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ACOUSTICAL SEALANT CHOICES
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ANIMAL ALLERGENS / PET DANDER
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ANIMAL or URINE ODOR SOURCE DETECTION
ROOF ICE DAM LEAKS
BATH & KITCHEN DESIGN GUIDE
BEST CONSTRUCTION PRACTICES GUIDE
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BLOWER DOORS & AIR INFILTRATION
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BLOWER FAN OPERATION & TESTING
BOOKSTORE - INTERIORS
CHINESE DRYWALL HAZARDS
FIREPLACES & HEARTHS
FLOOD DAMAGE ASSESSMENT, SAFETY & CLEANUP
FREEZE-PROOF A BUILDING
HEAT LOSS in BUILDINGS
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HOUSEWRAP AIR & VAPOR BARRIERS
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HOUSEWRAP at SILLS, SOLES, TOP PLATES
HUMIDITY CONTROL TO PREVENT MOLD
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ROOF ICE DAM LEAKS
INDOOR AIR QUALITY & HOUSE TIGHTNESS
INDOOR AIR QUALITY IMPROVEMENT GUIDE
INSULATION IDENTIFICATION GUIDE
INSULATION R-Values & Properties
KITCHEN & BATH DESIGN GUIDE
KITCHEN VENTILATION DESIGN
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 GASES 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
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ZONE VALVES, HEATING
Bath vent fan installation, troubleshooting, repair: this article series 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.
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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.
Some signs of excessive, uncontrolled bathroom moisture include:
Bathroom vent fan duct installation & routing suggestions
Don't terminate your bathroom exhaust fan ductwork as we illustrate in the photographs just below.
Our photo at left and two additional examples seen at below left show two typical bath vent fans that spills directly into each building's attic - both are improper vent fan installations.
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.
Reader 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.
Flex duct routing 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.
Vent the bathroom exhaust 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.
Reader 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
Our article BATHROOM VENTILATION cites the importance of venting bath vent fans to the outdoors, not into an attic or crawl space.
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.
Bath exhaust fan duct length specifications and restrictions are discussed separately at BATHROOM VENT DUCT LENGTHS.
Insulate the bathroom exhaust vent 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.
We 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.
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.
Bath vent fan duct slope: Bath fan vent ducting in most installations is sloped gently back towards the bathroom. Especially where flex-duct is used, it is important to to avoid a low spot that collects condensation moisture in the attic.
Not only does such accumulation risk leaks into the ceilings (mold hazards) but in freezing climates water accumulating in duct work can freeze, accumulate further, block the duct, and when temperatures rise, cause extensive leakage back into the building.
We like to slope the bath fan vent duct downwards towards its building exit in non-freezing climates or where insulation protects against freezing. 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 in this article.
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.
Don't just blow the bath (or kitchen) vent exhaust into an attic, nor, even more crazy, into a ceiling or wall cavity or into a crawl space.
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).
Venting bath fans through up the roof surface?: While we agree that building exhaust ventilation is most powerful and thus effective when it is routed vertically, we prefer to 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.
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.
Our photo (left) shows a typical attempt at venting a bath into or actually just below a ridge vent - this direction of vent exit may seem convenient but we don't like it much.
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.
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.
At DRYER VENTING we discuss the horrible crawl space shown at left. Below this home the clothes dryer, bath vent fans, and even a dishwasher drain all were dumping into a soaking wet crawl space below the building.
The mold contamination, mud, and stink were horrible.
Soaking a crawl space or attic also wets the insulation, ending its effectiveness and asking for more mold contamination (Mold in Fiberglass Insulation.)
While we don't want a (lint collecting) screen over a clothes dryer vent termination (that's a fire hazard) we do screen bath or kitchen exhaust vent terminations to keep out birds, bats, and rodents.
Our photo (left) illustrates a self-closing bath vent termination in a soffit; when the bathroom vent fan is in operation this cover opens to exhaust moist air; the cover closes by gravity or a light-weight spring when the blower fan is not operating, thus avoiding any cold air back-drafts into the building.
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 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.
Typically the bathroom vent fan 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.
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.
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Frequently Asked Questions (FAQs)
Reader Question: is it OK to vent a bath vent fan straight-up, vertically out through the roof? Is it ok to vent the bath vent fan through a larger duct size than the fan's outlet diameter?
I am going to install a new bath fan, I am having a new roof put on the house and decided now would be a good time to put the vent on the roof. My question is I got a vent for 6" ducting, I will need a reducer at the fan end to 4" Would this be a good size duct for the fan.? Also I an using metal ducting and it's about six feet from the fan to the roof, Should I angle the duct a little or would it be ok to go straight up.? D.K. 10/19/2013
You've raised several key topics, and your question helps us realize where we need to work on making our text more clear or more complete. A competent onsite inspection by an expert usually finds additional clues that would permit a more accurate, complete, and authoritative answer than we can give by email alone. For example on site I might notice something about your attic and roof structure, ease of routing venting, placement of insulation, and even very basic stuff like - where the heck is your home? Bath ventilation worries may be a bit different in a cold climate than in a warm dry one and different again in a wet humid climate. That said I offer these comments:
Local Climate Affects Good Bath Vent Fan Designs: freezing vs hot and humid
For freezing climates we don't want to risk ice accumulation in the vent system - ice can collect from freezing condensate that arrives inside the bath vent duct during hot steamy showers;
For hot humid climates we don't want to have condensate accumulation in air conditioning systems and A/C ductwork, but a bath vent run through a hot attic is less likely to raise that same issue.
Bath vent routing vertically up through roof - not my first choice
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:
The vertical 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.
Bath vent diameters & vent duct materials
The vent fan manufacturers installation instructions typically give maximum run lengths and recommended vent diameters for their products; long vent runs and vents that use plastic dryer-type flex-duct (not your case) cut the effectiveness of the fan by adding airflow resistance and thus increase the risk of accumulated moisture too. Metal duct work (your case) is in my opinion always a better installation: smooth interior means better airflow. Metal fan vent ducting also reduces the risk of duct crush or collapse.
I am guessing that for a very short bath vent duct run, going to a larger duct size is fine - it'd make no difference but you're probably not gaining a thing on a short run by using a 6-inch duct to vent a fan that expects to vent through a 4-inch duct.
Bath vent fan capacity
In my experience inspecting and troubleshooting buildings, I've seen many bath vent fans that seemed ineffective. A fan that nobody uses because it's too noisy means a bathroom that is rarely vented adequately (risking mold, smells, even wet insulation). A fan that is under-powered means even if the fan is used it doesn't do anything.
The fan capacity you need depends on the size of the bathroom being vented - usually calculated in cubic feet. That figure is matched against the fan manufacturer's recommendations for fan capacity measured in cubic feet per minute (CFM). The CFM rating of the fan in turn presumes that the vent routing, diameter, length, and number of obstructing turns and bends is within the company's specifications. In the article above we explain how to calculate the required bathroom vent fan capacity. 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
Sorry that these notes are a bit long on arm-waving and short on more specific details, but as we've got no information about your particular installation except what's in your original note, I have to stop here.
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