Common Basement Smells and Odors – The Problem and Solution

An unpleasant smell coming from the basement is never a good thing, and it is never something that you want to ignore. Some of these smells could be caused by something as serious as mold or sewer gas that could have some ill effects on your health if left too long.

Some of the most common unpleasant odors in the basement are caused by mold, mildew, sewer gases, or a dead decaying animal such as a mouse.

Sewer Gas

Not only can the smell of sewer gas be repulsive, but it can also be quite dangerous. Sewer gas is made up of methane, sulfur dioxide, carbon dioxide, nitrous oxides, and even a trace of gasoline could be present in both privately owned and municipal sewage systems. Large amounts of methane can become extremely flammable and in some cases, even explode.

Methane can also be dangerous for your health in large amounts. A low concentration of methane is usually not harmful to people or pets; however, in a high concentration, the methane gas will begin to displace oxygen from the air. In the beginning, as the oxygen first starts to dissipate, you might start to experience rapid heart rate, fatigue, and vertigo. As more oxygen gets removed from the air, you could begin to damage organs in your body, including the heart and brain.

What causes a sewer gas smell

  1. Problem: If you are smelling sewer gas from your basement, the most common reason is because of a dried up P-trap in the floor drain, sink drain, shower drain, or toilet.

    Solution: Sewer gas will enter your home through pipes connected to the main sewer system. The best way to help prevent sewer gas from entering the home is to make sure that your drains remain primed and to do this all you need to do is dump a gallon or two of water down the drain every couple of months, or sooner if you notice that it is drying out quickly.

    You can also flush the toilets and turn on sinks and showers to let them run for a minute or two, especially if they don’t get used very often. Doing so will re-prime the P-traps and keep that sewer gas from entering your home.

  2. Problem: Another common cause for the sewage smell is from a missing or broken cleanout plug found in your floor drain.

    Solution: Remove the cover to your floor drain and check to see if you are missing a cleanout plug, or if your plug is cracked or broken. If needed, you can purchase a replacement cleanout plug from your local hardware or plumbing supplies store for about $2.00 to $5.00. Keep in mind that not all drains have a cleanout plug, so if you can not see one, that is normal. You’ll know if you have a missing cleanout plug because you’ll be able to see down into a hole that goes on an angle.

  3. Problem: Sometimes, the wax ring seal, which is found between the toilet flange and the base of the toilet, can become broken and begin to leak. Sewer gas will make its way through the broken seal and into your home. These seals can sometimes wear out on their own, but normally they break if you have a wobbly toilet.

    Solution: The only way to fix a bad wax ring seal is to completely remove the toilet and replace the wax ring seal with a new one. To avoid having to replace this again, ensure that your toilet does not rock back and forth or side to side, try leveling the floor underneath the toilet, or wedge a plastic shim in place if you need to.

  4. Problem: Not often, but sometimes, the main sewer line could develop cracks or leaks within the seals or joints. This, of course, is a much more serious problem that would require the help of a plumber.

    Solution: Contact a local plumber and have them come out to the home to perform a hydrostatic water test, or send a camera down through the pipes to determine where the leak is.
Mold and Mildew

Mold and mildew have a very unique, strong, musty odor to them. Some people might also describe the smell as earthy, or compare it the smell of a pair of wet used socks.

Sometimes your mold problem might not be visible for you to see because it is behind a wall, wooden panel, or in the bottom of a cupboard. Your nose is generally the first line of defense when it comes to mold since your nose can pick up on the microbial volatile organic compounds that mold releases as it grows.

Finding the mold can sometimes be a little tricky, and then determining the source of the problem is something else. If you are unable to see the mold problem, the best way to find it is to use your nose and sniff around. You’ll notice if the smell begins to get stronger around a certain area.

What causes a mold or mildew smell

  1. Problem: One of the most common causes of mold growth in your basement is from high humidity or condensation. When you have high humidity, the water vapor found in the air will meet up with a cold surface such as an exterior basement wall, which will then turn that vapor into a liquid.

    Solution: If you find that your basement has a problem with high humidity, you should invest in a dehumidifier. A dehumidifier will pull the little water vapors out of the air before it has a chance to land on any cold surfaces.

    Keep an eye on the weather forecast in your area and on the days where there is a high percentage of humidity; you can try opening some windows in the basement to create a cross breeze while running your dehumidifier at the same time.

  2. Problem: Another common cause of mold growth is from a leaking pipe or drain hose. Over time corrosion on the inside of the pipe will begin to build up and create a clog which will restrict water from moving smoothly through the pipe. Clogs are one of the main reasons for a leaking water pipe.

    Drain hoses on things like an air conditioner or furnace can also get clogged with things such as dirt, sludge, dust or even an insect. When this happens, water will build up and flow out onto the floor.

    Solution: Once you have found out where the leak is coming from, you should immediately try to stop the leak from continuing. Some leaks might not be as severe as others and can easily be patched up with a silicone tape that you can buy from Amazon for under $10. Other leaks might be more serious and require you to shut off the water line so you can have the pipe or joint replaced by a professional.

  3. Problem: Water leaking through the outside basement walls. You might find that after heavy rainfall, or when the snow begins to melt, your basement walls begin to leak. The reason for this is because concrete tends to crack, and if the crack isn’t taken care of quickly, overtime that crack will get larger and eventually allow water to get through.

    Solution: Take a walk around the outside of your home and look for cracks in the concrete foundation. If you see cracks, you should immediately take action to patch them up or inject them with an epoxy resin.  

    Also, make sure that your downspouts are not clogged and that they are draining at least six to eight feet away from home. The same goes for your sump pump; this should also be drained at least six feet away and ensure that the earth next to the foundation is elevated slightly higher to allow water to run down and away from home.
Cat Urine

Cat urine is another odor that homeowners have noticed coming out of the basement. Cat urine has a very distinctive, pungent smell to it. As the urine begins to decompose, it begins to let off an aroma similar to ammonia.

What causes a cat urine smell

  1. Problem: Cat’s will often pee outside of their kitty litter box for different reasons, but one of the more common reasons is to mark their territory. Whatever the reason might be, the smell of cat urine is not pleasant, and it should be taken care of quickly before the cat makes a habit of it.

    Solution: Clean up the mess and eliminate the odor with a powerful stain remover. By getting rid of the smell it’ll not only be better for your nose, but it’ll hopefully stop the cat from peeing there again.

    Next, try to figure out why the cat is going on the outside the box in the first place. It could be several different reasons, for example, the litter box could be; too small, needs to be changed, has too much litter in the box, or you’re using a different type of litter.

  2. Problem: Sometimes, what you are smelling is not cat urine, but the early stages of mold growth.

    Solution: Take a good look around the basement. You might not even be aware of it, but you could have mold growing. One of the most common culprits is a wet underlay for your carpet.

    Also try looking underneath tables, couches, and boxes. Look behind bookcases and even pictures on the wall. Mold can grow anywhere that’s dark.

  3. Problem: It is not uncommon for neighborhood cats to come around to your home and spray on the foundation or front door, especially if you have cats or other pets in the house. When the smell travels into the house through cracks or gaps, it would seem as if it is coming from within the basement.

    Solution: To stop cats from coming around your house and spraying, you can leave orange peels scattered around the outside of the home, as well as use a repellent that contains citrus oil. Cats do not like the smell of citrus so it’ll keep them away from your property.
Dead Animal

You might not even be aware of it, but you could be sharing your home with a family of rodents or other pests.

Dead animals such as mice and other rodents let off a sulfur type smell as they go through the decomposition phase. This smell can make you and your family sick if it is left too long. Small animals like a mouse could end up smelling only for a few days as they decompose, while other larger animals like a squirrel or raccoon could smell for a few weeks.

The only way to get rid of this smell is to find the dead animal and remove it from your home. Trying to locate the dead animal, on the other hand, could be quite challenging. For this, you will need to use your nose and sniff around until you find where the smell is the strongest. Once you find the general area, look for flies and other bugs that might be collecting in a certain spot, this will help you pinpoint the general area where the body could be.

To prevent animals from dying in your home, the best way to do this is to block off any access they might have to get in your home in the first place. To do this, you will need to do a complete walk around the exterior of your home and seal any small cracks, gaps, and any other point of entry. Then do the same on the inside of the home and thoroughly check the exterior walls in the basement, and attic.

Once you have all of the access points blocked off, set up traps throughout the house to catch any animals that you might have now locked in. Do not use poisons to try killing them because once they take the poison, they could end up dying anywhere in the house, and you’ll have that dead animal small all over again. With traps, you can help control where the animal is caught to dispose of them.

How Can I Make My Basement Smell Fresh Again?

Once you have found and eliminated the source of the foul smell, you can begin to make your basement smell fresh again. Below are some tips that you can try doing.

  • Get rid of anything old cardboard boxes, clothes, books, and magazines that are sitting on the floor.
  • Scrub the floors, walls, and floor joists with borax solution.
  • Remove mold from furniture by using a combination of vinegar and baking soda.
  • Pull up and remove damaged carpet and underlay.
  • Use a dehumidifier to remove moisture from the air.
  • Use a deodorizing method to remove musty stagnant smells from the air.

How Do You Deodorize a Basement?

There are few things that you can try using to deodorize your basement and help remove those nasty odors. You can use any one of the mentioned below or separately try using a combination of them.

Activated charcoal is a great safe way to deodorize your basement and eliminate nasty smells. There is a company of out California that produces bamboo charcoal bags that will last up to 2 years. Just place a few bags around your basement, and they’ll immediately get to work deodorizing the space. Amazon has a bag of 8 on for $18.95. Click here for the current price.

White vinegar is another great way to help deodorize the air in the basement. To try this method, all you need to do is fill a few bowls of undiluted white vinegar and place the bowls around the area where you want to remove the odor. Let the bowls sit for 24 hours, and then discard the old vinegar. Refill the bowls with new white vinegar and let them sit for another 24 hours. Repeat this every 24 hours until the smell is gone.

Baking Soda can also work to help clear that musty smell in the basement. Fill a couple of bowls with baking soda and place them at different ends of the room. The baking soda will begin to absorb any moisture and odors that are in the air. Replace the baking soda every 3 to 4 weeks as needed.

Coffee grounds will work the same way as baking soda. Fill a few bowls with ground coffee and place them around the basement. You don’t have to use fresh coffee grounds, you can use used grounds, but make sure they are dried first before using them. The coffee grounds will absorb odors, and leave your basement smelling like coffee.


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