If poor ventilation is turning your bathroom into a Petri dish, it’s time to call in an HVAC pro to install a fan. There are two kinds to choose from: bath fans and remote fans, also known as multi-port ventilators.
So, how does each type work? Bath fans are installed directly in the bathroom and discharge moisture to the exterior via a duct, while multi-port ventilators are mounted elsewhere in the building, such as the attic, and use ducts to exhaust air from one or more bathrooms at the same time.
Wondering when it makes sense to go with a remote fan? In addition to being able to handle larger bathrooms, one popular advantage of a multi-port ventilator is that it’s extremely quiet. Since the fan is mounted in a remote location, very little sound is transmitted to the bathroom.
With either type of fan, you can also add efficiency by installing a timer circuit and occupancy sensor to ensure that the vent works when it should and for the right amount of time to properly wick moisture away. After a hot bath or shower has concluded, the vent should be running an extra 15 or 20 minutes; with the help of a timer, the job is done and you don’t have to worry about going back to flip a switch yourself.
Vent outdoors, not into the attic or house
Besides choice of fan, the other major factor is making sure that the fan is vented correctly and free of obstructions such as insulation. Efficient bathroom ventilators must vent to the outdoors, sending moisture back into the environment.
However, a lot of homes were built with the bathroom venting into the attic or back into the house, which only transfers the moisture and mold problem to another spot in the home.
When you dump humidity and moisture into an attic space, a couple of things happen. First of all, insulation becomes very ineffective. If you add just two percent moisture to insulation, it loses its effectiveness (R Value) by a third. So, for example, if you have R19 and put moisture into that, that leaves you with an R13 or less.
If you want to know if a fan vents out, look for the vent flapper (a.k.a. the damper) on the outside of the wall and see if it opens when you turn the vent on. Sometimes you’ll find that a builder has put in the bath fans and installed exhaust ports on the exterior, but never connected the two.
Make sure the fan can handle the bathroom size
In addition to choosing which type of fan is right for your situation and making sure it is properly vented, a third essential concern is getting a fan that’s powerful enough for the size of your bathroom.
For bathrooms up to 100 square feet in area, the Home Ventilation Institute (HVI) recommends that an exhaust fan provide 1 CFM per square foot 
(approximately eight air changes per hour) to properly ventilate the bathroom. For example, if the bathroom is 8 feet by 5 feet (with 8-foot ceilings), your bathroom area is 40 square feet. At 1 CFM per square foot, the minimum recommendation is a fan rated at 40 CFM.
For bathrooms above 100 square feet in area, HVI recommends a ventilation rate based on the number and type of fixtures present. In this calculation, figure 50 CFM each for the toilet, shower and bathtub, and 100 CFM for a jetted tub. To accommodate the total CFM demand in a bathroom of this size, you have two options.
Your first option is to install three 50 CFM fans: one over the tub, one in the shower and one in the water closet. This method is very effective, providing targeted ventilation when and where you need it.
Your second option is to install a single 150 CFM fan. The air will then be pulled through the entire room and exhausted at a central location.
For more information on bathroom ventilation, visit www.hvi.org.
What bathroom vent fan do you recommend? I heard it on your radio show but I failed to write it down.
I’d recommend you consider the Broan-Nutone line. Lots of options and some have a simplified “5 minute” installation.