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Figure 6-4x: (C) J Wiley, S Bliss Bathroom Ventilation Design Recommendations

Bathroom venting design specifications:

This article discusses the proper design, location, control, and ducting of bathroom vent fans and bath vent duct work. We also discuss the issue of bath vent fan noise, proper location of bath vent controls, and bath vent duct condensation problems.

This article series discusses current best design practices for kitchens and bathrooms, including layout, clearances, work space, and accessible kitchen and bathroom layout, clearances, turning space, grab bars, controls, etc.

We include advice on choosing and installing kitchen countertops, cabinets, and kitchen or bathroom flooring, sinks, and other plumbing fixtures and fixture controls such as faucets. A list of kitchen and bath product manufactures and sources is included.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

- Daniel Friedman, Publisher/Editor/Author - See WHO ARE WE?

Bathroom Ventilation Design Recommendations

Bath vent over shower © D Friedman at InspectApedia.com 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.

Our photo (above left) illustrates location of a bathroom exhaust vent over the shower stall - placed 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.

As noted in Chapter 6 of Best Practices Guide to Residential Construction:

Kitchens and bathrooms are key sources of indoor moisture and other pollutants. Kitchens produce particulates and atomized grease from cooking, and with a gas range, they also produce combustion by-products including nitrogen dioxide and carbon monoxide.

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.

See details at INDOOR AIR QUALITY IMPROVEMENT GUIDE.

Bathroom ventilation should be powerful enough to remove excess moisture before it has time to condense on cold walls and windows, potentially damaging finishes, or escape into wall or ceiling cavities, where it may lead to decay or peeling paint.

Ventilation Rate Required for Bathrooms

The minimum ventilation rate for bathrooms required by the 2003 International Residential Code (IRC) is 50 cfm intermittent or 20 cfm continuous (if part of a whole-house ventilation system). While this may be adequate for a small bath, the guidelines of the Home Ventilating Institute (HVI) are more suitable for larger rooms:

Table 6-12 Minimum Ventilation Rate per Fixture for Baths over 100 Sq.Ft.

 
Fixture Ventilation Rate
Toilet 50 CFM
Shower 50 CMF
Bath tub 50 CFM
Stea Shower 50 CFM *
Jetted Tub 100 CFM

Notes to the Table Above

 

Bath Fan Noise Issues

The biggest homeowner objection to bath fans, their noise, has been addressed with the introduction of whisper-quiet bath fans from a number of manufacturers.

Choose the quietest fan for the job, preferably one rated 1.5 sones or less (one sone roughly equals the sound of a quiet refrigerator motor).

Recommendations for Bathroom Fan Location

Bath vent over shower © D Friedman at InspectApedia.com For optimal ventilation, locate the exhaust grilles near the source to be ventilated, typically over the tub or shower.

In smaller baths, a single, central exhaust point is usually adequate, while in larger baths, multiple smaller fans (or a remote fan with separate pickups) will be more effective.

Bathrooms with enclosed toilet areas or steam showers should have separate exhaust grilles in those areas. Since all exhaust fans require makeup air, the bathroom door needs to be undercut to provide makeup air when bathroom windows are closed.

Our photo (left) illustrates location of a bathroom exhaust vent over the shower stall - placed 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.

Advice for Bath Fan Controls

The Home Ventilating Institute (HVI) recommends that a fan be left on for 20 minutes after use of a bath or shower to remove all excess moisture from the room and ductwork. A timer switch is the simplest way to accomplish this.

Another option is a dehumidistat switch, which will automatically operate the fan whenever the humidity level rises above a preset level. A manual override allows normal operation of the fan for other bathroom uses.

Bath Fan Ductwork Specifications

Examples of good, bad, and horrible fan duct work installations are found at our bath venting

home page: BATHROOM VENTILATION CODES SPECS

A ventilating fan must overcome the resistance, called “static pressure,” created by the ductwork, including transitions, elbows, and the wall or roof cap.

The advertised airflow of bath fans is typically based on a static pressure of 0.1 (inches of water gauge) although some also publish the airflow rating at 0.25 inches, which gives a better estimate of actual airflow in most installations.

Figure 6-4x: (C) J Wiley, S BlissA static pressure of 0.1 is roughly equivalent to 15 feet of straight, smooth 4-inch duct with a 100 cfm fan. A typical installation with about 20 feet of duct, two elbows, and a wall cap has an equivalent duct length closer to 80 feet (see Table 6-13).

Table 6-13

[Click any image or table to see an enlarged version with additional detail, commentary & source citation.]

How well a particular fan can overcome the ductwork’s resistance to airflow is highly variable and is indicated by its fan curve, available from the manufacturer. Inline fans mounted remotely and exterior fans are generally the most powerful.

A typical installation with two elbows and 20 to 30 feet of duct reduces the rated airflow of a standard fan by anywhere from 10 to 30%. With long runs, multiple elbows, or corrugated flex duct, airflow may be reduced by 50% or more. To ensure good airflow, follow these guidelines:

Condensation in ductwork is also a concern in cold climates. To avoid problems, insulate the ductwork to at least R-5 or run it below the ceiling insulation. Also keep any metal duct seams facing upward and slope the duct slightly toward the exterior outlet so that any condensation drains to the outside. Avoid any sags in the ductwork, which are potential pooling areas for condensation.

Kitchen and Bath Product Manufacturers, Sources, Associations

Bath Fan Manufacturers - where to get bath fan products, installation instructions, specifications:

This article includes excerpts or adaptations from Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) , by Steven Bliss, courtesy of Wiley & Sons.

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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2021-01-18 by (mod) - can bathroom exhaust ducts be combined to vent through a single common vent duct and termination?

Bramley

Thank you for the question about combining venting ducts. It is one that is mentioned from time to time.

There's not a single correct answer to whether ducts can be combined because we need to look at the manufacturer's specifications on the total allowable effective duct length which includes the effect of elbows and bends, and because combined bath vent ducts may not be approved by your local code inspector, and because even if approved there may be backdrafting issues.

I will do some further research and post it here.

But I think the question is going to turn on the vent capacity of the larger. The conducts the combined venting from multiple fans.

I don't think your local code inspector will specifically prohibit such an arrangement but we're going to need to be able to demonstrate that the capacity is adequate.

You'll also want to look at the vent routing and connection to be sure that when one good exhaust fan is running and another is off that there's no backdrafting.

Bramley

Thank you for the question about combining venting ducts. It is one that is mentioned from time to time. There's not a single correct answer to whether ducks can be combined because we need to look at the manufacturer's specifications on the total allowable effective duct length which includes the effect of elbows and bends.

I will do some further research and post it here.

But I think the question is going to turn on the vent capacity of the larger. The conducts the combined venting from multiple fans.

I don't think your local code inspector will specifically prohibit such an arrangement but we're going to need to be able to demonstrate that the capacity is adequate.

You'll also want to look at the vent routing and connection to be sure that when one good exhaust fan is running and another is off that there's no backdrafting.

Backdrafting problems when bath exhaust vents are shared are not entirely predictable but IF your local code inspector will approve connecting two or more bath exhaust vent fan ducts to a common exiting (presumably larger) duct line, and IF both exhaust fans have adequate capacity in cfm for that duct length, still you may need to install a duct backdraft preventer.

Watch out: although I have not found an explicit prohibition of shared bathroom exhaust vent ducts, at least some building code inspectors won't approve shared exhaust duct venting.

And watch out also: installing a backdraft damper may not work with some bath exhaust vent fans - check with the manufacturer of your vent fans.

On 2021-01-15 by Bramley - Can one combine the ducts of two bathrooms to avoid two penetrations on the wall?

Greetings - I've been looking through the different articles on bathroom venting and I don't see any info on combining the ducting for the vents of two bathrooms through the attic and through a wall on the side of a house.

Can one combine the ducts of two bathrooms to avoid two penetrations on the wall? I would think to do so, one would put a one way door in the duct of the first bath, then merge that duct with the second bath, then on out through the wall. Thank you for any consideration.

On 2020-11-26 by (mod) - bath exahust fan not working, mold problem

Kathleen

If there is already a bath exhaust vent fan installed in your bathroom it makes sense to have a licensed electrician inspect and repair the unit, or the switch, or wiring - a bit of diagnosis will tell what's wrong.

You'll want to clean off that moldy ceiling surface: any household cleaner will be fine.

If the ceiling is drywall and the actual drywall itself has been soaked and is moldy, surface cleaning is not likely to be sufficient and the ceiling may need to be replaced - don't immediately assume that, but

If there is more than 30 sqft. of mold OR if there have been leaks into the building ceiling, walls, floors, then further expert investigation and probably professional mold cleanup will be needed.

On 2020-11-26 by Kathleen

I live in apartment om 6 floor and the bathroom extractor fan is not working it is connected to the light as there is no window in apt bathroom the fan is on ceiling

Can you advice me on how to resolve the problem are these extractor fans work different to one on windows ?

It it a electrician or ventilation person who installs these type of fans ? There is mould appearing on ceiling over bath which i am concerned about !
Thanking you

On 2020-07-21 by (mod) - ok to vent bath exhaust out through soffit

Bath vent termination in soffit (C) Daniel FriedmanJosé

Yes you can vent down out of a soffit.

See details at BATHROOM VENT DUCT TERMINATION

On 2020-07-16 by José Garay

Is the bath exahust permited thruogh the sofit away from windows or openings?

On 2020-01-04 by Anonymous

Not that I'm aware of, but there are of course practical limitations such as avoiding cutting key framing elements. If your bath vent exhaust is going to send moisture up through a soffit and into the attic you may want to review that installation; it may be that blowing down will be sufficient; else up through roof may be a better option.

On 2020-01-04 by Jesse

Are there any code requirements or recommendations on how far away a bath vent should be from the edge of a roof?

I'm putting in a bathroom on second floor of an old house, and due to arrangement of the room, the bathtub is nearest the exterior wall. The bath fan is currently set to blow towards the exterior wall, which would put the vent about 2-3 feet from the edge of the roof. Should I reorient the fan and ductwork to blow the other direction, putting the vent about 5-6 feet away from the roof edge?

On 2019-05-16 by Darrell

How far should warm air be from exhaust fan
IMAGE LOST by older version of Clark Van Oyen’s Comments Box code - now fixed. Please re-post the image if you can. Sorry. Mod.

Question:

(June 15, 2014) Jean said:
An area of grouting on my tiled bathroom floor has started to become damp which makes the floor appear to be dirty.

I can dry it out with dehumidifier but the dampness returns as soon as this is switched off and the bathroom door is closed. This did not used to happen and I wonder if cavity wall insulation has anything to do with the problem.

Reply:

Jean I'd look for a leak behind or under the tile.

Question:

(Nov 27, 2014) divya said:
what would be the best ventilation system for a communal bathroom block having a high footfall?

Reply:

Diva

Probably a positive vent fan to outside operated when lights are switched on.

Question:

(Feb 9, 2015) Jims Finishing/contrating said:
New fan vented through roof but still getting condensation on exterior wall above shower. This is and 70's era home. Have vented many bath fans, Owner purchased and install fan which is over-sized but was in prior to our being called. Moisture problem started after new fan. Have since re-blown roof as Ice was forming in very cold weather.

Reply:

Jim

If the fan duct is not insulated condensate may be indeed forming and running back down the fan duct or even out at an upside-down joint.

Question:

Jim

If the fan duct is not insulated condensate may be indeed forming and running back down the fan duct or even out at an upside-down joint.

Reply:

Jim

What's the relationship between the area of condensation and the fan and its ducting?

Question: heating supply air register minimum distance to exhaust fan air inlet

Heating air supply outlet distance to return air or exhaust vent fan inlet (C) InspectApedia.com Darrell2019/05/15 Darrell said:

How far should warm air be from exhaust fan

[Click to enlarge any image]

Reply:

Darrell

Thanks for a great question: minimum separation distance between warm air supply and either a return duct OR as in your photo and example, an exhaust vent fan inlet as one might find in a kitchen or bathroom.

Short answer: four feet - 48" would be desirable minimum distance. But that's supported only by limited research.

Looking through model codes and HVAC practice guides I don't find a quantitative answer to the question as it's usually phrased: minimum distance between supply and return air registers.

Instead designers typically say: place the two registers at opposite sides of the room - that's going to get them as far apart as possible, thus avoiding a short circuit that simply sucks supply air back into the return register before room occupants can enjoy the supplied warm (or cool in A/C case) air.

But for a bath exhaust my OPINION is this short circuiting of heating or cooling supply air is less of a worry, principally because the exhaust fan is not, in most homes, left running constantly.

It's interesting that the actual degree of supply air short-circuiting into a return air register or into an exhaust fan inlet depends on more than the distance between the supply outlet register and return or exhaust fan inlet register.

Also pertinent are

- the direction of air flow out of the supply - affected by the register vanes and vane setting when they're adjustable

- the direction of air flow into the return or exhaust fan inlet - also affected by register inlet van direction and details

- the relative air velocity at the supply outlet and return inlet - in turn determined by the blower fans involved as well as a host of other factors.

Saadeddin (2016) wrote about this in a useful thesis paper:

Here is an excerpt from his conclusions

In conclusion, this investigation found that using round air diffusers is more efficient than using square air diffusers when placed 48 inches or less away from the return air grille.

In addition, round diffusers have better air circulation throughout the entire zone, as they distribute air following a 360 degree pattern.


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