Yours Is Probably Installed Incorrectly

A combination bath vent/light/heater rated for 110 CFM.

Indoor air quality is an especially important consideration during the colder months when windows are closed and we're bundled up in our Snuggies on the sofa. In newer construction especially, a good, tight envelope means that little air is exchanged from indoors to outdoors, resulting in concentrations of indoor air pollutants.

One room in the house, however, can create problems for you year-round, and that is the bathroom. Inadequately ventilated bathrooms, depending on why they are inadequately ventilated, can reduce the effectiveness of your attic insulation, encourage the growth of mold and mildew both in your bathroom and in your attic, be far noisier than necessary, and, of course, keep you from being able to use the mirror in your bathroom unless you blast it with your hair dryer.

Any of this sound familiar? There's a good reason.

Many local building codes specify bathroom ventilation practices that are flat-out wrong. Even books you pick up to help you know how to do-it-yourself are wrong. So, if you've installed your own, and it looks like the illustration here, don't blame yourself. But you'll probably want to fix it.

The illustration on the right is a perfect example of "standard practice." Its even a step above the building codes for this area in the 1970's-- those codes said it was perfectly OK to just hang that flexible plastic stuff from the attic joists and put the end of the hose up to the gable end of the house and call it good.

First, any venting hose or ductwork needs to be installed to an exterior surface that does not share air exchange with the attic. Through an exterior wall (as shown above) or, preferably, through the roof installations are how you should vent all that hot, wet air out of your home. You don't want that air immediately sucked back into the attic as can happen with a stick-the-hose-up-to-the-gable-end installation. If you cannot locate a vent hood outside your home in proximity to your bathroom, you have Work To Do.

[update: The United States has finally adopted a national building code by signing on to the International Code Council. Bathroom ventilation is required to be vented outside and not recirculated. Putting the pipe to a gable vent or soffit would allow for easy recirculation; hence, this is not allowed by the new National Code. In the Resources I have included a new link which provides guidance on best "green" building practices.]

Attic insulation subjected to moisture from a bathroom vent is less effective, causing your home to use more energy to heat/cool. A bathroom vent that is having a hard time pushing air out of your bathroom because of incorrect ducting also uses more energy than necessary.

This is where "standard practice" is frequently ill-advised and wrong. The installation manual for our bathroom fan specifies rigid pipe -- either PVC or rigid metal duct. Seams are to be either glued (PVC) or taped with foil tape (not duct tape). The reason it specifies rigid pipe is because to reach the rated extraction of air--in this case 110 cubic feet per minute (CFM)--the air must be free to flow. Any restrictions on that flow, such as interior "ribs" from the flexible stuff you see above or on flexible metal hose/duct cause the fan to work harder even while less air is making it out of your bathroom. Our particular fan recommends a run of 2 to 3 feet straight out of the exhausting unit with a shallow-angle (not 90°) bend and then up out the roof. Flexible hoses that are allowed to dip or weave their way out of the attic create additional restrictions to air flow. And yet, flexible hoses are frequently "OK" with local building codes.

Does it really make a difference? OH, YEAH.

We installed the bath exhaust you see at the top of this post a little over a year ago. It was rated to exhaust 110 CFM, 40 CFM more than the fan it was replacing, which just couldn't seem to keep the moisture down in the bathroom. Condensation would build up on the walls and occasionally even run down them, if enough people showered in a short enough window of time. The new fan reduced that condensation, but not by a great enough amount. (The mirror still fogged!) I knew that we had ductwork hung at the gable end of the house, because you can see the end of the hose, if you look hard enough. So we finally "upgraded" to ductwork (rigid metal in our case) that met the specifications of the fan's manufacturer.

The condensation issues were over and done the next day, which is terrific for


indoor air quality. It might be even better for yours, if you live in a radon-prone area and use well water. (Radon is a known carcinogen.) Municipal water systems tend to have water stored for a period before use, allowing some decay of the radon prior to use. (They may also employ aeration or filtering to remove radon.) Well water, however, doesn't receive this protection, since it is an "on-demand" system. Wells tapping crystalline rock aquifers are at much higher risk for radon contamination than sand or sedimentary rock aquifers. Radon degasses during water use, especially when hot water is used. So dishwashing, clothes-washing and showering all increase our exposure to radon.  Showering is definitely the highest-risk activity, however, especially if your bathroom is not well ventilated.

One more note on getting good airflow out of the bathroom during showering--it doesn't pay to have a door that is super tight. A gap at the bottom of the bathroom door is necessary to ensure that there is air to replace that which is being sucked out of the bathroom. Otherwise, it's just like when your vacuum cleaner hose is stuck flat to the floor--high-pitched engine whine! Inability to suck!

Send this one to your friends--radon is responsible for lung cancer in many, many people--about 21,000 people a year die as a result of radon-induced lung cancer.


Duke University


Florida Dept. of Health


CA Green Building Codes, 4.506.1

Leeks, Squash and Necessity

Leeks, Squash and Necessity