If you, like millions of people in the United States, have a house built on a crawl space, you are likely to have odors inside your house that came from the crawl space. This is not an unusual situation especially if your house is air conditioned.

The odors that you are likely to be getting from the crawl space  are:

• mold,
• dead animal odor,
• rodent/pet urine and feces.

Even if your crawl space is very dry most of the time, on very hot and humid days, the warm and humid breeze blows into the multiple screened vents in the foundation. If the cool area under your house  is below the due point temperature of the outside air, condensation forms on all the cool surfaces which feeds any mold or mold spores in the area. If there is rodent or pet urine in the area, the high humidity will cause it will begin to producing mercaptin gas (what we know as urine odor).

If your crawl space, like most, is not really dry, these conditions exist almost all the time.

The question is…How do these odors get into your house?

The answer is pretty simple though not obvious. When your home is heated or air conditioned, the barometric pressure (air pressure) inside you home is slightly less than the outside air. This is because the air handler draws air out of your home, heats or cools it and forces most of it back into your home, though some leaks out of the pipes. Another condition exists when the air is being cooled. Cool air has less volume than warm air which accounts for much of the apparent loss of air.

Because your crawl space is well vented to the outside air, it is the same pressure as outside air. Because nature abhors a vacuum, air outside your home will make every effort to get inside your home via any large or small air leak available.

These air leaks can be large or small and include but are certainly not limited to:

• Plumbing clearance holes in the floors and walls.
• Electrical outlet boxes, ceiling light fixtures, and switches.
• The gap between the heat/AC fittings in the floors or walls.
• Leaks in the return air ducts.
• Leaks around windows and doors.

All of these leaks allow outside air into your home and bring in odors with them especially the leaks from the crawl space.

How can these leaks be stopped?

In most cases they can not be stopped, but the work around isn’t all that complicated because the goal is to stop the air in the crawl space from entering your home not to stop all the air from entering your home.

One solution though not the best one would be to pressurize your home with a fan blowing in from outside.  Then all the leaks would leak out instead of in. The obvious problem with this is that it would be forcing hot or cold air into your home that the heating and air condition system would have to deal with.

A better solution is to close all but 1 vent in the crawl space and install a small quiet fan blowing air out of the last vent. One would likely think that this would have very little effect on the air pressure in the crawl space but in fact it can and does.

Unfortunately, you can’t stop into the local hardware store and pick up a fan setup for this application but they are pretty easy to make.  Once you find a quiet (usually a shaded pole low speed motor) fan that will fit in the vent and a piece of wood or aluminum to mount it on you are almost there. There is a manufacture of these fans called Tjernlund Products Inc. They do have distributors and are available in some area and on line.  These are NOT the cheap foreign made fans that will stop working in a month or two. You do not need or want the humidity controlled version.

The main clue is to cover the hole on the outside with a metal window screen material (usually copper)  and cover that with 1/4″ hardware cloth to keep the bugs and critters from getting in.  The fan should be left to run 24-7-365.

If you have urine or feces or dead animal smells, you will want to treat the contaminated areas with OdorXit Concentrate, sometimes several times to get it down into the dirt or concrete to treat the urine that migrated there.  DO NOT USE A PRESSURE WASHER!!!! Especially on concrete. Driving the source of the smell deeper with high pressure water is not a solution.

If there is a heavy musty and/or moldy  smell you will want to treat the crawl space with OdorXit CLO2.  Usually 25 grams per 1000 sqft will do the job unless you have lots of visible mold blooming on what ever is in the crawl space. Wet cardboard and clothing are the worst. Remove them before activating the CLO2 packet(s).

I will have more to say about pet odors and mold in crawl spaces but the ventilation issue is really the big one.