Hot Topics: Humid Basement Causes Mold

Lead Image

Here on we enjoy providing a place where home improvement novices and experts can come together to share ideas and advice. Inside our Forums, users can browse threads to see what exchanges are taking place on a topic of interest or start their own dialogue by posting something for the community to take part in. With over 250,000 members and counting, this resource is quite active so each week we highlight one of the conversations that may just help you with that next DIY project.

Original post: Humid basement causing mold problems

hcliffe Member

Hello, I live in St. Louis where it is hot and humid all summer long. I have a two-story, 2,600 square-foot house with a full walkout basement. The back basement wall is framed, not concrete. The basement is mostly finished with several rooms. I had no humidity or mold problems (that I knew of) until about five years ago. I noticed the basement room doors were sticking. I have a large collection of vintage clothing and wood furniture in the basement. I noticed that it was starting to mold. I have no leaks in the basement and it does have a working sump pump with drain tile that does run quite a bit when it rains.

I installed a dehumidifier about three years ago. It runs non-stop and shows 70-75 percent humidity most of the time. Last year I installed a second dehumidifier in another room. It shows 60 percent most of the time.

This summer, in another room I have found mold on several of my items. I am now leaving that door open but the humidity stays close to 70 percent in there.

My AC unit is about three years old. I believe it is sized correctly for the house. Main floor humidity is around 55 percent. I have no cold air returns in the basement, but do have three air ducts blowing AC into the basement.

What to do? Do you think installing one or two cold air returns will help? Just install another dehumidifier? Any idea why my basement stays so humid as soon as the humidity outside rises?

Pilot Dane Group Moderator

I would read up on water problems in basements. Easiest are things outside like making sure the ground is graded and sloped to direct all water away from the home. Make sure you have gutters and all downspouts have pipes to carry the water at least 10 feet away from the home. If you have a foundation perimeter drain make sure it is clear and functioning. After that it gets into more extensive things that can require excavating around the basement to install a perimeter drain and a damp proof coating to the outside of the foundation/basement walls.

airman.1994 Member

What size and brand are the dehumidifiers? What is the temperature of the basement? How many square feet is the basement?

hcliffe Member

Thanks for the replies so far.

I have read up on water problems but am still somewhat at a loss. Two years ago I added some soil to an area that was not sloping away from the home, now it is. At that time I also found a drain that had collapsed from tree roots. I re-routed that and it now drains away from the home. They are not all 10 feet, but with the current grade that would be hard to do without running a pipe a long distance underground through tree roots.

My basement is about 1,300 square feet. It is sectioned into one main room and four smaller rooms. In the main room I have a GE 50 pint dehumidifier. In one of the smaller rooms where most of my vintage clothing is located, I have a GE 30 pint dehumidifier. The basement temperature is between 68 and 70 degrees Fahrenheit during the summer.

Bud9051 Member

Hi hc, You are dealing with two sources of moisture beyond what you and others living there generate.

  1. All homes lose air to the outside which gets replaced by that hot humid air you mentioned. Just checked St. Louis and it is 86 degrees Fahrenheit at 61 percent RH. Using a temperature and humidity calculator when that outside air cools down to 71 degrees Fahrenheit, its moisture capacity goes up to 100 percent RH.
  2. The other source is, as discussed, moisture passing through your concrete floor and foundation and you mentioned the sump does run quite a bit when it rains so we know the soil outside is on the wet side.

First step should be to pick up one or a few humidity meters. You will also want to measure the temperature at the same time in each location. Be sure to check the outside temperature and RH at the same time.

Supply and return air flow should be matched as closely as possible to avoid pressurizing or depressurizing any space. Those pressures will increase the amount of outside air coming into your home.

That will get you started.


fastback Member

My home is on a lot with a very high water table. Adding fill for a significant slope had little effect. I hope using a brand name isn't against any rules but I installed an Aprilaire 1850 dehumidifier in my basement. This thing is rated for a 3,000 square-foot home. Since doing so there has been a very noticeable decrease in the basement/home humidity as measured with a meter. You may wish to check if there is any warranty issues if you install it yourself.

I would also suggest cutting some vents into your basement return ducts. If you use closeable ones you could shut them in the winter.

hcliffe Member

More good comments and input, thanks, everyone. I do have a meter to measure humidity in the basement and I check it regularly. I'll be working soon to install at least one cold air return in the basement. However, it begs the question: Should I have my supply vents open in the basement during the summer? I do not need them open for comfort as it stays cool enough with them closed. However, I thought opening them would help move the humid air through the HVAC system and remove some of the humidity. But perhaps the vents being open is causing more humid air to be pulled into that space?

Bud9051 Member

Just to note, outside, inside house, and inside basement humidity readings cannot be compared as each is "relative" to the immediate temperature.

To read the rest of the thread, look here: