Solid metl exhaust fan duct terminating in a roof soffit (C) Daniel Friedman Eric GalowBathroom Exhaust Fan Terminations at Walls & Roofs
How should the bath vent fan duct terminate, how should it be closed, screened, & at what clearance distances from other vents?

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Bathroom exhaust fan termination fittings, locations, & clearance distances: this article describes the proper closure or termination of bathroom exhaust fans & fan ducts to prevent drafts, heat loss, leaks, or even bird or rodent pest entry to the building.

We also review recommended clearance distances between the bath exhaust duct end opening and other building features such as a gas fired water heater exhaust opening.

This article series explains how to install bathroom exhaust fans or vents, the vent ducting, the vent termination at the wall, soffit or roof, vent fan wiring, bath vent duct insulation, bath vent lengths, clearances, routing, and we answer just about any other bathroom ventilation design or installation question you may have.



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Bath Vent Fan Duct Terminations & Clearance Distances & Termination Covers / Screens

Bath vent spills into attic (C) D FriedmanDon't terminate your bathroom exhaust fan ductwork as we illustrate in the photographs shown at left and below.

Our photo at left and two additional examples seen at below left show two typical bath vent fans that spills directly into each building's attic - both are improper vent fan installations.

Terminate the bath vent duct outside at gable end or eaves: horizontally through the building wall at a building gable-end wall (first choice), or if the gable end is too distant, terminate the bath vent down through a building soffit at the roof eaves.

Don't just blow the bath (or kitchen) vent exhaust into an attic, nor, even more crazy, into a ceiling or wall cavity or into a crawl space.

If you were not convinced that failure to properly spill bath vent air and moisture outside can lead to attic mold, notice our photo (below left).

Brown mold was found growing on the attic side of roof sheathing in this new home only where the bath vent moisture was spilling into the soffit (but not outside).

Mold over bath vent fan exit (C) Daniel Friedman Mold over bath vent fan exit (C) Daniel Friedman

Reader Question: what kind of ductwork can be used to terminate the run of piping from the fan to the soffit?

For soffit ventilation, what kind of ductwork can be used to terminate the run of piping from the fan to the soffit? Does the existing soffit mesh of a newer home need to be cut away and replaced with some other register, or will the existing mesh allow a 110cfm fan to operate efficiently? Thanks, great site! - Anonymous

Solid metl exhaust fan duct terminating in a roof soffit (C) Daniel Friedman Eric Galow Solid metl exhaust fan duct terminating in a roof soffit (C) Daniel Friedman Eric Galow

Reply:

When we install a vent fan out through an attic and down out of the soffit, we like to use solid metal ductwork (photo above left) to maximize air flow and minimize resistance, keeping the run as short as possible. Certainly use of flexible metal ducting and even plastic flex duct are permitted in many jurisdictions but in our opinion those are less effective choices.

And I would buy a proper rodent-proof vent opening cover (photo above right) and cut a hole in the soffit to install that device, connecting it to the duct.

In my photo at above left we had not yet trimmed the metal duct to proper length to protrude through the roof overhang or eaves at the proper distance to fit the vent opening cover )shown at above right). That device is automatically opened by pressure of the exhaust fan and snaps shut when the fan is off, avoiding possible back drafts through the bath exhaust vent fan system.

If you just drop the end of the duct into the soffit bay that is in turn covered by perforated panels, I worry that a substantial portion of moisture and vented air will just blow back into the attic.

Venting bath fans through up the roof surface?: While we agree that building exhaust ventilation is most powerful and thus effective when it is routed vertically, we prefer to avoid venting bath fans up through the roof, both to avoid an extra roof penetration (and leak risk) and to avoid condensate leaks into the bathroom ceiling.

Droopy attic ducting of a bath fan (C) Daniel Friedman Mold over bath vent fan exit (C) Daniel Friedman

Terminating exhaust fan duct at the ridge vent: our photo (above left) shows a typical attempt at venting a bath into or actually just below a ridge vent - this direction of vent exit may seem convenient but we don't like it much. In the photo (left) the droopy flex-duct will certainly invite bath moisture to condense and run back to the home's ceiling rather than exiting at the ridge. Snow cover on the ridge will prevent this vent from working.

Terminating exhaust fan duct on the roof surface: at above-right we show an ugly bathroom exhaust vent installation through the roof surface using a laundry dryer sidewall vent cover. Not only were the roof shingles torn up and sealed again to leave a leak-risk around the vent penetration of the roof surface, but because this roof exhaust vent was installed on a home in a snow-climate, in winter with snow cover on the roof the vent is likely to be blocked.

Sources of makeup air for a bath vent fan (C) Daniel Friedman

Venting a bath exhaust fan straight up: Our sketch shows a bath vent fan exiting up through the roof.

The through-roof vent approach gives us another roof penetration, a possible leak spot, and it almost assures that condensing moisture will drip down the vent duct and into the bathroom ceiling.

Considerations when venting a bathroom exhaust fan straight up are discussed at BATHROOM VENT THROUGH ROOF

A direct through-wall bath vent fan design may be preferable if the building roof shape, bath location, or other details make it difficult to exhaust a ceiling-mounted bathroom exhaust fan.

In other words, some bathroom locations and designs such as first floor baths in a multi-story home, are vented out thorough the building sidewall not up through the attic.

Dryer vent spilling into wet crawl space (C) D FriedmanDo not vent bath fans into a crawl space: you're only putting more moisture into an area where it is already going to be a problem, inviting mold growth on wood surfaces and hidden mold growth in building insulation.

At DRYER VENTING we discuss the horrible crawl space shown at left. Below this home the clothes dryer, bath vent fans, and even a dishwasher drain all were dumping into a soaking wet crawl space below the building.

The mold contamination, mud, and stink were horrible.

Soaking a crawl space or attic also wets the insulation, ending its effectiveness and asking for more mold contamination (Mold in Fiberglass Insulation.)

 

Protect the bath fan duct outlet at the building exterior, using approved screening or a louvered fitting so that you do not invite birds or rodents into the building through the ductwork.

Bath vent termination in soffit (C) Daniel FriedmanWhile we don't want a (lint collecting) screen over a clothes dryer vent termination (that's a fire hazard) we do screen bath or kitchen exhaust vent terminations to keep out birds, bats, and rodents.

Our photo (left) illustrates a self-closing bath vent termination in a soffit; when the bathroom vent fan is in operation this cover opens to exhaust moist air; the cover closes by gravity or a light-weight spring when the blower fan is not operating, thus avoiding any cold air back-drafts into the building.

Watch out: an improperly installed bath or kitchen vent fan can draw sewer gases or other odors from outdoor sources right back into the building. See Backdrafting & Sewer/Septic Odors for details.

Watch out: inspect the kitchen (KITCHEN VENTILATION DESIGN), bath (BATHROOM VENTILATION DESIGN), and especially laundry dryer vent outside screen regularly and clean or clear any blockages such as by debris, dust, lint, leaves, or anything else. We have found clothes dryer vents completely blocked with lint and debris.

A blocked clothes dryer laundry vent is a fire hazard.

See CLOTHES DRYER VENTING for examples of vent duct terminations, screens, and safety concerns.

Bath Vent Termination Cover

As you can read in detail at BATHROOM VENT CODES, at the outdoor bathroom vent duct termination you'll need to install a vent termination cover that

  1. Closes to prevent back-drafting when the fan is not running. Typically the cover closes by gravity and works fine.
  2. Prevents critters from entering or nesting in the duct system.

    Unlike a clothes dryer vent (where lint clogging is a fire safety issue) you can screen a bath vent outdoor termination with 1/4" or 1/2" mesh (2006 IRC [305.5]) but that won't prevent a cold backdraft if wind is blowing towards the opening.

    Non-residential buildings such as dormitories can use a mesh with openings no smaller than 1/4" nor larger than 1"

It's smart to insulate the vent ducting too, reducing condensation in the duct from exposing the warm moist bath air to cooler crawl space (or attic) air.

Bath Vent Fan Duct Termination Clearance Distances

Question: 5/15/14 Kevin said: restrictions on how close a bath vent can be to water heater sidewall vent?

Are there any restrictions on how close an exhausting bath vent may be placed to an exhausting vent for a gas water heater? Both exhaust vents will terminate outside through a side wall. Thanks.

Reply:

Kevin,

Clearances for gas fired appliance sidewall vent terminations are given in detail at DIRECT VENTS / SIDE WALL VENTS

You will see that the required distances range from 1 foot to 7 feet depending on what's being cleared-from.

If the bath exhaust fan vent termination location respects the gas fired exhaust (chimney / flue) vent clearances you should be ok.

Question: minimum clearance from bathroom window to exhaust fan outlet

(Apr 9, 2014) Ryan said:

If I ran my bathroom exhaust fan duct outlet out of my soffit is there any minimum clearance from the bathroom window so it will not draw in the odors if someone happened to open the window. My house came with a bathroom exhaust fan that automatically turns on when the overhead shower light is turned on. Right now it vents into the attic (bad) and I want to vent it out the soffit(good)

Reply:

Ryan you'd want to be 10 feet from the nearest operable window if you want to avoid odor intake; that is not a code specification it's an opinion.

Question: bath was venting into attic, now we have mold and peeling paint

(June 1, 2016) JCRego said:
After having a new bathroom installed on our second floor and a second remodeled, within a year we noticed mold and paint peeling. After checking the ductwork in the attic from each of the bathrooms, we noticed they were never tied into the duct from the masterbath vent fan that dumps the hot air thru. a vent tp the outside of the roof.

The ductwork was rather laying on the attic rafters dumping hot air. The original company fixed it by connecting both vents ducts to the master baths ductwork but denied that it would've caused mold and paint damage. If I have the heat on in the winter, both bathrooms have the attic above them and the ductwork is dumping hot air from 2 bathrooms vents into the attic, wouldn't it cause extra condensation in the bathrooms thus why we had mold and paint peeling?

Reply:

I'm not quite clear on this problem JC. If the bath vent duct is of sufficient size and the runs are not too long, and as long as there is no backdrafting when just one fan is running, the combined vent duct system ought to work; there can of course be other errors: dips that collect water, leaks, inadequate insulation, forst or ice formation in winter.

Follow the water: find the leak or water source by following the mold concentrations backwards in the ceiling or attic.

Beware of and replace insulation that was wet.

Question: bath fans spill into attic - cause of mold?

(June 1, 2016) jcrego said:

When the ductwork from 2 separate fans each from a different bathroom below the attic was dumping hot air in the attic (before the contractor fixed it and connected the ducts to the outside of the roof via a vent in the roof) --could the hot air build up in the attic plus our furnace heat was on have caused excess heat build up in these two bathrooms below the attic and then if so could the hot air buildup in the attic cause mold and paint peeling in each of these bathrooms?

Reply:

Well certainly venting bath exhaust into the attic can cause condensation, moisture, mold problems, year round. In winter you're more likely to see frost on the under side of the roof deck if you live in a freezing climate; any time of the year you may find

- mold growth or moisture stains on the under-side of the roof deck ranging from stains around nails to large black areas to visible mold
- moldy insulation onto which water dripped from melting frost or in which moisture formed from high attic moisture
- water stains and mold on the attic side of the ceiling drywall below

It's not the "hot air" per se, that's at fault but the moisture. Yes that can cause peeling paint below. But the peeling paint is the less worrisome problem. Go to the attic and pull up insulation over the worst peeling paint area to look for moisture stains or mold; moldy drywall and insulation need to be replaced.

Use the page bottom CONTACT link to send us photos for comment if you find something interesting.

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