Should a bathroom fan hose be vented into an attic? If so, how is it done?
During my home inspections, I have come across many bathroom vents incorrectly installed into attics.
A poorly vented bathroom fan can lead to moisture damage, mold, and other issues ---- but it can be done correctly with a few crucial steps.
In this article, you will learn...
- Why you should never terminate a bathroom vent inside an attic
- How to properly vent a bathroom fan duct to the outside through an attic
- The real reason you should insulate a bathroom vent pipe contained in an attic
- Why mold is the real culprit of bathroom attic venting
- Why excess moisture in an attic may lead to poor air quality
Can I Vent a Bathroom Fan Into My Attic?
No, you should not vent a bathroom fan directly into an attic. However, you can vent a bathroom fan through an attic while it terminates on the roof or gable end.
It seems like such an easy solution, just leave a bathroom vent hose in an attic. It’s all outdoor air anyways, right?
Let me tell you, improperly ending a bathroom vent inside an attic can lead to “unintended consequences” --- and builders have stopped this bad practice many years ago. Prior to the 90’s, builders commonly vented bathrooms into attics.
On my home inspections, it is still a common sight for me to see bathroom vents going directly into the attic --- spewing moisture. The problem with this approach is that all of the moisture in the air goes onto the wood sheathing, insulation, wood rafters, and wood ceiling joists --- which can lead to mold growth and rot.
Especially during cold weather months, the excess moisture from a bathroom vent can build up frost on the underside of the roof sheathing, and eventually this ice will melt and leak onto the insulation and drywall ceiling.
How Do I Properly Vent A Bathroom Fan Into An Attic?
The correct way to vent a bathroom fan through an attic is to terminate the vent either to the roof or to the gable wall.
Either way, the vent will have to go directly to the outside, and installed with a vent hood cover. The vent hood will have a little flap that opens when air is blowing through the vent, but at other times it will remain closed.
The flap will help prevent outdoor air from going into the home when the bathroom vent is not on. The flap will also prevent birds from nesting in the bathroom vent which is extremely common.
A bird’s nest in a bathroom vent will greatly reduce or eliminate the effectiveness of a bathroom exhaust fan. It's also important to note that if you install a roof vent cover for a clothes dryer, you must remove the metal screen because it will catch lint and may turn into a fire hazard.
Roof Vent Covers
It is also important to note that vent hood covers for the roof are a different and specialized type of cover. You can’t just take a normal hood cover and install it on the roof. The roof covers are made from metal, and are much more robust, and somewhat more expensive.
My absolute favorite vent hood cover for roofs is the Builder's Best Roof Vent Cap.
The Builder's Best Roof Vent Cap actually has some protection against corrosion because it is powder coated and galvanized --- which is really important when it's on the roof. And it comes with the required flapper, as well as pre-drilled holes for an easier install.
What About Venting To The Soffit Vent In My Attic?
Even though it is not my preferred installation, it is possible to vent your bathroom fan directly to the soffit vent. In this way, you avoid messing with the roof and the associated leak risks.
The downside of soffit vent installations is that it disrupts some of the air flow of the soffit venting.
Soffit vents work because hot air rises, and it creates a suction effect when working in tandem with the ridge vent. There is a possibility that some of that bathroom moisture will get sucked into the attic due to negative attic pressure and air moving into the soffit vent.
There are actually special bathroom fan vent covers designed to be installed on the soffit vent.
I really like the Dundas Jafine Soffit Vent Cover because it has a spring-loaded damper (flap) with a flush mount style so it blends into your soffit vent. The Dundas Jafine Soffit Vent Cover fits 4" , 5" and 6" bathroom ducts and the damper is designed to keep working in the presence of frost.
Can I Vent My Bathroom Duct To The Ridge Vent?
Another very common thing I see during home inspections is that the bathroom vent is vented just below the ridge vent. This is probably a better thing than venting it to the soffit vent, but it can still cause problems.
When the air is warm outside, this powers the natural soffit-ridge vent system, by pulling outside air from the soffits and then through the ridge vent. But how about when the air is cold? That natural suction system is no longer active.
Also, the vent is always terminating a few inches BELOW the ridge vent, so when it is venting, all of the air is not going directly the outside. Much of the air is bouncing around the edge of the ridge vent, and the wood sheathing. So venting it to the ridge vent can still lead to mold growth and wood damage.
How Do I Prevent Moisture When Venting a Bathroom Into An Attic?
When a bathroom vent is making a long horizontal run in an attic, there is always a risk of condensation or water droplets. I always recommend installing an insulated bathroom vent, which is a just a normal vent surrounded by fiberglass insulation.
This will greatly help reduce the potential of condensation forming which may lead to drywall damage or mold problems.
Condensation forms when two different temperatures meet --- such as a cold drink on a very hot day --- water droplets forms on the outside of the glass.
If you are sending warm interior air through the bathroom vent while attic is very cold, then water may form. In addition to insulating your bathroom vent, you want to avoid U-shaped bends in the vent, because this will allow condensation on the inside of the vent to drip towards the low point of the vent hose.
I personally recommend the Dundas Jafine Insulated Flex Duct which is a 4-inch duct and 24-feet in length. The Dundas Jafine Insulated Flex Duct is wrapped in fiberglass insulation and it also has an outer vapor barrier. The vapor barrier helps prevent damage due to ultraviolet light.
The Truth About Attic Bathroom Venting
The lowdown on venting your bath fan into an attic is that it can be done, but it should terminate on the outside.
I would by lying if I told you that it will cause problems, it may be totally fine. In fact, I have seen numerous houses that haven't yet had signs of mold with the bath fan venting incorrectly.
A Few Possible Issues
However, there are a few real potential downsides when a bathroom vent is improperly terminated inside an attic:
- mold growth
- wood rot and structural deterioration
- moist bathroom air is sucked back into the home interior
- excess moisture attracting rodents and pests
It isn't that difficult to do a vent installation properly, if you have all the equipment and some homeowner DIY skills.
But if you don't own a sawzall (or know what it is) then you may want to get some professional help. There's no reason to waste this weekend risking your neck on a ladder or walking around a dusty attic.