Question? Just ask us!
Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair
InspectAPedia ® Home
ACOUSTICAL SEALANT CHOICES
AIR BYPASS LEAKS
AIR LEAK DETECTION TOOLS
AIR LEAK MINIMIZATION
AIR LEAK SEALING PROCEDURE
AIR TEST FOR MOLD: ACCURACY
AIR TEST SAMPLING CASSETTE STUDY
ALLERGEN TESTS for buildings
ALLERGENS in BUILDINGS, RECOGNIZING
ALLERGY & MOLD IAQ PRODUCTS
ALLERGY TESTS for PEOPLE
ALLERGY TEST ACCURACY
ANIMAL ALLERGENS / PET DANDER
ANIMAL ENTRY POINTS in buildings
ANIMAL ODORS IN buildings
ATTIC LEAKS, CONDENSATION & MOLD
BATH & KITCHEN DESIGN GUIDE
BEST CONSTRUCTION PRACTICES GUIDE
Best Interior Finish Practices
BLOWER DOORS & AIR INFILTRATION
BLOWER FAN CONTINUOUS OPERATION
BLOWER FAN OPERATION & TESTING
BOOKSTORE - INTERIORS
CHINESE DRYWALL HAZARDS
FIREPLACES & HEARTHS
FLOOD DAMAGE ASSESSMENT, SAFETY & CLEANUP
FREEZE-PROOF A BUILDING
HEAT LOSS in BUILDINGS
HEAT LOSS DETECTION TOOLS
HEAT LOSS INDICATORS
HEAT LOSS PREVENTION PRIORITIES
HEAT LOSS R U & K VALUE CALCULATION
HOUSEWRAP AIR & VAPOR BARRIERS
HOUSEWRAP INSTALLATION DETAILS
HOUSEWRAP at SILLS, SOLES, TOP PLATES
HUMIDITY CONTROL TO PREVENT MOLD
HUMIDITY LEVEL TARGET
ICE DAM PREVENTION
INDOOR AIR QUALITY & HOUSE TIGHTNESS
INDOOR AIR QUALITY IMPROVEMENT GUIDE
INSULATION IDENTIFICATION GUIDE
INSULATION R-Values & Properties
KITCHEN & BATH DESIGN GUIDE
LIGHTING, INTERIOR GUIDE
MOLD RESISTANT DRYWALL
MOLD RESISTANT CONSTRUCTION
MOLD on or in CARPETS
MVOCs & MOLDY MUSTY ODORS
MYCOPHOBIA, STAINS MISTAKEN for MOLD
MYCOTOXIN EFFECTS of MOLD EXPOSURE
NOISE / SOUND DIAGNOSIS & CURE
ODORS & SMELLS DIAGNOSIS & CURE
ODOR DIAGNOSIS CHECKLIST, PROCEDURE
STAIN DIAGNOSIS on BUILDING INTERIORS
STUCCO WALL METHODS & INSTALLATION
SWEATING (CONDENSATION) on PIPES, TANKS
THERMAL TRACKING & HEAT LOSS
VENTILATION in BUILDINGS
WALL SIDING TRIM & FINISHES
WALL FINISHES INTERIOR
WALL CONSTRUCTION BARRIER vs CAVITY
WATER BARRIERS, EXTERIOR BUILDING
WATER ENTRY in buildings
WIND WASHING INSULATION At EAVES
WINDOWS & DOORS
WINTERIZE A BUILDING
WOOD, COAL STOVES & FIREPLACES
WOOD STOVE SAFETY
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.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
Bathroom vent fan duct routing suggestions
Don't terminate your bathroom exhaust fan ductwork as we illustrate in the photographs just below.
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.
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
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
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
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
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
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.
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
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
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.
Frequently Asked Questions (FAQs)
Questions & Answers on bathroom vent fan and fan ducting installation procedures, codes, standards.
Ask a Question or Enter Search Terms in the InspectApedia search box just below.
Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
References for Bathroom Vent Fan Installation