Can your bathroom fan vent into the attic or soffit?
I decided to write this quick blog after having a discussion with a fellow contractor recently in which we both disagreed where a bathroom fan could properly vent and then seeing a textbook case on this the other day. From the other contractor's experience he believed the bathroom fan could be vented into the soffit or even in some cases out of a gable wall, while I disagreed and noted that it had to be directly vented to the outdoors per the code.
Since we are all human and we all make mistakes I decided not only to pull the code book myself, but also call my local building inspector and ask him in case I was missing something or there were changes in the recent codes.
Inevitably when he called me back he referred me to the exact code which I had located in Chapter 15 – Exhaust Systems in the International Residential Code (IRC). Note: I am personally referencing my 2009 code book and while the codes have since been updated, this code specifically has not changed.
The inspector stated as does the code on Section M1501.1 – Outdoor Discharge Termination – “The air removed by every mechanical exhaust system shall be discharged to the outdoors. Air shall not be exhausted into an attic, soffit, ridge vent or crawl space.”
Section M1502.3 - Duct Termination - goes on to say “…Exhaust shall terminate not less than 3 feet in any direction from openings into buildings. Exhaust duct terminations shall be equipped with a backdraft damper.”
In a nutshell the inspector said that a bathroom or dryer vent since both are exhausting moist air can NOT be vented into a soffit per the code. Second, doing so would also likely not allow the installation of a termination with a backdraft damper. Thirdly it would be within 3’ of an air intake back into a building assuming they had either vented soffit or vented drip edge.
Could the bathroom vent be exhausted out of a gable wall? The answer: it depends. Is there a gable vent within 3 feet of where this vent will exhaust? If so, then the answer is no.
What if there is no gable vent?
Technically then per the code the answer is Yes. However, doing so would be bad practice. Why? Picture this, in the winter time when the homeowner is taking a nice hot shower and the fan is running exhausting moist hot air out the hose and into the vent, upon entering the cold attic space and cold hose, the moisture would condense within the pipe filling to some degree with water. Eventually this would leak over time causing water stains on the ceiling and the belief that there is a leak in the roof.
Now I have seen this many times first hand, not only when visiting homeowners to inspect roofs, but even on an old house of mine where after I moved in I examined the attic space to find a bathroom fan vented into the gable wall via a 25’ long dryer hose. In the winter the hose sagged like a slinky between each floor joist as it was full of water. I literally walked the water out of the house dumping several gallons of water out onto the side of my house. That was quickly rectified!
So, if you’re having discussions with a roofer and they are telling you that you can exhaust your bathroom fan into your attic space or the soffit itself, you now know this is incorrect and does NOT meet the current building code. Doing so allows moist air to be sucked back into the home potentially causing mold buildup.
These photos were taken on 3-2-2019 at a friends house in Taunton after he called me to look at a mold issue. After inspecting it I would have to say that this is the best display of how a house can pull moist air back into the home causing mold on the plywood underlayment. You can clearly see that the mold is the worse where the fan is exhausted into the soffit and decreases as you get further away from it. Approximately 8' on either side of this vent there was no visible mold