Poor Ventilation
If you have poor ventilation, it could be the cause of mildew stench. The moisture is trapped and is building up on your walls and inside your ducts, causing mold and mildew to form. To fix the problem, install proper ventilation---such as a fan or window---in the areas where moisture may be occurring. In most cases, you can get rid of simple mildew on the walls by mixing a cup of bleach with a gallon of water and wiping down the area.
Excess Moisture
If excess moisture is seeping into your home, an earthy, mildewy smell may occur. This smell can begin even before mildew actually forms. If you know poor ventilation is not the cause of your mildew smell, start checking for excess moisture throughout your home. Excess moisture, not caused by poor ventilation, is typically found in places where leaks occur, such as on your roof, in your basement or under your sink. If none of these places appear to be the culprit, check all of your water pipes for possible leaks.
Excessive Heat
If simple excess moisture is not causing the wet, earthy smell, check for excessive hot spots or warm areas within your home. Mold and mildew, like many types of fungus, tend to grow in warm, damp places and areas where air conditioning or cold air cannot reach, such as hot attics, inside washing machines, around windows and in damp basements. When searching for the culprit of your mold and mildew smell, check spots that are both warm and wet first, since such spots are more likely to be the birth place of any mold and mildew growth, than an area which is only hot and not wet.