In many houses, bathroom ventilation doesn’t present problems. A fan or fan/light combination is installed in the bathroom ceiling, then vented through the roof or through a soffit or side wall.
With the flip of a switch, odors and excess moisture are easily vented outside. But what happens when a building’s design makes it difficult or impossible to vent a bath fan to the exterior?
As it turns out, there are many situations that can make it difficult to install a bathroom fan that vents to the exterior. Solving this problem calls for some construction creativity, smart product selection, and the installation skills of an experienced HVAC contractor.
Dealing with Bathroom Ventilation and Building Code
Before delving into solutions to difficult bathroom ventilation problems, it’s helpful to understand some basic history and code requirements. Even before indoor plumbing, folks understood that outhouses needed ventilation.
When bathrooms moved indoors, ventilation was required not just to remove odors, but also to exhaust excess moisture. We all know how much moisture can be produced by taking a hot shower—just think about the fogged mirrors and the condensation that forms on windows and walls, especially when it’s cold outside.
Today, the building code that applies in most municipalities calls for bathrooms to be vented by means of an exhaust fan or an operable window. The window ventilation option is a minimum standard that can’t be viewed as effective or reliable. Depending on someone to open a window to vent excess moisture is a dubious proposition—especially in cold weather. A vent fan will always exhaust moisture more effectively.
Signs of Bathroom Ventilation Problems
Houses built (and renovated) today are more airtight, and more highly insulated than those built in the past. The mandate to “build tight and insulate right” provides some important benefits: a higher level of interior comfort (especially during temperature extremes), savings on fuel and electricity, and lower levels of carbon emissions.
But tight construction creates a much greater potential for indoor air pollution. Mold spores are among the most hazardous indoor air pollutants, and mold is a direct result of excess moisture. Mold is a serious health issue, causing a wide range of respiratory ailments and allergic reactions. It will also damage and destroy common building materials like wood and gypsum board.
Mold in a bathroom can be caused by a plumbing leak, but splotchy stains on wall or ceiling surfaces usually indicate inadequate ventilation—too much moisture in bathroom air. This can also cause mold to form on wood or insulation in an attic space above a bathroom. Even if you can’t see mold, the unpleasant smell is a telltale sign of its presence.
While there are other reasons to solve bathroom ventilation problems—the inconvenience of foggy mirrors, for example— preventing mold is the most compelling reason by far.
Basic Guidelines for Bath Fans
Before delving into difficult bathroom ventilation problems and how to solve them, let’s go over some basic details that apply to bathroom fans. Understanding these elements will help you make the right decisions about bathroom ventilation, whether straightforward or complicated.
Buy the right fan
Bath fans are sized according to the volume of air they can move, measured in cubic feet per minute, or cfm. The rule of thumb is that you need 1 cfm for every square foot of floor area in your bathroom. It’s smart to err on the high side, however—especially in a bathroom that gets heavy use or one with a high ceiling. Better fans are engineered to run quieter than low-priced versions. Look for a sone rating of around 1, as opposed to 3 or 4.
Consider special features
A bath fan that includes a light will eliminate the need to install a separate ceiling fixture. A fan with variable speed control enables you to get extra exhaust power when you’re generating extra humidity—like when taking a long shower or using a jetted tub.
To ensure that the fan operates when it’s needed, you can buy a bath fan equipped with a humidistat; this feature enables you to have the fan turn on automatically when a certain humidity level is detected.
This is a good option in a rental apartment or whenever you’re not sure the fan will be used as it should be. The point here is that all fans are not created equal. Opting for special features can help you cope with challenging ventilation conditions.
Use metal or PVC for ductwork
Avoid using inexpensive flexible duct (“Flexduct”) to vent a bathroom fan. It can crush or tear easily, which will impede air flow. By fabricating bath fan duct runs from rigid metal or PVC plastic, you’ll get the best long-term performance from your fan.
Keep duct runs as short and straight as possible
This rule applies to any type of ductwork. As a duct run becomes longer and/or more convoluted with direction changes, the fan has to work harder to move air. If you can’t avoid a long run or elbows in the ductwork, make sure to upsize your fan.
Options for Fixing Bathroom Ventilation Problems
All of the options discussed below have been used to solve difficult bathroom ventilation problems. But these unconventional installations are best handled by an experienced HVAC contractor.
- Install a recirculating fan in a half-bath. This type of bath fan doesn’t exhaust air to the exterior. Instead, it passes bathroom air through a filter medium that removes odors. A recirculating bath fan would be a workable option for a half-bath because there’s no shower to fill the space with moisture.
- Reduce noise with an inline fan. As a duct run gets longer or includes more elbows for changing direction, it takes more blowing power to exhaust air to the exterior. More powerful fans also tend to produce more noise. But you can minimize the noise by installing an inline fan somewhere along the duct run that’s away from your living space.
- Vent under the floor to get outside. Let’s say you have a bathroom located away from an exterior wall and it’s not possible to vent the fan through the roof. This might be the case with a bath located under a stairway, or in a renovated building with high ceilings. Ask your HVAC contractor about fabricating a duct run that extends from a fan mounted in a bathroom wall, down through the floor, and (between floor joists) out to an exterior wall. Mount the fan high on the wall to better capture warm, moist air. Fabricate the duct run from rigid metal or PVC material. And install a more powerful fan to overcome natural convection and exhaust the air down and out.
- Build a soffit to hide ductwork. This option will allow you to hide the duct run for a bath fan inside a soffit built along an interior wall.
- Install commercial ductwork. This option might be suitable in a situation where a number of smaller apartments have been built in a larger building with high ceilings. Instead of fabricating a number of separate long duct runs that extend to the exterior, bathroom ductwork could feed into a single long run of large-diameter commercial ductwork. Leaving this industrial-scale ductwork visible can be a positive aesthetic feature, just as it is in some restaurants and retail stores.