Bathroom Vent Fan Codes, Installation, Inspection, Repairs

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Bath vent fan codes, installation, diagnosis & repair:

How to install, specify or improve bathroom venting, reduce indoor condensation, avoid bathroom mold. Bathroom vent fans, required bath vent fan capacity, fan noise and sones.

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 bathroom exhaust vent codes, specifications, advice.

We explain 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.

We discuss bath vent routing, insulation, slope, termination, airflow rate requirements and other specifications. We also describe bathroom vent fan ducts, where to route vent air, duct condensation, ceiling leaks; Photographs of bad or ineffective bath fan installations.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Why is Bathroom Ventilation Needed?

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.

And as Steven Bliss writes in a companion article


“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? These conditions are inviting an attic mold problem too.

2015 IECC Section R403.6 Mechanical Ventilation (Mandatory)

The building shall be provided with ventilation that meets the requirements of the International Residential Code or International Mechanical Code, as applicable, or with other approved means of ventilation. Outdoor air intakes and exhaust shall have automatic or gravity dampers that close when the ventilation system is not operating.

Guide to Installing Bathroom Vent Fans & Fan Ductwork

Bath vent spills into attic © D Friedman at

Some signs of excessive, uncontrolled bathroom moisture include:

  • Stains, thermal tracking, or mold growth on bathroom walls and ceilings, possibly also on bath vanities and cabinets
  • Damaged bathroom window trim
  • Bathroom window condensation, and frost forming on bathroom windows during freezing weather
  • Frost found in the attic on the roof underside over the bathroom area during freezing weather
  • Stains and mold growth found on the roof sheathing in attics or roof cavities over bathrooms
  • Damaged or curled roof shingles concentrated over a bathroom
  • Leak stains in bathroom ceilings, especially around ceiling penetrations for light fixtures or ceiling vent fans

Bathroom vent fan duct installation & routing suggestions begin just below



Bathroom vent fan duct materials: flex duct vs solid metal

Vent fan spilling into attic © D Friedman at  IRC M1506.1 Duct Construction

Where exhaust duct construction is not specified in this chapter, construction shall comply with Chapter 16.

IRC Section M1601 Duct Construction

IRC M1601.1 Duct Design

Duct systems serving heating, cooling and ventilation equipment shall be installed in accordance with the provisions of this section and ACCA Manual D, the appliance manufacturer’s installation instructions or other approved methods.

Flexible plastic vent fan ductwork: shown at above left is a common use of un insulated, flexible ventilation fan duct. In this installation the duct is improperly installed, spilling directly into the attic space of the building.

This duct material is least costly at the time of installation but may be most costly when a combination of accumulated condensation and duct damage leaks into the building insulation or ceiling cavity.

IRC M1601.1.1 Above-Ground Duct Systems

Above-ground duct systems shall conform to the following:

  1. Equipment connected to duct systems shall be designed to limit discharge air temperature to not greater than 250°F (121°C).
  2. Factory-made ducts shall be listed and labeled in accordance with UL 181 and installed in accordance with the manufacturer’s instructions.
  3. Fibrous glass duct construction shall conform to the SMACNA Fibrous Glass Duct Construction Standards or NAIMA Fibrous Glass Duct Construction Standards.
  4. Field-fabricated and shop-fabricated metal and flexible duct constructions shall conform to the SMACNA HVAC Duct Construction Standards‒Metal and Flexible except as allowed by Table M1601.1.1. Galvanized steel shall conform to ASTM A 653.
  5. The use of gypsum products to construct return air ducts or plenums is permitted, provided that the air temperature does not exceed 125°F (52°C) and exposed surfaces are not subject to condensation.
  6. Duct systems shall be constructed of materials having a flame spread index of not greater than 200.
  7. Stud wall cavities and the spaces between solid floor joists to be used as air plenums shall comply with the following conditions:

    1. These cavities or spaces shall not be used as a plenum for supply air.
    2. These cavities or spaces shall not be part of a required fire-resistance-rated assembly.
    3. Stud wall cavities shall not convey air from more than one floor level.
    4. Stud wall cavities and joist-space plenums shall be isolated from adjacent concealed spaces by tight-fitting fireblocking in accordance with Section R602.8.
    5. Stud wall cavities in the outside walls of building envelope assemblies shall not be utilized as air plenums.

Source: IRC SECTION M1601 DUCT CONSTRUCTION (2015) [PDF] Utah state code, retrieved 2019/10/03 original source: codes as adopted by the state of Utah

Uninsulated fan ducts in an attic (C) Daniel Friedman

Flexible metallic exhaust fan ductwork: shown at above right is flexible metal exhaust fan ductwork.

This material is more smooth-surfaced than the plastic product shown at above left and by its flexibility, can eliminate the need to install many elbows in the system.

The installation shown is too long and should have been insulated.

Metal bath exhaust fan duct installation in new construction (C) Daniel Friedman Bath exhaust fan in insulated ceiling after foam insulation installation (C) Daniel Friedman

Metal vent ducts: kitchen vent fans require, and good bath vent duct design also uses solid metal ducting, not flexible “dryer vent” material. Solid ductwork has a smoother interior surface that improves airflow, though it is indeed more trouble and a bit more cost to install.

Solid metal vs flex duct for kitchen and bath vent fans: For optimum venting use insulated 4″ or larger metal ductwork rather than flexduct that may sag, giving you areas that
collect water and risk leaking into a ceiling below. Kitchen vent fans require metal ductwork for fire safety.

Our photo at above left illustrates a solid metal bathroom exhaust duct along with the bath vent housing installed in a cathedral ceiling during new construction. The ceiling cavity between the I-Joists was later insulated with solid foam, as shown at above right.

Because this is a sloped cathedral ceiling it was not possible to slope the fan ductwork back down towards the shower below the fan. Instead we vented this fan out through the soffit in the roof eaves. (/ventilation/Bathroom_Vent_Termination.php)

Smooth metal duct also maximizes vent fan airflow performance. But flex duct and insulated flex duct are approved by bath vent fan manufacturers are very widely used for fan venting
because of its ease of installation and low cost, as we describe next.

Question: using spray foam to seal around a bathroom vent fan

(Mar 12, 2014) John said:

Hi. I sprayed insulation foam ( not the fire block ) around the bathroom vent fan in an attempt to seal small air leak from the attic . Is this a fire hazard ? Thank you



You should be OK if – the following:

– foam did not enter the fan enclosure
– the fan installation data including stickers inside the fixture specify that it can be installed in direct contact with building insulation.

I’d have preferred using a fire-resistant foam, but if the above conditions are met you should be ok. Take a look at the bathroom vent fan duct installation in the photographs above and you’ll see a successful bath vent installation in a foam-insulated cathedral ceiling.

Bathroom Venting Codes & Vent Fan Codes

Bathroom Ventilation Requirements Summary

The model building codes adopted by most jurisdictions typically require bathroom ventilation to be provided either by an operable window (3 sq. ft. or greater) or by mechanical means – a bathroom vent fan (20 continuous or 50 cfm intermittent, vented to the building exterior.

Lighting as well as mechanical ventilation are both required where there is not an adequate operable window.

Below we excerpt key bathroom ventilation code specifications. More complete bathroom venting design and installation specifications such as discussions of vent duct lengths, insulation, slope, air supply, wiring, are listed


The model ventilation codes, building codes, mechanical codes, plumbing codes do not generally discuss exhaust vent duct direction and routing such as “up” or “down” through attics or crawl areas, but all of the model codes require that the vent fan must exhaust to the building exterior:

  • Residential code: R303 Ventilation air from the space shall be exhausted directly to the outside.
  • Mechanical code: M1501.1 Outdoor discharge. The air removed by every mechanical exhaust system shall be discharged to the outdoors.
  • M1507.1 General. Where toilet rooms and bathrooms are mechanically ventilated, the ventilation equipment shall be installed in accordance with this section.
  • M1507.2 Recirculation of air. Exhaust air from bathrooms and toilet rooms shall not be recirculated within a residence or to another dwelling unit and shall be exhausted directly to the outdoors.
  • M1507.3 Exhaust Rates for Residential Dwellings – Oregon 2011 Code adopted from IRC

Table M1507.3 Exhaust Rates for Residential Dwellings: Kitchens / Bathrooms – Toilet Rooms

Table M1507.3 Exhaust Rates for Residential Dwellings IRC Excerpt from Oregon  - cited & discussed at

[Click to enlarge any image]

Source: M1507.4 Prescriptive Exhaust Duct Sizing – Oregon 2011 Code adopted from IRC

Table M1507.4 Prescriptive Exhaust Duct Sizing

Table M1507.3 Exhaust Duct Sizing for Residential Dwellings IRC Excerpt from Oregon  - cited & discussed at

Watch out: be sure to see the duct length limits given at BATHROOM VENT DUCT LENGTHS


IRC RESIDENTIAL SPECIALTY CODE – CHAPTER 15 EXHAUST SYSTEMS 2017 FLORIDA [PDF] modeled from the International Residential Code or IRC

IRC RESIDENTIAL SPECIALTY CODE – CHAPTER 15 EXHAUST SYSTEMS 2011 OREGON [PDF] modeled from the International Residential Code or IRC

M1507.4 Local exhaust rates

Local exhaust systems shall be designed to have the capacity to exhaust the minimum air flow rate determined in accordance with Table M1507.4.


Area to be

Exhaust Rate
Kitchens 100 CFM intermittent or 25 CFM continuous
Bathrooms-Toilet Rooms Mechanical exhaust capacity of 50 CFM intermittent or 20 CFM continuous

Note that for bathrooms the ventilation specification is for mechanical exhaust and is not considering ventilation through a bathroom window.

Bathroom Venting Exhaust Fan CFM Adequacy

The model codes are specifying the minimum bathroom vent fan capacity needed. If your bathroom includes a steam bath, steam shower, or jetted tub, sources of additional moisture, you will need to increase the total bathroom exhaust vent capacity in CFM.

Industry experts recommend 60 cfm to 80 cfm for small bathrooms and 200 to 300 cfm for a large bathroom with a steam generator

. And if your vent fan ducting is long or includes bends that too will require added fan capacity.

Watch out: higher CFM bath fans, and some low-cost cfm bathroom ventilators can be so noisy that nobody wants to operate them – leading to indoor moisture, mold, even rot problems. Details about how to determine the necessary vent fan capacity in cubic feet per minute and about bath vent fan noise control options are


Watch out: while recirculating or in my view faux-vent fans are sold by some building suppliers, they are not permitted for bathrooms, are ineffective, and will probably leave the bathroom wet and smelly.

2018 Bath Exhaust Vent Duct Lengths

Exhaust duct length vs diameter vs CFM per the building codes and discussion of use of fan duct calculators are given


2006 ICC Ventilation Code, Chapter 4, Ventilation

The U.S. state of Virginia adopted the ICC model ventilation code that we cite here.

Required Bathroom vent fan ventilation rate in CFM

This ventilation code specifies a bathroom exhaust airflow rate for toilet rooms and bathrooms (not served by a window of adequate size) of

25 or 50 cfm/ft for hotels, motels, resorts and dormitories, and a slightly more modest 20 / 50 cfm for private dwellings.

Rates are per room unless otherwise indicated.

The higher rate shall be provided where the exhaust system is designed to operate intermittently.

The lower rate shall be permitted where the exhaust system is designed to operate continuously during normal hours of use. – [Table 403.3, Minimum Ventilation Rates]

This model ventilation code also specifies that the ventilation system

Exhaust openings shall have a minimum and maximum size in louvers, grilles and screens, measured in any direction of not less than 1/4″ and not more than 1/2″

Intake openings in residential occupancies shall be not less than 1/4″ and not more than 1/2″

Intake openings in other than residential occupancies shall be not less than 1/4″ and not more than 1″ – [Table 401.5, Opening Sizes in Louvers, Grilles and Screens Protecting Outdoor Exhaust and Air Intake Openings]

2018, 2015, 2009 & 2006 IRC Bathroom Ventilation Code Excerpts for Ventilation

Section R303.3

Where no window: 20 cfm continuous, 50 cfm intermittent, vent to building exterior

The 2006 (or later) International Residential Code IRC R303.3 Bathroom Ventilation – requires that

when natural ventilation requirements (3 sq ft of windows 50% operable) can’t be met,

the bath should be vented by either a 20 cfm continuous vent fan or a 50 cfm vent fan that is intermittent (or switched).

Exhausting of the bath vent fan must indeed be to the building exterior.

Exhaust Air from toilet rooms and bathrooms shall not discharge into attic, crawl space or other areas inside building.

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

Example Model Ventilation Codes

New York City Title 27 / Subchapter 12: Light, Heat, Ventilation & Noise Control Code

A very detailed building code guideline beyond the IRC I cited below is the New York City housing code from which I excerpt this Bathroom Ventilation Code:

§[C26-1207.3] 27-759 Bathrooms and toilet rooms.-
Bathrooms and toilet rooms shall be ventilated as follows:

(a) When ventilated by natural means, the natural
ventilation sources shall comply with section 27-749 of
article six of this subchapter and shall have an
unobstructed free area of at least five percent of the
floor area.

In no case shall the net free area of the
ventilation sources be less than one and one-half square
feet except that in occupancy groups H-1 and H-2,
provided the ventilation opening conforming with
section 27-749 of article six of this subchapter may be
in a vent shaft provided that the net free area of the
opening is not less than three square feet.

The vent shaft
cross-sectional area shall be increased by one-fifth of a
square foot for every foot of height, but shall not be less
than nine square feet in area and open to the outer air at
the top; or, the vent shaft may be open at the sides
above the roof with louvers providing an equivalent net
free area at the top, equal to the area of the shaft.

(b) By individual vent shafts or ducts constructed of
noncombustible materials with a minimum cross-sectional
area of one square foot and one-third additional square
foot for each additional water closet or urinal above two
in number. The upper termination of such ducts shall be
equipped with a wind-blown ventilator cap.

(c) When a bathroom or toilet room is not ventilated by
natural ventilation as required by this section, it shall be
mechanically ventilated as follows:

(1) Rooms containing only one water closet or urinal
shall be mechanically ventilated by an exhaust system
capable of exhausting at least fifty cubic feet of air per
minute. Means shall be provided for air ingress by
louvres in the door, by undercutting the door, or by
transfer ducts, grilles, or other openings.

(2) Rooms containing more than one water closet or
urinal, and any auxiliary spaces such as those used in
hand basins, slop sinks, and locker rooms, shall be
mechanically ventilated by an independent exhaust
system capable of exhausting at least forty cubic feet of
air per minute per water closet or urinal. The outdoor
air supply shall conform to the requirements of article
seven of this subchapter.

(3) Toilet exhaust systems shall be arranged to expel air
directly to the outdoors.

Bathroom & Kitchen Vent Air-Flow Measurements & Standards

Loose blower assembly pulley or belt reduces airflow Carson Dunlop AssociatesQuestion: can you measure the air flow at my New York City Apartment Kitchen & Bathroom Vents to Show They’re Inadequate?

I come to you because I need a professional of HVAC.

I moved in in a new building on Harlem – NYC – mid December, and since then I have a problem with the exhaust fans in the kitchen and the bathroom. The strength of the aspiration seems very low and so the fans are not efficient. (ie: the bathroom stays humid)

I asked the management of the building to take care of this but they just did a test with a tissue that stays on the grill. But this test does not mean the strength is OK.

I want to be sure I am right, so I would like to know if you guys can come over to measure the flow of the air with a flood hood or another dedicated appliance.

Do you know a serious, reliable, reasonable price contractor who could provide this service ? – Anonymous by private email 2017/08/01

Sketch above was provided courtesy Carson Dunlop Associates a Toronto home inspection, education, and report-writing firm.

Reply: Recommendations & OPINION

Window open stack effect (C) Daniel FriedmanA tissue is a reasonable way to see which way air is moving – in or out of a register, as we discuss


or in and out of a window in a high-rise building (photo shown here and discussed further


But it is impossible to translate the thin tissue of that observation to an objective measure of air flow rates nor to a reliable statement about the adequacy of a vent system since even the most slight air flow will move a light tissue against the intake grille of the vent.

In NYC there are quite a few HVAC companies, some of whom might be willing to send over someone to measure your exhaust fan performance but frankly I’m doubtful that the cost is worth the benefit.

Exhaust fans in the U.S., especially in older buildings, are typically of very modest capacity – to put it politely. Historically or at least before the city’s adoption of modern energy and ventilation codes, New York City (NYC) buildings vented into an air-shaft, by windows or by convection and without electricity.

I suspect the cost of even a simple site visit and report (probably $250 to $400.) would cost not much less than the cost to install a higher capacity fan, even more-likely if the higher-capacity fan were also a low-sones-rated (that is to say less noisy) unit. A Panasonic ceiling vent fan unit itself is about $125. U.S. + installation. Figure $400+ in NYC.

Most of those HVAC engineering companies are interested in working on larger projects involving building air conditioners and are not interested in vent fan systems in an individual apartment.

A local HVAC installation company (not an engineering firm per-se) may have a qualified design engineer on staff who could do what you ask, but they too are not very interested in small jobs and may price consulting accordingly -unless you are going to install a more expensive air-to-air heat exchanger unit (HRV or a more-sophisticated “ERV” – energy (enthalph) recovery unit, – loosely-called “heat -recovery-ventilators”.)

Here is an NY City company familiar with ventilation codes – you could call to see what they charge and what they will do (and let me know what you’re told)

  • New York Engineers, 275 West 39th Street, 2nd Floor New York, NY 10018 212-575-5300,

If you own or intend to have a long term lease on the apartment, those improvements may be worth doing.

But when you rent, usually the landlord is not legally bound to improve your bath venting. The landlord may not be complying with city mechanical codes on ventilation (given below) but they still may not be too anxious fix inadequate ventilation unless faced with a larger issue than one tenant who wants proper design, ventilation, and indoor air quality.

Note: For a helpful discussion of just what constitutes “adequate venitlation”, see Offermann, Francis (Bud) J., P.E., C.I.H., ASHRAE & Mark Nicas, Ph.D., MPH, C.I.H., USE WITH ADEQUATE VENTILATION ? [PDF], ASHRAE Journal, May 2018.

I suspect that only if you could make a case that the present unit is dangerous as a fire hazard or unsafe (creating an expensive mold contamination problem) might you get anywhere without having to hire a lawyer.

If you take a look in the ductwork and see an accumulation of grease in a kitchen exhaust I’d consider that a potential fire worry that might get action.

You could buy your building manager an air flow measuring device –


for less than the cost of a service call. Then let your building super use the measuring tool instead of a piece of tissue – if they’re willing to do it. (The gift of the cfm tool might be taken as generous and prompt action or as annoying and … depending on personalities. )

Compare the readings at your kitchen and bath vents with what NY City building mechanical code requires:

  • Kitchen ventilation fan requirements: 100 cfm (intermittent) or 25 cfm (continuous) (cfm = cubic feet per minute)
  • Bathroom ventilation fan requirements: 50 cfm (intermittent) or 20 cfm (continuous)

At BUILDING CODE DOWNLOADS I provide links to New York City codes that specify that mechanical ventilation is required for kitchens and bathrooms regardless of whether there is a window or not (Chapter 12, INTERIOR ENVIRONMENT) and a link to the city’s mechanical code where the engineering firm whose page I gave above got those vent-rate requirements. There you’ll find

  • NEW YORK CITY MECHANICAL CODE (2008) [PDF] adapted from the ICC Mechanical code, Chapter 4, VENTILATION, retrieved 2017/08/01,original source:
  • NEW YORK CITY CODE: INTERIOR ENVIRONMENT [PDF],. Chapter 12, INTERIOR ENVIRONMENT, retrieved 2017/08/01,original source: viewer.html?file=2014CC_BC_Chapter_12_Interior_Environment.pdf&section=conscode_2014

    1201.1 Scope.The provisions of this chapter shall govern ventilation, temperature control, lighting, yards and courts, sound transmission, room dimensions, surrounding materials and rodent proofing associated with the interior spaces of buildings

  • NEW YORK STATE HVAC ENERGY CODE REQUIREMENTS [PDF] for Heating, Ventilation, & Air Conditioning Code & Specifications retrieved 2017/08/01, original source: Energy/RmanHVAC.pdf

and other codes and standards.

Also see more specifications for ventilation of baths and kitchens at

Bath Vent Fan Duct Duct Damage Protection

Damaged bath exhaust vent duct © D Friedman at Protect the bath exhaust fan exhaust vent from damage during installation. Our photo (left) illustrates damage we found in a building ceiling where the exhaust vent duct had been torn during installation. A result was moisture and ultimately mold contamination in the bathroom ceiling.

Bathroom Vent Fan Duct Length Specifications

Bathroom vent fan duct length restrictions: keep the fan duct length as short and straight as possible.

Some manufacturers require a minimum distance between the duct outdoor termination and the fan assembly; a review of installation guides for several bathroom vent fan models did not come up with a maximum distance. Long vent fan duct runs reduce the ability of the fan to move air.

Discussion of maximum and minimum bath fan duct run distances or lengths are


Bathroom Exhaust Fan Heat Recovery Ventilator Systems

This bathroom vent fan topic has moved to BATH EXHAUST FAN HEAT RECOVERY

Bath Vent Fan Wiring Connections

Simple fan only wiring for a Nutone Model 8814R and similar bath vent fans (C)  adapted from Nutone Typically the bathroom vent fan motor is powered by the bathroom ceiling light fixture circuit; some installers, particularly in hotels or rental units, hard-wire the bath exhaust vent fan to force it on when the bathroom ceiling light is on – thus assuring that the vent fan is in fact used. If the bath vent fan is noisy this forced-on status can annoy everybody.

In the simplest case there will be just three fan wires to connect: black to black (the hot or live or power wire), white to white (the neutral wire), and ground to ground, typically a bare ground in the house wiring to a green ground wire in the fan housing wiring set.

The most general fan wiring instructions will state: “Hook up the bath vent fan wiring by removing the electrical junction box cover. Next connect the house wires to the fan wires: ground to ground, neutral white to neutral white, and hot black to hot black.”

[Click to enlarge any image]

But when the fan includes also a light and perhaps an electric heater there are actually three circuits to complete, each with its own controlling switch.

At BATHROOM FAN WIRING you will find detailed electrical wiring connections for a typical fan, light, & heater combination vent fan system, adapted from installation instructions for the Delta Breez Model RAD80L installation manual and other bath fan installation guides cited there.

Bathroom Vent Fan Ductwork Insulation

This topic has moved to a new article at BATHROOM VENT DUCT INSULATION

Bathroom Vent Fan Duct Routing


Uninsulated fan ducts in an attic (C) Daniel Friedman

Bath exhaust fan duct length specifications and restrictions are discussed separately


See BATHROOM VENT DUCT TERMINATION for details about how to terminate the bath exhaust vent duct. The fan must vent to the building exterior.

Bath Exhaust Fans Routed Up Through the Roof

Sources of makeup air for a bath vent fan (C) Daniel FriedmanThis discussion is now found


I 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:

A vertical bath exhaust fan duct 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.

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


Reader Q&A – also see the FAQs series linked-to below


If I understand your original question, you have a bath exhaust that is discharging into a roof cavity or space not to outside.

In no case is it acceptable to just dump any exhaust fan directly into an attic. It needs to exhaust to the exterior.

this is only a toilet room, adjacent to the master bath which contains a ventilation fan that vents externally. How are half bathrooms handled? Any differently? Our house passed final inspection when the house was built 7 years ago, but failed in this area when the buyers hired a home inspector.

Bath exhaust vents need to be directed to the outdoors, not into an enclosed building cavity.

The answer to your code question is given in the article above on this page


Is it okay to vent a ceiling fan into the attic for just a toilet room? What is the IRC code referencing this?


The exhaust vent duct should not terminate in an attic or roof space; it needs to vent to the building exterior.

What is California code on bathroom exhaust fans, primarily at the point of how the tubing and what tubing is connected to. Is it legal to have the flex tube vented to a dormer vent but the tube ends 1ft short of the actual vent and not connected directly to it?

Its ok if the length, slope, etc. are following the manufacturer’s guidelines.

Is it acceptable to use 4″PVC pipe as an exhaust duct in a small Powder Room?

It will go straight up through an insulated interior wall into the attic, and then outside via roof vent installed by a roofer.

Or do I need to use a duct of other material ? Please advise.
[email protected]

Example of Broan Maximum Bath Vent Fan Duct Length Calcualtor - cited & discussed at


Thank you so much for asking. I’m embarrassed to report that reading our two main articles on bath exhaust fan duct length here and at the link I give below, I found no clear answer.

The reason that there is no one right answer to “how long can I make my bath exhaust vent duct run?” is that … it depends on

– the fan model (its size and CFM rating)

– the vent material (smooth vs. corrugated or worse)

– the vent diameter

– the number of elbows

– the required or “design” flow rate in CFM (from your local code, such as 40 CFM).

The exhaust vent must comply with local codes that in turn are typically adopted versions of

the 2015 IRC Section M1506.2

or the 2018 IRC Section M1504.2 (ft)

So we need to know the Brand and model of your exhaust fan and then to look at that manufacturer’s specific installation instructions (that will include duct length and length-equivalent numbers for each elbow or bend)

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

Also, though I emphasize that following the instructions for your specific exhaust fan are the first place to check, the fan manufacturers also include exhaust vent fan length calculators.

For example Broan’s calculator is at

There you’ll see that for higher CFM flow rates some fan models won’t be acceptable period while others (according to Broan and I doubt this) are shown as “NL” or “no limit on length”

Example: for a 40 CFM flow rate and a 3″ smooth duct diameter and using just 2 elbows
Broan Fan Model EZ80N can run up to 34 ft.

But this isn’t enough: Industry experts recommend 60 cfm to 80 cfm for small bathrooms and 200 to 300 cfm for a large bathroom with a steam generator

When we change the design CFM to 60 we see that with a 3″ duct this fan WILL NOT WORK period.

When we then change the DUCT DIAMETER to 5 inches, the calculator says yeah, you’re good, no limit.

Watch out: using these fan calculators you can quickly get in trouble: that means having a fan that won’t actually work or that will give a fire hazard – if you do not watch every parameter you enter with great care. Some of the fan models will show “no limit” to duct length (that I question) but look again: the calculator may have changed the vent duct diameter from 3″ up to 7″ or 8″.

I’ll add more data tables, code citations and examples to the article above.

Thanks for asking

Finishing out basement bathroom the only wall I can run my exhaust outside is 32’ away can I use 3” glued pvc and can I have a run of this distance


I’m not sure what kind of fan we’re discussing, Matt, but certainly if an exhaust fan is too close to a supply or return air register for a heating system, when the exhaust is running it may interfere with the delivery of heated air to the space.

A typical guideline will require that the exhaust fan inlet be at least two meters or six feet away from the hearest HVAC register. Some inspectors require a 10 foot separation. Section M1602.2 of the 2006 IRC is an example.

Is there a requirement for seperation between ac/ heat registers and the exhaust fan housing


You cannot necessarily put all plumbing vents out through a roof at a single point – the distances involved may be too great to jam all of the plumbing vents together – it depends on the building size, plumbing routing, etc.

Is it possible and up to code to put all you vents together so there is only one stack coming out of the roof


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

We moved this discussion to a full article


Proper Bath Vent Fan Duct Slope & Slope Direction

This topic has moved to a new article at BATHROOM VENT DUCT SLOPE

Bath Vent Fan Duct Terminations & Clearance Distances

This topic has moved to a new article at BATHROOM VENT DUCT TERMINATION

Bath Vent Fan Duct Termination Clearance Distances

This topic has moved to a new article at BATHROOM VENT CLEARANCES

Continue reading at BATHROOM VENTILATION DESIGN Best-Practices, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see BATHROOM VENTILATION CODE FAQs – questions and answers posted originally on this page.

Or see these

Building Ventilation Articles

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Publisher – Daniel Friedman

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