It’s a question a lot of people ask, especially those living in multi-story rooms. For example, should the whole-house dehumidifier be located on the first floor or the upper floors? And, if it’s the upper floors, which one specifically? For instance, should the dehumidifier be on the second or third floor if you live in a three-story house?
People with basements have the same dilemma. Do they place the dehumidifier in the basement or the upper floors? If you live in a single-story building with a basement, should you have the dehumidifier in the basement or the main floor?
Relative Humidity in a Multi-Story Building
Perhaps we should begin by understanding the differences in relative humidity on the different floors in a multi-story building.
Relative humidity in the home on a typical day is about 35% to 50%. Of course, the actual value depends on where you live, air velocity, and other factors such as actual temperature and pressure. The good news is that any value between 30% and 50% humidity is excellent for a healthy person. However, anything lower or higher than this range can begin to cause issues.
However, in multi-story buildings, the relative humidity isn’t evenly distributed. The upper floors are typically more humid.
The reason is simple – the stack effect. The stack effect states that warm air is lighter, thus always rises, and its place is taken up by denser, cold air. Since summers are hot, the air inside your home will naturally rise, and its place is occupied by colder air entering the house from the ground and outside. Typically, the second floor will be 1% to 5% more humid than the first floor.
The hot, humid air will rise through to the uppermost floors, often making the top-most floors the hottest parts in the home. The problem is worsened by the fact higher floors are more exposed to sunlight.
Why the Second Floor Could Be Even More Humid
Your upper floors can become even more humid under certain conditions, including poor insulation, leaky ductwork, and an oversized air conditioner.
We’ve already mentioned the stack effect. It’s most pronounced in poorly insulated homes. During the summer, hot, unconditioned air will enter the home through cracks and openings, which is instantly pushed up the house to the upper floors due to the stack effect. The heat can come from the air outside or even the ground surrounding the basement.
This problem isn’t so pronounced in homes with sound insulation because very little (hot) air enters the home from outside.
Many multi-story homes have a separate air conditioning system that explicitly serves the upper floors. For instance, a two-story building will likely have a dedicated air conditioning system for the second and first floors.
Leaky ductwork can pull excess humidity from your attic and into the AC system. Over time, the rate of moisture entry can exceed the AC’s moisture removal capacity, causing moisture buildup in the upper floor.
An Oversized Air Conditioner
An oversized air conditioner causes short-cycling, where the AC runs for shorter periods than usual. For example, whereas a normally-sized air conditioner runs for 10 minutes then shuts off for 10 minutes before resuming operation, a short-cycling unit may run for just five minutes before cycling off.
Unfortunately, short cycling is very bad for dehumidification. It doesn’t give the air conditioner’s dehumidifier enough time to remove all the moisture in your home. This, combined with the stack effect, often results in warmer, damper upper floors.
So, Where Should You Put the Dehumidifier?
The short answer is the second floor. As we’ve seen, the second floor is likely to be a damper. Furthermore, issues like poor insulation and leaky ducts are more likely to affect the second than the first.
However, this doesn’t mean that everyone should have their dehumidifier on the second floor. It may sound counterintuitive. However, it’s easier to digest when you consider that throughout, we find that the stack effect is a significant reason your upper floors are usually damper and hotter. It means that the moisture primarily comes from the lower floors.
Therefore, one might argue, and rightly so, that addressing moisture issues on the first floor would automatically improve relative humidity on the second floor.
So, what do you do? Experts recommend finding unique solutions to your humidity issues. Don’t follow what others do. For example, never place the dehumidifier on the first floor because your neighbor has done it, or have it on the second floor because your favorite blog says so. Instead, find what works for you.
Factors to Consider in Dehumidifier Placement
The following are several factors to help you determine whether to have the dehumidifier on the first or second floor and the exact position where the dehumidifier would serve you best.
Always place the dehumidifier on the floor with the worst moisture issues. Why? Because dehumidifiers are designed to remove moisture, they’re most effective when working from the source of the moisture.
Consider that moisture comes from somewhere. The standard home typically comes from outside the house, the kitchen, the bathroom, and the basement. Having the dehumidifier on the floor where these rooms are located means the dehumidifier can remove more moisture per unit time. In general, have it on the floor with the most moisture.
By now, you likely know that moisture levels are directly related to temperatures inside the home. Therefore, you’ll likely have the worst humidity levels in areas with the highest temperatures. However, we’ve also found that moisture tends to originate from areas with low temperatures.
Therefore, to address the root cause, identify the coldest floor (upper or lower) with the highest moisture.
Another critical issue is airflow. Humidifiers, like other HVAC systems, work best in areas with unimpeded airflow. That’s because humidifiers work by drawing air from the house, extracting moisture, and returning dry air to the same room. If the airflow is impeded, it may not draw air at a high rate, meaning moisture extraction is hindered.
To this end, choose the floor and room with the best airflow. If the upper floor doesn’t offer good airflow, have it on the lower floor and vice versa.
What about the Basement?
If you have a basement (about 15% of homes in the US do), you should consider having a dehumidifier there. However, we recommend investing in a special-purpose basement dehumidifier since you also want a dehumidifier on the second floor.
The reason is that basements are one of the primary sources of humidity problems in the home. This is because warm air entering the basement from the surrounding soil is very damp. And, due to the stack effect, the moisture always finds a way to the top floors. Worse still, basements are typically poorly ventilated. So, they can accumulate a lot of moisture.
Above all, if the basement has mold and related odors resulting from excess humidity, the issue will gradually spread to the entire house.
The best place to have your dehumidifier in a two-story house is the second floor. However, you’re allowed to locate the dehumidifier on the first floor if the first floor is damper, colder, or offers better airflow circulation. Most importantly, always consider investing in a separate dehumidifier for the basement.