In addition to new construction and remodeling, our company offers home energy assessments and any related energy-performance upgrades that may be needed. So we spend lots of time in attics fixing air-sealing and insulation problems, as well as addressing issues with bathroom fans. We often find that fans are vented directly into the attic area, or that the ductwork is poorly installed.
When hot, moist air is dumped into a vented attic in the winter, the water vapor generally condenses on the roof sheathing, starts mold growth, and eventually leads to structural damage. We also find that even when bathroom fans are exhausted to the outside, they're usually vented with flex duct, which is very restrictive to airflow.
A Common Complaint
The clients on one of my current remodeling jobs pointed out mold growing on the ceilings of their two bathrooms. The master bath was 110 square feet and had a sink, toilet, shower, and whirlpool tub. The shared bath was 40 square feet and had a sink, toilet, and shower. There was an odd-shaped patch of mold in the middle of the shared bath's ceiling, and mold and mildew around the edges. Both bathrooms had 70 cfm fans.
Basic fan sizing. The first instinct of many homeowners is to assume that poor bathroom ventilation is caused by an undersized fan. Often this is not the case.
Properly sizing mechanical ventilation systems can get complicated fast. (The process is outlined in ASHRAE standard 62.2, "Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings," which is a good resource.) But there are some rules of thumb that work pretty well for most residential bathrooms. For example, a guideline published by the Home Ventilating Institute (hvi.org) says that for bathrooms up to 100 square feet, you should provide at least one cfm for every square foot of floor space; above 100 square feet, HVI advises adding the requirements for each fixture, with toilets, showers, and bathtubs requiring 50 cfm each and jetted tubs 100 cfm. Another common recommendation is to ensure eight air changes per hour for bathroom ventilation; for normal 8-foot ceiling heights, this works out pretty close to one cfm per square foot.
Of the two fans in this house, one seemed to be adequately sized, and the other too small. Yet both bathrooms had mold problems. Because I do energy assessments, I have a blower door and a couple of manometers, which measure air pressure. A manometer used in conjunction with an exhaust-fan flow hood can measure the air pressure at the bath fans to within 10 percent accuracy.
The flow hood (the Exhaust Fan Flow Meter, energyconservatory.com) is basically a plastic pan you place around the fan (or register); it has weatherstripping on the edges to make a tight seal and a sliding door that adjusts to accommodate various fan sizes. With the fan running, the manometer measures the air pressure inside the hood with respect to pressure in the bathroom, from which you can calculate the airflow between the two areas of pressure. I found that the master-bath fan was drawing 41 cfm and the shared-bath fan 43 cfm.
If you don't have a manometer, there's a simple test you can do: Turn on the fan and see if it can hold a single sheet of dry toilet tissue against the grill. If so, the fan is probably drawing at least 40 cfm.
On climbing into the attic, I discovered that the two fans were connected to the same roof jack via a tee with no dampers in it - the master fan through a run of flex duct more than 23 feet long, the shared-bath fan through 26 feet of flex duct. It looked like the builder had tried to minimize the number of roof penetrations and to conceal the vent on the back of the house.
There were also long runs of unsupported duct - places where condensed moisture could settle in the low spots. I've seen cases where these puddles freeze into solid blocks during the winter and block the airflow. (Also, in rare cases, standing water can spawn Legionnaires' Disease, so it's a good idea to be careful and wear a respirator when removing flex duct.)
Where the flex duct exited the fans, the fiberglass batt insulation had been loosely stuffed over the top. This made it easy for cold attic air to get underneath the batts, creating a cold ceiling where bathroom moisture condensed and caused mold to grow. It also explained the oblong patch of mold in the center of the shared bath ceiling.
Basic duct design. To precisely size ducts, you have to figure out the air resistance, or static pressure, of the pipe and fittings, which involves either a lot of math or a static-pressure calculator (there's one at the HVI website). Fortunately, for the typical residential job, this is not necessary. I try to keep duct runs and fittings to a minimum, and when in doubt, I can always contact the technical support line of the fan manufacturer.
I prefer smooth metal duct, which has less friction loss than either metal or plastic flex duct. The most important point to remember is that fittings create a lot of static pressure; for example, in one equivalent-length chart, a 90-degree elbow is counted as 15 feet of straight duct and a wall termination jack as 30 feet.
Because they are approximations, published equivalent-length numbers vary, as do recommendations for total duct length. In fact, manufacturers test their fans to a predetermined static pressure, not a specified length of duct. So it's always a good idea to check with the fan manufacturer if you think the duct run is getting too long. My rule of thumb is that if I use more than a straight 10-foot section of rigid duct, the actual flow rate of the fan will be less than the rated value.