Over the coming months, I plan to address several aspects of HVAC. Today, I want to focus on the “V” in HVAC, ventilation, specifically, bathroom ventilation in existing homes. Many homeowners have come to recognize the importance of bathroom ventilation, for obvious reasons. But what many people don’t realize is that beyond disagreeable odors, there are also significant health risks associated with improper bathroom ventilation.
Showers and Steam
Here’s a very common scenario. You step out of a nice hot shower and you have to wipe off your bathroom mirror to see yourself because of all the steam. You chalk it up as a minor inconvenience. But consider this: if your bathroom mirror is fogging up, where else is all the excess moisture from your shower going?
The answer, of course, is that it accumulates behind the walls and tile work as well as your tub or shower, not to mention underneath your bathroom floor, with potentially bad results. Think mildew and mold. And while a lot of people really like all the steam that results from a hot shower, mold spores are an entirely different story.
The good news is that you don’t have to give up hot showers to protect your family from the health risks associated with mildew and mold. Instead, install adequate fans in your bathrooms – and keep them running long enough to achieve proper ventilation.
Proper Ventilating Equipment
Twenty years ago, building codes only required bathrooms to have a window or a fan. There was no consideration for getting rid of indoor pollutants, never mind excess moisture. Now there is more information and standards are more stringent. Specifically, the Home Ventilating Institute has established standards for bathroom fans according to size and setup.
For a powder room with toilet and sink, the HVI standard for a bathroom fan is 50 CFM per square foot. The HVI standard for a full bathroom with toilet, sink and shower or bathtub is 100 CFM per square foot, while HVI guidelines for a bathroom with a Jacuzzi or similar larger tub are 150 CFM. However, prior to us getting into Home Performance 12 years ago, we’ve never seen bathroom fans that adhere to these recognized standards.
Another consideration is that bathroom fans are rated based on factory conditions that often don’t exist in actual homes. Restrictions in ductwork in particular may require sizing up to a more powerful fan or purchasing a fan with adjustable controls. For example, ductwork that runs through many twists and turns may reduce the effective output of an 80 CFM rated fan to 50 CFM. As a result, you may need to size up to an 80 CFM rated fan in a room that would normally need a 50 CFM rated fan.
Let the Fan Run
But having the right fan won’t completely solve the problem. Many people only run their bathroom fans while they’re actually in the bathroom – averaging between 10 minutes and half an hour. In reality, you should allow the fan to run for at least an hour after a shower to ensure that excess moisture is vented out of the room.
You may resist the idea of running your bathroom fan for that long because of the noise. But today’s fans are much quieter than older fans. For an extra $50 or $150 you can purchase a quiet bathroom fan with adjustable CFM that can run as long as it needs to run to get the job done. And really, $50 or $150 isn’t a lot to spend when you’re considering your family’s health.
Ventilating an Interior Powder Room
Some homes feature powder rooms near the main living areas, which is really convenient for entertaining or for guests. You may assume that a powder room isn’t an option for your home due to the need for getting fan exhaust to the outside wall. But ventilating interior powder rooms isn’t really that difficult. In such cases, your home’s joists are your friends. Many joists have about 14 inches of open space, which is plenty of space to accommodate 4-inch round ductwork for a bathroom fan, unless you have plumbing pipes or recessed lights already installed in that space. After verifying, simply run the ductwork to the back or the side of the house and drill a hole in the exterior wall. Just be sure the hole is in the correct bay!
If you have to run ductwork through an unconditioned space, such as an attic or crawlspace, you’ll need to insulate the ductwork in that unconditioned space. For instance, the air temperature in an attic can run as high as 140 degrees Fahrenheit. Running uninsulated ductwork through that attic effectively pulls 75 degree conditioned air into that space, and the contrast in temperature creates condensation, which creates mold.
If you’re not confident about installing ductwork for a bathroom fan, you really should call in professionals to do the job, especially when you’re dealing with an older home. Most homes have drywall, which is fairly easy to replace, but older homes often have plaster, which is much more of a challenge.
In later articles, I’ll address other ventilation aspects that can affect your health and indoor comfort. In the meantime, remember to allow your bathroom fans to run after taking those hot showers. Your family’s health depends on it.
Latest posts by Rob Minnick (see all)
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