate vent termination with a screen or backdraft damper. (The vent may exit through the roof or through the side of the structure)
Ducting should be securely attached to vent termination and fully sealed against air leaks.
Commonly Found Defects
Fan vents directly into the attic.
Ducting has come loose from vent termination and now blows into attic.
Common gray "duck" tape is used to connect and seal the vent ducting (primary cause for duct work coming loose). "Duck" tape dries out over time, becomes brittle, falls apart and fails. You should use a proper clamp to secure ductwork to the fan and vent termination. Metal aluminum tape designed for ductwork can be used to air seal the duct work where needed.
Plastic flex duct is damaged/torn and leaks air into the attic.
Vent is terminating through a passive roof vent and not air sealed. *This one happens a lot. Someone just hangs it close to a regular roof vent (used for attic ventilation not exhaust fans) and figures that it's good enough to have it blowing in the general direction of going outside.
Short and to the point. Vent fans are designed to move air and moisture outside, please make sure yours are doing their job.
Thanks for reading and I hope this saves you some troubles or money down the road.
Cheers,
Brennan
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