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. However, it can be done safely and 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
Let's get started!
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.
- Control moisture automatically
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- How bath fans with auto moisture control can protect your bathroom
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.
What About Venting To The Soffit Vent In My Attic?
Sometimes builders and contractors place the end of a bathroom vent directly to the soffit vent which is also an unwise course of action. The soffit vent is pulling air in from the outside, and it works in tandem with the ridge vent at the top of the roof.
Since there is a negative pressure pulling in air from the outside through the soffit venting, and if there is a bathroom vent ending there, the moisture from the bathroom will be pulled back into the attic --- and may lead to mold damage and wood rot. And the problem is that homeowners rarely go into the attic, so mold may be growing in the attic for a long time before it is discovered.
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. At homeinspectorsecrets.com, 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.
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.
The Bottom Line On Bathroom Fan Attic 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.
But the potential cost of dealing with a large mold issue can by very expensive, as well as the possibility of dealing with wood rot and deterioration. Also, attics are never perfectly sealed from the interior of the home, so some of that moist air sent into the attic will get recirculated back into the interior of the home.
Anyways, it isn't that difficult to do it properly, and a homeowner with some DIY skills can do it on a Saturday afternoon.