Bathroom Ventilation Fan Installation & Improvement Guide
InspectAPedia® -
How to improve bathroom venting, reduce indoor condensation, avoid bathroom mold
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 bad or ineffective bath fan installations
Notes on kitchen and laundry vent ducts and outside screens
Questions & Answers on bathroom vent fan and fan ducting installation procedures, codes, standards
Bath vent fan installation, troubleshooting, repair: this article 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.
InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers nor with topics or services discussed at this website.
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.
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 (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.
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
Guide to the Basics of Installing Bathroom Vent Fans
Bathroom vent fan duct routing suggestions
Don't terminate your bathroom exhaust fan ductwork as we illustrate in the photographs just below.
Our photo at left and two additioal exapmles seen at below left show two typical bath vent fans that spills directly into each building's attic - both are improper vent fan installations.
Vent to outside: 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.
Do 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.
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. We illustrate a metal duct bath fan vent installation at Questions & Answers below
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.
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).
Venting bath fans through the roof surface?: We 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. 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.
Avoid through-roof bath vent exits: Our photo (left) shows a typical attempt at venting a bath into 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.
Our sketch (below) 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.
Do 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. See Mold in Fiberglass Insulation.
Slope the bath fan vent duct downwards towards its building exit - this will avoid condensation accumulating inside the ductwork and dripping back into the building ceilings or insulation. We illustrate a down-sloped bath vent duct installation at Questions & Answers
Insulate the bath vent ductwork: to minimize the condensation that will otherwise occur as warm moist bathroom exhaust air passes through cool ductwork in the building attic or roof cavity.
Our photo (above left) and Carson Dunlop's sketch (above right) shows a long routing of bath vent fan ducts across an attic floor to an exit at the building soffit or eaves and the condensation of moisture in such vent lines. These ducts should be insulated (or replaced with pre-fab insulated ductwork or flexduct), and a better installation would have shortened that very long duct as much as possible, exiting to a soffit closer to the bathroom. Long vent fan duct runs reduce the ability of the fan to move air.
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.
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.
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), bath (Bathroom Ventilation), and especially laundry dryer vent outside screen regularly. 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 details.
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.
Bathroom Vent Fan Capacity & Vent Fan Noise Suggestions
Fan capacity measured in CFM: Bathroom vent fan capacity is measured in CFM (Cubic Feet per Minute) of air that the fan can move. Keep in mind that the CFM capacity of a bathroom vent fan will be reduced by long or irregularly exhaust ducting.
The bath vent fan CFM capacity should be slightly more than the number of square feet of floor of the bathroom area being vented. Multiply the bathroom floor area in square feet by 1.07. For example, a 10' x 10' square bathroom floor area (100 sq.ft. of area) multiplied by 1.07 yields a 107 CFM bath fan requirement. Since bath vent fans are not sold at exactly such odd numbers, buy a vent fan whose capacity is the nearest number just above the calculated CFM required.
Required Bath Vent Fan CFM = (SQFT of bath floor area) x 1.07
Use a still larger CFM bath vent fan if your bathroom includes facilities that generate extra levels of moisture such as whirlpool bath tubs, Jacuzzis™, steam bath, or if occupants (like my daughter Mara) like to take very long hot showers.
Fan noise measured in sones: When purchasing a bathroom vent fan also ask about its sones rating. Sones is a measure of appliance noise. Sones is a linear scale of loudness. Higher sones means a noisier fan. Since bath fan noise often annoys building occupants, a noisy fan will often not be used as much as it should. One "sone" is about as loud as a typical home refrigerator.
Four "sones" is about as loud as normal conversation. Orchestral music has a loudness ranging from 1 to 64 Sones. A higher CFM fan will also have higher sones, but as fan motor and housing quality vary, you may find a higher CFM fan that produces lower sones than some of its competitors. People's subjective experience of sound is a bit complicated. For example, the perceived noisiness of a fan depends also on the frequency of the sound that it is emitting.
Look for a high CFM low-Sones bath vent fan.
Design for Very Quiet Kitchen and Bath Vent Fans
Vent fan manufacturers such as Fantech provide remote fan models. The fan blower/motor assembly is located remote from the air intake in the area to be vented, minimizing fan noise in the building.
For example a bathroom ceiling vent register may be connected by an eight foot or longer duct to a fan located in the building attic where noise transmission to the interior of the building is minimized. Of course the attic vent fan/motor is also connected by further ductwork to blow vented air and moisture outdoors.
Provide Bath Vent Fan Makeup Air
For the bath fan to be effective, and considering that usually the fan is running while the bathroom door is closed, provide an opening to allow makeup air to enter the bathroom from an adjacent building area. Makeup air might be supplied by any of the following:
A warm heating air supply register in the bathroom (if your building is heated by warm air)
A supply grille covered opening cut near the bottom of an interior wall between bathroom and adjoining room, or in the bottom of a bathroom door.
A supply grille covered opening cut near the top of an interior wall between bathroom and adjoining room, or in the top of a bathroom door.
How Proper Bath Vent Fans Save Energy & Lower Heating Bills
Our photo (left) illustrates location of a bathroom exhaust vent over the shower stall - a location chosen to maximize the effectiveness of exhaust of moisture during shower use. We used a GFCI circuit to assure safety for anyone changing bulbs in this fixture. In additional photographs below we illustrate the duct routing for this bath exhaust. Photo courtesy Galow Homes.
Sure, when a bath exhaust fan is running to remove unwanted excess moisture it is also removing warm air from the building.
Air to air heat exchangers and exhaust fans are available that can move air into or out of a building while keeping the heat (or cooling) indoors, but we suspect that such equipment is not cost-justified in a private residence.
An air to air heat exchanger ventilator may be very appropriate for a public building, large dormitory bathroom, or gymnasium.)
But using a properly vented bathroom vent fan wastes less heat than is lost by opening a bathroom window.
To remove the same amount of bathroom moisture as will be handled by a bath exhaust fan a window would need to be open considerably longer than the bath fan must be run.
Frequently Asked Questions (FAQs) on bathroom vent fan and fan ducting installation procedures, codes, standards
Question: what is the maximum horizontal bath vent fan duct length through an attic?
You note for the best bath vent fan duct termination to route the duct: horizontally through the building wall at a building gable-end wall (first choice), or if the gable end is too distant ... . what is a distance the would be within you recommendation?
Reply: Bathroom Vent Fan Vent Ducting Installation Advice Details from InspectAPedia
Our bath vent fan duct installation photo (left, courtesy Galow Homes) shows use of solid metal 4" ductwork conducting the fan exhaust to an outdoor soffit under a cathedral ceiling roof.
Following the sloping roof/ceiling and exiting into the soffit, this fan vent duct slopes down away from the vent fan which you can just see in the photo upper right corner. The ceiling cavity was later filled with solid foam insulation (second photo below).
Any condensate in the ductwork drips to outside - a possible winter icing worry, but because of the solid foam insulation there should be little condensation in the fan duct. Also we insulated the full outdoor soffit bay where the duct exits the building to avoid condensate icing.
We haven't found a bath vent fan manufacturer's spec nor building code or ASHRAE specification limiting the horizontal run length for bathroom vent fan ducting,
perhaps because the normal size of residential buildings means that the duct run length will be within the operating
capacity of currently-sold bath vent fans. But we do have some specific
suggestions for a good bath fan vent duct installation and some duct length specifics as well:
Maximum Vent fan duct length: In general, make the bath vent fan duct run through the attic as short as possible, and avoid elbows and bends as much as possible.
Vent piping should slope
gently back towards the bathroom in most installations, to avoid a low spot that collects condensation moisture in the attic.
Minimum vent fan duct length: But there is an exception to this advice: for remote vent fans such as the Fantech model described below, in order to make
the vent fan as quiet as possible, mount the fan itself as far as possible from the intake register in the bathroom. Fantech
recommends a minimum of eight feet of insulated flex duct between the exhaust inlet register or grille and the fan motor. The
company does not cite a maximum exhaust duct length.
Solid 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. 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.
Flex duct installation 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.
Take a look at the vent fan installation manual for the particular bathroom vent you are installing. Because bath and kitchen
vent fans vary in their power, the length of the vent ducting could provide enough airflow resistance to a small vent fan that it
would not work properly.
The bath vent fan installation manuals we reviewed did not specify a maximum exhaust duct length, they just say route the duct to the outdoors. If you contact your bath vent fan manufacturer directly and speak with a technical support contact, s/he should be able to give you a maximum recommended exhaust duct length for their fan model.
Insulate fan ducts: In un-conditioned space such as an attic, where otherwise your fan duct will be exposed to cold attic air in winter, use insulated solid metal ducting or insulated flex duct to avoid condensation buildup, corrosion
or leaks into the structure.
Watch out: Electrical wiring should be done by a licensed, qualified expert. If the fan is installed over a tub or shower, it's electrical
circuit should be GFCI protected. Never put electrical controls where they can be reached from a bathtub or bathroom shower.
Watch out: Combustion air interference: The possible safety hazards caused by any building vent fan and combustion air for heating appliances are discussed in detail at COMBUSTION AIR and at
COMBUSTION AIR for TIGHT buildings. Fan manufacturers such as Fantech and Nutone all recommend similarly (quoting Fantech):
The combustion airflow needed for safe operation of fuel burning equipment may be affected by this unit’s
operation. Follow the heating equipment manufacturer’s guidelines and safety standards such as those
published by the National Fire Protection association (NFPA), the American Society of Heating, Refrigeration,
and Air Conditioning Engineers (ASHRAE) and the local code authorities.
Question: recommended distances for horizontal bath vent duct routing
You note the best Terminate the bath vent duct: horizontally through the building wall at a building gable-end wall (first choice), or if the gable end is too distant. what is a distance the would be within you recommendation. - DGB family
Reply:
Good question. Naturally we want to keep the fan duct run as short as possible. I reviewed various industry sources and did not see a maximum allowable distance, but typically we see it's 10 - 12 feet or less.
Longer duct runs provide more air resistance, a problem you can mitigate by using solid metal ducting or metal flex duct rather than the more sinuous plastic and wire flexduct often found in these installations. The bath vent exhaust duct shown in our photo (above left) continued to snake across the building attic - too long, too many turns, too much up and down variation in slope - it was an ineffective installation that collected condensate inside the ductwork.
Question: effect of draught-proofing the whole perimeter of a bathroom door on the bath fan operation?
What will be the effect of draught-proofing the whole perimeter of a bathroom door on the fans operation? - Rob Findlay
Reply:
If the bathroom is air-tight the vent fan will have trouble exhausting moisture during hot showers etc.
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
Reply:
Tony 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.
Question: minimum distance required between bath vent fan exhaust and outside main gas line and meter
is there a minimum distance from your outside main gas line and meter that a bathroom fan can be vented out to at the side of your house? - Lee
Reply:
Lee: I haven't seen a gas code specification for the required clearance between a bathroom exhaust vent fan outlet and an incoming gas line or pipe at the building, perhaps because an exhaust vent opening should be venting only one-way: from the building interior to the outdoors. Second, gas piping should not be leaking, and if it is, you should smell the gas leak and repair it immediately.
However if you are concerned that the exhaust fan flapper that should close to prevent back-drafts of outdoor air into the building could malfunction and admit leaking gas from an LP or natural gas pipe or from the gas vent found at an outdoor natural gas meter, if you match the TEN FOOT clearance distance required between an air conditioner air intake and an LP gas tank, since that is the largest clearance that applies for most residential building conditions you should be ok.
We discuss gas piping and gas tank, regulator clearance and various building features in our article titled "LP Gas Tank Inspection & Reporting " - you can find the article by searching InspectAPedia for that title.
Question: ice accumulation around the bath vent cover on the outside wall
I have a brand new house that was built last summer. I noticed that the bathroom vents right below the roof line on the wall. The problem is there's an ice accumulation around the vent cover on the outside wall.
This does not seem normal to me, or is it? I should specify that I haven't even started using this washroom yet, since my shower has not yet been installed. I went up in the attic and they used a flexible vent and there's insulation on top of it. Is heat loss normal when the fan is not operating? Could this be the cause of the ice buildup outside at the vent cover? - Marc
Reply:
Marc, it sounds as if warm moist air is exiting at the vent and you're seeing ice accumulation as a result. I agree that we don't want house air venting itself through the bath vent duct when the bath vent fan is not even running = that's an unnecessary heat loss.
But if the ductwork is routed all sloping "up" from the vent fan location this might happen, in particular if the bath vent fan system does not include an automatic closing mechanism indoors or outside at the wall. Take a look at the vent on the exterior wall (if you can safely do so in this weather and at that height) - see if there is a closing mechanism?
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
Reply:
When we install a vent fan out through an attic and down out of the soffit, we like to use solid metal ductwork 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 and cut a hole in the soffit to install that device, connecting it to the duct. 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.
Questions & Answers regarding this article
Questions & Answers on bathroom vent fan and fan ducting installation procedures, codes, standards.
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Additional technical contributors & reference sources for this article are listed below.
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Eric Galow, Galow Homes, Lagrangeville, NY. Mr. Galow can be reached by email: ericgalow@gmail.com or by telephone: 914-474-6613. Mr. Galow specializes in residential construction including both new homes and repairs, renovations, and additions.
References for Bathroom Vent Fan Installation
Fantech Installation, Operation, and Maintenance Manual, PB Series Premium Bath Fans. These fan models use a remote fan motor and
are available in 4" duct and 6" duct models. Web search 7/26/11 - original source http://fantech.net/docs-resi/412889-pb-install.pdf
Contact Fantech:
Fantech in the United States
10048 Industrial Blvd.,
Lenexa, KS 66215
Phone: 800.747.1762; 913.752.6000
Fax: 800.487.9915; 913.752.6466
www.fantech.net; info@fantech.net
Fantech in Canada
50 Kanalflakt Way,
Bouctouche, NB E4S 3M5
Phone: 800.565.3548; 506.743.9500
Fax: 877.747.8116; 506.743.9600
www.fantech.ca; info@fantech.ca
Nutone Bathroom Exhaust Fan/Light Combination Installation Instructions, Model 8663RP, 8673RP, 8664RP suitable for use
in shower or tub enclosure when used with GFCI protected branch circuit. Suitable for use in insulated ceilings.
Nutone, 4820 Red Bank Road, Cincinnati, Ohio 45227, web search 07/27/2011, original source:
http://www.nutone.com/PDF/InstallGuides/8663RPins61784.pdf
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.
"Energy Savers: Whole-House Supply Ventilation Systems [copy on file as /interiors/Energy_Savers_Whole-House_Supply_Vent.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11880?print
"Energy Savers: Whole-House Exhaust Ventilation Systems [copy on file as /interiors/Energy_Savers_Whole-House_Exhaust.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11870
"Energy Savers: Ventilation [copy on file as /interiors/Energy_Savers_Ventilation.pdf ] - ", U.S. Department of Energy
"Energy Savers: Natural Ventilation [copy on file as /interiors/Energy_Savers_Natural_Ventilation.pdf ] - ", U.S. Department of Energy
"Energy Savers: Energy Recovery Ventilation Systems [copy on file as /interiors/Energy_Savers_Energy_Recovery_Venting.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11900
"Energy Savers: Detecting Air Leaks [copy on file as /interiors/Energy_Savers_Detect_Air_Leaks.pdf ] - ", U.S. Department of Energy
"Energy Savers: Air Sealing [copy on file as /interiors/Energy_Savers_Air_Sealing_1.pdf ] - ", U.S. Department of Energy
Fiberglass: Indoor Air Quality Investigations: Health Concerns About Airborne Fiberglass: Fiberglass in Indoor Air from HVAC ducts, and Building Insulation
Humidity: What indoor humidity should we maintain in order to avoid a mold problem?
Re-Bath, tub lining products is a bath tub relining manufacturer and distributor located in Tempe, Arizona - see rebath.com
Slips, Trips, Missteps and Their Consequences, Second Edition, Gary M. Bakken, H. Harvey Cohen,A. S. Hyde, Jon R. Abele, ISBN-13: 978-1-933264-01-1 or
ISBN 10: 1-933264-01-2,
available from the publisher, Lawyers ^ Judges Publishing Company,Inc., www.lawyersandjudges.com sales@lawyersandjudges.com and also from the InspectAPedia Bookstore (Amazon.com)
How to Clean Moldy Wood Framing & Sheathing How to clean/seal mold from/on exposed lumber or plywood subfloor or roof sheathing indoors - some suggestions based on our field and laboratory research
"Weather-Resistive Barriers [copy on file as /interiors/Weather_Resistant_Barriers_DOE.pdf ] - ", how to select and install housewrap and other types of weather resistive barriers, U.S. DOE
Weaver: Beaver Board and Upson Board:
Beaver Board and Upson Board: History and Conservation of Early Wallboard, Shelby Weaver,
APT Bulletin, Vol. 28, No. 2/3 (1997), pp. 71-78, Association for Preservation Technology International (APT), available online at JSTOR.
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