Bathroom ceiling vent fan, heater, light combination (C) Daniel Friedman Bathroom Ventilation Exhaust Fan
Installation / Repair / Building Codes

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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 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? These conditions are inviting an attic mold problem too.

Guide to Installing Bathroom Vent Fans & Fan Ductwork

Bath vent spills into attic © D Friedman at

Some signs of excessive, uncontrolled bathroom moisture include:

Bathroom vent fan duct installation & routing suggestions begin just below at BATHROOM VENT DUCT MATERIALS


Bathroom vent fan duct materials: flex duct vs solid metal

Vent fan spilling into attic © D Friedman at  Uninsulated fan ducts in an attic (C) Daniel Friedman

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.

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 Vent Fan Duct Routing


Uninsulated fan ducts in an attic (C) Daniel Friedman

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

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 at BATHROOM VENT UP THROUGH ROOF

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 at BATHROOM VENTILATION DESIGN

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

We moved this discussion to a full article at BATHROOM VENT DOWN THROUGH FLOOR / CRAWL AREA

Bathroom Venting Codes & Vent Fan Codes

Bathroom Ventilation & Lighting 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 at BATHROOM VENTILATION CODES SPECS - home.

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:

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 at BATHROOM VENT FAN SIZING

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.

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]

2015, 2009 & 2006 IRC Bathroom Ventilation Code 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.

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 at RETURN AIR, INCREASE, or in and out of a window in a high-rise building (photo shown here and discussed further at AIR MOVEMENT in BUILDINGS).

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)

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 - see AIR FLOW MEASUREMENT CFM - 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:

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

and other codes and standards.

Also see more specifications for ventilation of baths and kitchens at

Bathroom Vent Fan Ductwork Insulation

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

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

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.

Details about maximum and minimum bath fan duct run distances or lengths are at BATHROOM VENT DUCT LENGTHS.

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.


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Bathroom Ventilation Articles

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