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Where to install bath fans to improve air quality in home
- Ken Nelson
A common question I'm asked is this: "I've purchased a continuous run bath fan for my two-story home, should I install it upstairs or down? "
The answer is yes. Either location will immediately improve air quality in your house. Depending on where you live and number of occupants in the home, one location might be better than the other.
But make no mistake, just adding continuous ventilation on either floor will immediately improve your Indoor Air Quality (IAQ). Installing the fan downstairs will help to eliminate many of the IAQ issues that come through the front door or up through the crawlspace.
Allergens, dust, pet (dander), radon and general front/back door traffic will draw organic and airborne particles into your house. The low pressure zone created by continuously moving air out of the house near the source of openings will give the entire house relief from these transient particles.
Installing a continuous run fan upstairs will address moisture in the house. Moisture is introduced through showering, laundry, dish washers, and general human transpiration and perspiration as well as outside air that migrates in. Additionally, the larger the population in the house, the more potential there is for moisture in the air.
Moisture will always be more prevalent upstairs than down. While you would think water would be heavier than air and tending to migrate towards the lower floor, the opposite is actually true. Moist air is lighter than dry air.
How so you ask? Pull out your handy copy of the Periodic Table of Elements. The air we breathe is mainly nitrogen (73 percent) and oxygen (21 percent); water vapor is hydrogen and oxygen (h2o). Increasing humidity means proportionally greater amounts of hydrogen than nitrogen in the air. Hydrogen (#1 on the table) is a lighter element than nitrogen (#7 on the table), which is why water vapor rises.
Additionally, as air temperature increases its ability to hold water increases. In fact, at 100 degrees, air has ten times (10x) the capacity to hold water as it does at 30 degrees. The challenge in our homes is that this warm moist air is going to go somewhere. One of the principles of physics is that "heat goes to cold". Meaning moisture is literally going to be drawn to any temperature difference in walls, ceilings, closets, or drawers. Like a magnet these cooler objects will attract the moisture which could then condense or turn back to water, providing the perfect environment for mold and mildew to thrive.
By installing the continuous run fan upstairs, you'll provide a pathway for this moisture to be evacuated from your house before it ever has a chance to land. So either location is good, both would be best.
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Also, ceiling mounted bathroom room fans should never be run unless it is during the summer while the shower is being used. The last thing we want to do is to create a negative pressure zone at ceiling level while flushing a toilet. All of the contaminated air that spills out of the toilet are further dispersed throughout the bathroom as the contaminant ladin air makes it's way to the negative zone created by the exhaust fan.
Utilizing an ERV/HRV with vents mounted low on the wall or in the floor, behind the toilets, is the best way to remove contaminated air from toilets while at the same time bringing in fresh outside air to prevent an overall negative pressure in the house which will bring in air from sources we are not controlling.