Older homes often lack bathroom exhaust fans. In the old days, if the bathroom was smelly or steamy, you were supposed to open a window to air it out.
This isn’t a very logical ventilation method, especially when temperatures are below zero, or when the weather is 90°F and humid. Yet this time-honored method of bathroom ventilation is still enshrined in our building codes. According to the 2009 International Residential Code (sections R303.3 and M1507.3), a bathroom with an operable window does not need to have a bath exhaust fan.
Why do we need exhaust fans?
In spite of the code’s archaic loophole, builders should install an exhaust fan in every bathroom or toilet room — even when the bathroom has a window.
A bath exhaust fan can perform several functions:
- It can exhaust smelly air, allowing fresher air to enter the bathroom.
- It can exhaust humid air, allowing dryer air to enter the bathroom.
- When operated for 24 hours per day or when controlled by a timer, it can act (in some cases) as the most important component of a whole-house ventilation system.
Designing an exhaust-only ventilation system is a topic unto itself, and is beyond the scope of this article. For more information on exhaust-only ventilation systems, see Designing a Good Ventilation System.
Where does the makeup air come from?
When the bathroom door is closed and the fan is operating, where is the makeup air coming from?
If the bathroom has an exterior wall, some of the makeup air is coming from the exterior — for example, through leaks around the window or baseboard.
Some of the makeup air is probably coming into the bathroom from other rooms in the house, via the crack between the bottom of the door and the flooring. Of course, if the bathroom fan is exhausting 50 cfm, then 50 cfm must be simultaneously entering the building. If some of the makeup air is entering the bathroom through the crack under the bathroom door, an equivalent volume of exterior air must be entering other rooms of the house through a variety of random cracks in the home’s envelope.
Unfortunately, many exhaust fans pull some of their makeup air through a nearby crack: namely, the crack between the housing of the fan and the ceiling drywall. You don’t really want attic air to be entering the bathroom through this route, so fan installers need to remember to seal the drywall crack around the fan.
What happens if homeowners don’t operate their fan?
Homeowners sometimes worry that if they fail to operate their exhaust fan, the bathroom ceiling will stay damp and every surface in the room will soon grow mold. That’s not necessarily the case; in fact, some bathrooms without exhaust fans stay dry and mold-free for years.
What factors affect mold growth?
- A small bathroom is more at risk for mold growth than a large bathroom.
- A bathroom that includes a shower that is used frequently is more at risk than a bathroom with a rarely used shower.
- Bathrooms with airtight walls and ceilings are more at risk than bathrooms with lots of infiltration and exfiltration.
- In cold climates, bathrooms with poorly insulated surfaces are more at risk than bathrooms with well-insulated surfaces.
All building inspectors encounter moldy bathrooms at some point. The extreme case is a small bathroom used daily by many members of a large family. In many cases, the main factor leading to mold growth is missing insulation above the ceiling. During the winter, cold drywall stays damp. That’s why ceiling mold often occurs near exterior walls, where insulation tends to be thin. Repairing insulation defects helps prevent mold: the insulation keeps the drywall warm, reducing opportunities for condensation or moisture absorption.
Needless to say, even bathrooms that aren’t at high risk for mold growth still need a fan.
Testing an exhaust fan
Just because a fan makes a comforting noise (or an irritating whirr), doesn’t mean it’s moving any air. Many such fans are totally ineffective.
The classic home inspector’s method for testing a bath fan is the toilet-paper test: Remove one square of toilet paper from the handy roll nearby; then turn on the fan, and see if the fan is strong enough to hold the square of toilet paper tight to the ceiling grille.
In some cases, the toilet paper falls on the floor. With a little bit of investigation, you should be able to figure out whether the duct in the attic is crushed or whether the termination is filled by a bird’s nest.
If you are installing a bathroom exhaust fan in a new home, it’s important to get an accurate measurement of the fan’s air flow rate. This is a requirement for most high-performance homes programs, including the Energy Star Home program. To measure the fan’s air flow rate, you’ll need measuring equipment — for example, the Energy Conservatory’s . For more hints on bathroom fan commissioning, see Is Your Ventilation System Working?
Achieving 50 cfm
According to most building codes, a bathroom without an operable window must have an exhaust fan with a minimum ventilation rate of 50 cfm (assuming intermittent operation) or 20 cfm (assuming continuous operation).
In the past, many builders and code officials interpreted this to mean that the fan should be rated at 50 cfm. There’s a problem with this approach, however: once a duct system is installed, a 50 cfm fan might only be moving 25 cfm. (The reduction in airflow rate is due to the static pressure of the duct system.)
Most green building programs now require that the airflow rate through a bath exhaust system be verified to determine that the fan and its associated ductwork are moving at least 50 cfm. (This rate of airflow is usually plenty, by the way. You don’t really need a big honking fan in your bathroom.)
Some green builders have been surprised to learn that their 110 cfm fans are failing the 50 cfm performance test. In other words, they didn’t realize that the static pressure of a duct system can seriously affect airflow rates. What’s going on? All of the usual culprits are implicated:
- The use of flex duct instead of smooth-walled duct.
- The use of ductwork with too small a diameter.
- Too many elbows in the ductwork.
- Sloppy installation practices leading to convoluted ductwork.
In a 2013 blog, Allison Bailes told the story of a developer who installed bath fans rated at 110 cfm in several new Energy Star homes. The developer assumed that a 110-cfm fan would be powerful enough to overcome sloppy duct installation and still pass the 50 cfm test. That approach didn’t work very well, since 4 of the 9 fans flunked the test.
Let’s say that your fan is hooked up to a funky duct system, and it’s only pulling 35 cfm. There are two solutions to this problem: you can swap out the 110 cfm fan for a more powerful model — say, a 200 cfm fan — or you can fix all the duct problems. (A good duct system has a large diameter, smooth walls, and few elbows.) Either approach will work, but the latter approach is preferable — because the system will use less energy and will be quieter.
Fan makers have begun to respond to reports that builders are failing the 50 cfm airflow test by offering more powerful fans. For example, ads for Panasonic’s EcoVent fan boast that the fan includes a booster switch that a builder can flip to ramp up the fan’s speed if the fan fails its airflow test. While that solution is easy for the builder, a better solution would be to fix the funky ductwork.
Static pressure and fan ratings
Every duct system has static pressure that limits air flow. Most exhaust manufacturers rate fan performance at two different static pressures: 0.1 inches of water column (i.w.c.) and 0.25 i.w.c. Since the installed duct system for a bath fan is likely to have at least 0.25 i.w.c. of static pressure, you should ignore the unrealistic numbers in the 0.1 column when you’re looking up fan ratings.
Some fans have lower airflow rates (in cfm) at 0.25 i.w.c. than at 0.1 i.w.c., as one might expect. Other (more sophisticated) fans — those with electronically commutated motors — have airflow ratings that are roughly equivalent at the different static pressures. These ratings are counterintuitive. Here’s the trick: these models automatically ramp up the fan in response to duct systems with a high static pressure. At higher static pressure, these fans are more effective than less sophisticated fans, but they also use more power (in watts) to achieve the full airflow they provide.
Ideally, you want to keep your maximum duct length to 10 feet or less — 20 feet in a pinch — with no more than three elbows. (Each elbow has the equivalent resistance of 15 feet of smooth metal duct, so you don’t want any extra elbows.)
Smooth duct is preferable to flex duct. If you decide to use flex duct instead of smooth-wall duct, the static pressure of your duct system will increase.
If your duct system is especially long or convoluted, or you ignore the advice in this article and install flex duct instead of smooth-wall duct, you will need to specify a more powerful fan to account for the high static pressure of your duct system. (For more information on this issue, see .)
Choosing an exhaust fan
Perhaps your exhaust fan is noisy. Perhaps it failed the toilet paper test. You’ve decided that it’s time to buy a new exhaust fan.
A good exhaust fan:
- Is properly sized: neither too wimpy nor too powerful.
- Is energy-efficient.
- Is quiet, since a quiet fan is more likely to be used than a noisy fan.
How many sones?
Fan noisiness is measured in sones. The lower the sone rating, the quieter the fan.
Higher static pressure makes fans noisier, so exhaust fan manufacturers list sone ratings at 0.1 and 0.25 i.w.c. For a realistic sone rating, look at the rating at 0.25 i.w.c. It shouldn’t be hard to find a Panasonic or Broan-Nutone fan rated at less than 1 sone at 0.25 i.w.c.
If you want an efficient fan, look for an Energy Star model. Qualifying fans are divided into two categories.
Fans rated at 89 cfm or less must have a minimum efficiency of 1.4 cfm per watt when tested according to the HVI 916 test procedure.
Fans rated at 90 cfm or more must have a minimum efficiency of 2.8 cfm per watt.
The Energy Star program also establishes maximum sone ratings. Fans rated at 139 cfm or less must have a maximum sone rating of 2.0 sones when tested according to the HVI 915 test procedure. Fans rated at 140 cfm or more must have a maximum sone rating of 3.0 sones.
Anyone interested in researching specifications for bathroom exhaust fans should probably consult the data published in the .
Installing an exhaust fan
It’s important to follow the installation instructions provided by the manufacturer of your bathroom exhaust fan. In addition to following those instructions, remember these principles:
- Verify that the bathroom door has enough of an undercut to allow air to enter the bathroom when the fan is running.
- Remember to seal the crack between the fan housing and the ceiling drywall with caulk.
- Verify that the backdraft damper in the fan operates smoothly, and hasn’t been taped shut at the factory.
- Plan ahead to make sure that duct runs are as short as possible.
- Install the fan so that the duct outlet is aimed in right the direction — toward the planned exterior termination. If it’s pointed the wrong way, you’ll have to start out the duct run with two 90° elbows, and that’s a bummer.
- In many cases, it’s a good idea to install ducts with a larger diameter than the duct outlet on the fan. If the duct is longer than usual, use 6-inch duct instead of 4-inch duct.
- Use galvanized duct or thin-wall PVC pipe. Joints in galvanized duct should be secured with sheet-metal screws and sealed with HVAC tape. To reduce noise transmission and simplify installation, many installers use a short (generally 2 feet or less) length of flex duct between the fan and the rigid ductwork.
- In a cold climate, attic ducts must be installed in a way that limits condensation. A duct that will terminate at a gable wall should first rise to an elbow that is high enough above the fan to allow the installation of a long run of horizontal ductwork that slopes slightly toward the exterior. (This encourages any condensation to find its way out of the house instead of dripping from your ceiling.) Wrapping the ducts with vinyl-faced fiberglass duct insulation reduces condensation.
- Support ducts every 3 feet with hangers or strapping to prevent sagging.
- Verify that the louvers (if any) on the wall termination or roof termination are operating smoothly.
If possible, exhaust fan ducts should terminate at a gable wall. Roof terminations can work, but you’ll usually encounter fewer problems if you put a hole in your gable wall than if you put a hole in your roof.
In no case should a duct be terminated in an attic or at a soffit. (Soffit terminations grow icicles during the winter, and allow humid air to be sucked into the attic in all seasons.)
How should an exhaust fan be controlled?
Operating a bath exhaust fan exacts an energy penalty:
- It takes electricity to run the fan.
- The fan pulls exterior air into your home through cracks, and this exterior air needs to be heated during the winter and cooled during the summer.
Because of the energy penalty associated with fan operation, fans should be no more powerful than necessary, and should be operated only as long as required.
The simplest way to control a bath fan is to wire the fan to come on with the bathroom light. This method works, sort of, but isn’t ideal. It’s better if the bath fan runs for 5 or 10 minutes after every shower.
This can be accomplished with a “delayed off” switch (for example, the Lutron Maestro) that keeps a fan running for a set amount of time after the switch is turned off. The delay is user-adjustable.
It’s also possible to install a humidity-sensing switch that turns a fan on whenever the indoor relative humidity reaches a preset level. This type of switch usually includes an override switch allowing the fan to be turned on regardless of the humidity level. These switches can be irritating; they often require seasonal adjustments, since normal indoor humidity levels are often higher during the summer than they are during the winter.
Some fans — for example, Panasonic’s WhisperSense fan — include sophisticated controls that incorporate motion sensors, “delayed off” features, and humidity controls. For more information on the WhisperSense fan, see .
Operating your fan
Some experts have proposed rules for operating bathroom exhaust fans. Every family is different, however, so it’s hard to establish rules that apply to everyone. There’s no reason to run a bath fan for 15 minutes every time someone uses the bathroom.
If your bathroom seems damp, you probably want to run the bath fan more often, or run it longer after every shower.
If your bathroom seems dry and pleasant, you may not need to operate your fan as much as you are now.
Martin Holladay’s previous blog: “A Canadian Editor Questions Passivhaus Dogma.”