Flex duct bath exhaust vent fan terminating straight up through the roof surface (C) InspectApedia Bathroom Ventilation Fan Duct Routing
Routing a bath vent duct down & out or up through an attic or roof & out

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Bath vent fan duct routing: up, through roof, down through floor or crawl area?

This article describes routing bath exhaust fan duct upwards through an attic or roof space or downwards through a floor or crawl space. In all cases the ducting needs to conduct the exchaust to the building exterior and needs to terminate in an animal-proof vent cover. Which direction is better? Up or down? It depends.

This article series describes how to install bathroom ventilation systems, fans, ducts, terminations. We include bathroom venting code citations and the text also explains why bathroom vent fans are needed and describes good bath vent fan choices, necessary fan capacity, and good bath vent fan and vent-duct installation details. \

We discuss bath vent routing, insulation, slope, termination, airflow rate requirements and other specifications.

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Where to Route Bathroom Exhaust Vent Ducts: up and out, down and out, over and out?

Wet attic around bad bath vent fan (C) Daniel Friedman

Ventilation in bathrooms is important to prevent moisture damage to wall and ceiling surfaces, decay of wood trim, saturation of building insulation, and mold contamination.

Here we will discuss: bathroom vent fans, required bath vent fan capacity, fan noise and sones. Bathroom vent fan ducts, where to route vent air, duct condensation, ceiling leaks; Photographs of both successful and unsuccessful or downright horrible bad bathroom exhaust fan or vent or duct installations.

Steven Bliss writes in a companion article at BATHROOM VENTILATION DESIGN, "Bathrooms produce moisture, odors, and VOCs from aerosols and various personal hygiene products.

Effective spot ventilation in these areas is critical for maintaining healthy levels of indoor humidity levels and an overall healthy indoor environment."

Especially in bathrooms where a shower is used, large amounts of moisture are added to room air and are concentrated in this area.

Our photo (above-left) shows a horrible bathroom ceiling vent fan ductwork job: multiple ducts sprawl around in the attic, all joining to terminate at an attempted through-roof vent that has fallen back into the attic.

Notice how wet the roof sheathing is in our photograph? These conditions are inviting an attic mold problem too.

Bathroom Vent Fan Duct Routing

Uninsulated fan ducts in an attic (C) Daniel Friedman

Flex duct routing details: If you are using flexible fan duct, stretch the flexduct tight to keep it as straight and smooth inside as possible.

Long sloppy bending flexduct runs significantly reduce the performance of the vent fan. Connect the flexduct to the fan itself using plastic ties, or second best, duct tape. Keep all connections tight and avoid air leaks.

Vent the bathroom exhaust to outside: Both the model building codes (BATHROOM VENT CODES) and every manufacturer's bath vent fan installation guide that we reviewed emphasized: make sure that the bath vent fan carries moist air all the way outside of the building.

Bad bath vent design (C) Daniel FriedmanDo not spill bath vent air into the building attic or roof cavity where it will condense on and damage building insulation, roof sheathing, possibly framing, and where it will certainly encourage mold growth.

Reader Question: dangers of wet bath exhaust vent air re-entering the attic

Isn't there a danger of wet bathroom exhaust air re-entering the attic through the soffit vents if the fan is exhausted through the soffit? - Tony



Our article BATHROOM VENTILATION CODES SPECS cites the importance of venting bath vent fans to the outdoors, not into an attic or crawl space.

The question about moist air reentering an attic through soffit vents after it has been blown out of an exhaust vent opening is a fair one, but I don't think that's likely to be a significant building moisture source. Once blown at any velocity into outdoor air, the moist bath vent exhaust air is diluted significantly.

Or speaking from empirical experience, having inspected several thousand homes and having looked very carefully at moisture and mold stains and patterns in attics and under roofs, I've not found any instances of back-venting of problem moisture into the attic through the soffit vents near the bath exhaust vent that presumably is blowing out through the same soffit or a nearby building vertical wall.

Bath exhaust fan duct length specifications and restrictions are discussed separately at BATHROOM VENT DUCT LENGTHS.

Bathroom Vent Fan Routing Down & Out Through a Floor or Crawl Space

Bath vent spills into attic (C) D FriedmanQuestion: OK to exhaust a bathroom fan under a crawl space or addition?

2016/09/12 Todd said:

We have built an addition to our house. The bathroom fan vent was run to the outside of the house but was removed as the addition surrounded vent.

The contract nicely wrapped the exhaust hose around the fan and installed the roofing with out re plumbing it.

All the floor joists have some kind of wiring or vents running through them. Would it be alright to exhaust the bathroom fan air under the addition as there is to vents on either side of the addition.

I would have to cut a hole through the wall to get to the next room to install the vent outside. This is a down stairs bath.

Reply: yes but ...


I don't have a complete understanding of the situation. In general, you can try venting a bath vent fan "down" but the combination of down-direction, elbows, and total run length may mean that it's ineffective.

Take a look at the installation manual for your fan brand and model. The manufacturer will give guidance on the total duct length permitted. Adding elbows and bends increases the "effective" duct length (shortening what's allowed) as would blowing "down".

Details about length restrictions on exhaust ducting are at BATHROOM VENT DUCT LENGTHS

I've seen OPINION by some builders and web page authors that venting a bath vent "down" is "best" and that it "respects the laws of physics". I'm not sure what physics class those authors attended.

Warm air wants to rise in a building or in ductwork, so pushing that warm bathroom air "down" and out is harder for a bath fan than blowing it upwards and out. The authors who like to blow down are perhaps mixing up the importance of avoiding backdrafts with the importance of providing a vent duct size, routing, length that make the fan work effectively.

Perhaps some confusion about up or down venting also arises because it IS good practice for an overhead fan duct to slope down 1/8 to 1/4" per foot so that any condensation in the fan drips out of the wall vent or soffit vent rather than back into the bath ceiling.

I also prefer using solid metal duct or solid PVC piping as a bath vent fan duct rather than thin mylar or plastic flex duct - you'll get better and safer airflow.

Bath Vent Termination Cover

At the wall 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

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.

Details incuding allowable mesh opening sizes for bath vents are at BATHROOM VENT DUCT TERMINATION

Bath Vent Indoor Air Intake Location

Locate the bath vent fan high in the bathroom - high on an exterior wall when you can, or on the ceiling where it can pick up steamy air from a bath shower.

Watch out: as we warn elsewhere, if a bath fan or light is close to or can be touched by someone in a tub or shower it must be on a ground fault circuit interrupter (GFCI) protected circuit lest your installation kill someone who's naked and wet.

Bath Exhaust Fans Routed Up Through the Roof

Sources of makeup air for a bath vent fan (C) Daniel FriedmanReader Question: is it OK to vent a bath vent fan straight-up, vertically out through the roof? Is it ok to vent the bath vent fan through a larger duct size than the fan's outlet diameter?

I am going to install a new bath fan, I am having a new roof put on the house and decided now would be a good time to put the vent on the roof.

My question is I got a vent for 6" ducting, I will need a reducer at the fan end to 4" Would this be a good size duct for the fan.?

Also I an using metal ducting and it's about six feet from the fan to the roof,

Should I angle the duct a little or would it be ok to go straight up.? D.K. 10/19/2013


You've raised several key topics, and your question helps us realize where we need to work on making our text more clear or more complete. A competent onsite inspection by an expert usually finds additional clues that would permit a more accurate, complete, and authoritative answer than we can give by email alone.

For example on site I might notice something about your attic and roof structure, ease of routing venting, placement of insulation, and even very basic stuff like - where the heck is your home? Bath ventilation worries may be a bit different in a cold climate than in a warm dry one and different again in a wet humid climate. That said I offer these comments:

Local Climate Affects Good Bath Vent Fan Designs: freezing vs hot and humid

For freezing climates we don't want to risk ice accumulation in the vent system - ice can collect from freezing condensate that arrives inside the bath vent duct during hot steamy showers;

For hot humid climates we don't want to have condensate accumulation in air conditioning systems and A/C ductwork, but a bath vent run through a hot attic is less likely to raise that same issue.

Bath vent routing vertically up through roof - not my first choice

Flex duct bath exhaust vent fan terminating straight up through the roof surface (C) InspectApediaI prefer to run a bath vent to outdoors via a horizontal line that goes across an attic and out through a gable-end wall or one that vents down and outside through a roof overhang or soffit.

The vent run needs to be designed to drain any condensate outside not back into the bathroom ceiling; in a freezing climate I'd insulate such a vent line as well; If we run a bath vent vertically up through a roof we have two risks I'd prefer to avoid:

The vertical run guarantees that any condensation runs back down into the fan (risking damaging the wiring or fan motor) and back into the bath or bath ceiling.

The vertical run also means another roof penetration. I prefer to minimize the number of roof penetrations on any building since every penetration is a potential leak point, more so if the penetration flashing is not installed correctly.

Bath vent diameters & vent duct materials

The vent fan manufacturers installation instructions typically give maximum run lengths and recommended vent diameters for their products; long vent runs and vents that use plastic dryer-type flex-duct (not your case) cut the effectiveness of the fan by adding airflow resistance and thus increase the risk of accumulated moisture too.

Metal duct work (your case) is in my opinion always a better installation: smooth interior means better airflow. Metal fan vent ducting also reduces the risk of duct crush or collapse.

I am guessing that for a very short bath vent duct run, going to a larger duct size is fine - it'd make no difference but you're probably not gaining a thing on a short run by using a 6-inch duct to vent a fan that expects to vent through a 4-inch duct.

Bath vent fan capacity

In my experience inspecting and troubleshooting buildings, I've seen many bath vent fans that seemed ineffective. A fan that nobody uses because it's too noisy means a bathroom that is rarely vented adequately (risking mold, smells, even wet insulation). A fan that is under-powered means even if the fan is used it doesn't do anything.

Typical bathroom ventilation codes such as the 2006 IRC specify that a bathroom (toilet) vent fan must provide 20 cfm if the fan runs continuously and 50 cfm if the fan runs intermittently, presumably switched on and off manually or by a timer.

The actual fan venting capacity (in CFM) that you need depends on the size of the bathroom being vented - usually calculated in cubic feet. That figure is matched against the fan manufacturer's recommendations for fan capacity measured in cubic feet per minute (CFM). The CFM rating of the fan in turn presumes that the vent routing, diameter, length, and number of obstructing turns and bends is within the company's specifications. In the article above we explain how to calculate the required bathroom vent fan capacity

Also, for bathrooms over 100 sq ft, the HVI recommends a ventilation rate based on the number and type of fixtures as shown in Table 6-12 - data discussed in more detail at BATHROOM VENTILATION DESIGN

Sorry that these notes are a bit long on arm-waving and short on more specific details, but as we've got no information about your particular installation except what's in your original note, I have to stop here.

Bath Exhaust Fan Routed Down Through Cathedral Ceiling or Down-Sloping Roof to Soffit

This topic has moved to a separate article found at BATHROOM VENT DOWN in CATHEDRAL CEILING

Vent Fan Termination: Exaust to the Outdoors

Watch out: do not simply terminate a bath vent fan duct in an attic as shown in our photo, nor can you just dump the exhaust vent into a crawl space nor into a closed wall, floor or ceiling cavity. The exhaust vent must terminate outdoors. Otherwise you're inviting a moisture, mold, rot, insect problem in the building.

See BATHROOM VENT CODES for the requirement to terminate vents outside.

See BATHROOM VENT DUCT TERMINATION for details about how to terminate the bath exhaust vent duct.


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