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.
Ken Nelson is the Northwest Regional Sales Manager for the Panasonic Eco Products Division, specializing in ventilation solutions for residential and multi-family living environments. Over the past four years, Ken has spoken throughout the Northwest, teaching and training builders, building science advocates and professionals on the physics of moisture and air movement in homes of all sizes, types and age.