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Posted by Cris Carl | Dec 19, 2010

Guide to Bad Basement Smells

Got a stench in the basement? We'll help you figure out what it is and how to fix it.

Generally, if we are talking about a smell in your basement, it usually isn’t a pretty one. What you smell may be mold, mildew, cat urine (or feces), dead mice, or sewer gas. If you have an older home, there is a good chance you battle with water in your basement, which leads to some of these problem smells. Even if you have a sump pump installed in your basement, it may not be enough to combat mold and mildew. You may also have blockages in plumbing, drains, venting, or heating systems. Here, I will go over some of the major culprits of basement smells and offer suggestions on how to fix or lessen the problem.

Mold and Mildew

Mold and mildew are caused by the growth of bacteria that arises with unchecked and stagnant moisture. Mold and mildew can grow slowly over time, or rapidly if you have a major leak. Mold has an unforgettable tangy, earthy, smell of decomposition. Mold is always in the air unless you live in an extremely arid region. The more moisture, the more mold. Basements, being underground, can become moist though cracks in walls, windows, plumbing, condensation, etc.

Many people are allergic to mold and may notice an increase in symptoms after a rain. The best way to deal with mold and mildew is to have your basement as dry as possible. Invest in a dehumidifier, seal any leaks or cracks the walls or floors, or invest in gutters so that rain drains away from your house.

Depending on the age of your home and how well you and previous owners have kept up maintenance, you may always have a slightly musty smell to contend with.

Getting rid of mold and mildew usually entails throwing out carpeting, furniture, or any absorbent materials such as fabrics and cardboard. Then you will need to clean thoroughly with a diluted bleach solution (10 to 1 mix).

Smaller items, when placed in bright sunshine for a couple of days will often lose both the smell and the mold.

Sometimes, if the problem is severe enough, you may need to look into professional remediation.

If your furnace and vents haven’t been cleaned in awhile, you may have a build-up of dust, debris, dead bugs and mice, and of course, mold. You should have your vents cleaned every one to two years.

Sewer Gas


If it smells like poop, rotten eggs, or garbage in your basement, you have a big problem. Sewer gas is an overpowering smell that is also toxic and in the right amounts – explosive. Sometimes the odor may only be present when the house is closed up and the heat or air conditioning is on.

The smell is created by a build-up of waste materials found in either a public sewer system or in a septic system. Plumbing is meant to keep the gases inside the pipes, so if you are smelling sewer gas, you may have a clog or a crack in the plumbing system.

You may also have a clog or crack in your venting pipes.  Most homes usually have venting pipes that are part of your plumbing that poke through your roof. Venting pipes are not for exhaust, but for intake of fresh air. When a lot of enters a drain pipe it pushes air in front of it towards your sewage or septic system. The vents replace the lost air.

You may be smelling sewer gas as a result of plumbing fixtures whose traps have gone dry. For example, if you are going to be away for awhile, you may want to have someone flush your toilets for you. Water evaporates quicker than you may think.

Sewer gas problems can be very serious and will likely require the help of a professional. One of the best detection methods professionals’ may use is having smoke forced through your plumbing/septic system.

Cat Urine


Pungent and pervasive. I doubt there is anyone alive who doesn’t know the smell of cat urine. There are plenty of products out there that claim to kill the smell, but you will likely end up going through a similar process as with mold.

You will often need to tear out any wood or wallboard that has been sprayed. Throw out everything that has the smell attached to it. Baking soda spread liberally and vacuumed up a few times will also help. The use of Kilz or stain on well-cleaned walls or wood will also help.

Dead Animals


Lastly, you may smell something with a sickly-sweet smell of decomposition. You may have dead mice or bats in a crevice or wall. Unfortunately, unless you are remodeling anyway, time is the only cure.

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