I know the feeling. You peer into the attic with a flashlight, and you wonder if the bathroom fan is vented correctly, or maybe you are considering venting directly into the attic interior.
The biggest problem of venting into an attic is the risk of mold growth and wood deterioration.
Moisture and wood don't mix, and it can lead to structural decay, even a roof collapse.
And virtually all building code in the USA requires that bath fans vent to the exterior.
In this guide, you will learn…
- Why you should never terminate a bathroom vent inside of an attic
- How to properly vent a bathroom fan duct to the outside
- The costs of venting a bath fan to the exterior
- Why you should use a spring-loaded cover when venting to a soffit vent
Keep reading to learn all the details of venting a bathroom fan into an attic...
Can I Vent A Bathroom Fan Into My Attic?
No, you should not vent a bathroom fan directly into an attic. The problem with venting into the attic is that the moisture-rich air may form droplets on the wood sheathing, insulation, rafters, and ceiling joists, leading to mold growth. In addition, the excess moisture in the attic may also lead to wood rot, wood degradation, and may even cause a roof collapse.
I invite you to read my guide on your bathroom fan venting options (5 choices) right here for more information.
Mold growth on attic truss due to excess moisture in the attic due to improper venting.
When Warm Air Mixes With The Cold
Since the attic is not a temperature-controlled part of the house, the warm air from the bathroom will mix with the colder air in the attic, and condensation will occur.
This hot-cold phenomenon also happens when you have a cold beverage on a hot day. Water droplets form on the outside of the glass because the hot and cold are mixing. I urge you to read my guide on stopping bath fan condensation issues here.
Excess moisture can sometimes be so bad that it can even build up frost on the underside of the roof sheathing in cold climates, leading to ice melt dripping onto the insulation and drywall ceiling. But it doesn't have to get super-cold for moisture to form inside the attic; the temperature just has to drop below the dew point.
Where Should A Bathroom Fan Vent To?
Bathroom fans should always vent to the outside. And virtually all U.S. states have building code that prohibits bathroom vents from exhausting into the interior of a home. You can read my full guide on bath fan venting code here.
Here is a direct quote from the International Residential Code that prohibits venting to anywhere but the outside.
The air removed by every mechanical exhaust system shall be discharged to the outdoors in accordance with Section M1504.3. Air shall not be exhausted into an attic, soffit, ridge vent, or crawl space.
The only exception is that older homes (such as built in the 1960's) most likely have grandfathered-in status and are exempted.
The correct way to vent bathroom fans into attics is to terminate the vent to the outside either to a gable wall, the roof, or to a soffit.
Either way, the vent will have to go directly to the outside, and installed with a vent hood cover.
Exterior Hood Cover
The vent hood will have a little flap that opens when air is blowing through the vent, but it will remain closed at other times.
The flap will help prevent outdoor air from going into the home when the bathroom fan is not on. The flap and screen will also prevent birds from nesting in the bathroom vent which is very common for bath vents (and dryer vents). Our guide on installing roof vents for bathroom fans goes into detail.
A bird’s nest in a bathroom vent will greatly reduce or eliminate the effectiveness of a bathroom exhaust fan.
Custom Roof 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 plastic hood cover and install it on the roof. Custom roof covers are made from metal, and are much more robust, and somewhat more expensive.
Most roof vent caps are powder-coated and galvanized—which is really essential when it’s on the roof. They also come with the required flapper (to stop air infiltration) and a screen to keep out pests.
Read Also >> The Best Way To Vent A Bathroom Fan?
How Much Will It Cost To Vent Into Attic?
The cheapest part of installing a new bathroom vent will likely be the supplies. You can get a 25-foot insulated bathroom vent and an exterior cover for around $100 or less.
Most of the cost will be in the labor. Also, if you are venting up to the roof, then a roofer may be needed which will cost more than if you are venting to a gable wall or soffit.
Installing a new vent through an attic is going to cost on the low end around $200 just for labor, and may be as much as $500 to $750 if it is a complicated vent that goes to the roof.
Read Also >> Roof Venting Without Soffits
Can A Bathroom Fan Vent Into A Soffit?
Even though it is not my preferred installation and isn't recommended by us, it is possible to vent your bathroom fan directly to the soffit vent.
But I am not talking about placing a bathroom vent on top of the soffit vent. Sometimes builders and homeowners place the vent loosely above the soffit vent, and this is not permitted by code, since it is still venting indoors. You can read our article on installing bathroom vents through soffits here that uses a special cover.
However, if the bathroom vent is vented through the soffit, and terminated with an exterior cover—then it should pass code since it is going outdoors.
Spring-Loaded Soffit Vent Covers
When venting to the soffit, homeowners should use specially-made soffit vent covers. These bathroom fan vent covers are designed to be installed on the soffit vent. Soffit vent covers usually include a spring-loaded damper with a flush-mount style so it blends into your soffit vent.
The downside of soffit vent installations is that it may disrupt some of the air flow of the soffit venting. Soffit vents work because the warmer attic air (compared to the outside) rises, and it creates a suction effect when working in tandem with the ridge vent.
To prevent this problem, I recommend closing off the nearby soffit venting next to the soffit vent cover to prevent the air from going back into the attic.
Can You Vent A Bathroom Duct To The Ridge Vent?
Another very common thing I see during home inspections is that the bathroom vent is terminated just below the ridge vent.
The homeowner or builder hangs the bathroom duct several inches below the ridge vent, securing it with straps. Technically, this won't pass building code, and home inspectors will always call them out.
Even though it is near the ridge vent, the bathroom moisture will still hit the roof sheathing, and some will fall back into the attic. So venting to the ridge vent can still lead to mold growth and wood damage, and it isn't recommended.
Read Also >> How To Choose Bathroom Exhaust Fans?
How To Stop Condensation When Into Attics?
When a bathroom vent is making a long horizontal run in an attic, there is always a risk of condensation or water droplets even if it terminates to the outside. I always recommend installing an insulated bathroom vent, which is a just a normal vent surrounded by fiberglass insulation.
The insulation will greatly help reduce the potential of condensation forming which may lead to drywall damage or mold problems.
How Condensation Forms
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.
Avoid Dips In The Duct
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.
There are pre-wrapped ducts with fiberglass insulation that also include vapor barriers that helps prevent condensation problems.
Final Thoughts On Bath Vents Into Attics
Venting a bath fan into an attic can be done, but it should terminate on the outside.
I would by lying if I told you that it will always 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.
The climate of the home can also make a big difference, and a home in south Florida is far less likely to have condensation problems than a home in Michigan with cold winters.
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