Humidity, mold and the musty basement

Most mold in a house is not from a flood. It is from ordinary summer air, warm and full of water, meeting a surface that is cool enough to make that water condense: the basement wall, the cold-water pipe, the back of a closet on an outside wall, the underside of the roof deck over a bathroom. Mold spores are in every house; what they need is that moisture and a few days. Take away the moisture and they stop.

The number

Buy a cheap hygrometer and put it in the basement. Under 50 percent relative humidity, mold does not grow and dust mites do not thrive. Over 60, both do. Summer basements without a dehumidifier commonly sit at 70 or more, and the musty smell is the mold telling you so.

Getting it down

• A dehumidifier sized for the space, set to 50, draining to a floor drain or a condensate pump so nobody has to empty it. A whole-house unit tied into the ductwork does the same for the whole house and costs more.
• The air conditioner. It dehumidifies as it cools, but only when it runs long enough. A system that is too big cools the house in ten minutes and shuts off before it has taken the water out, which is why an oversized air conditioner leaves a house cold and clammy. A Pro who sized by load calculation will not have done that.
• Bath fans, run for twenty minutes after every shower, vented outside. A timer switch makes it automatic. The range hood on when the pots are boiling.
• The dryer vented outside, all the way, with no leaks.
• Fix the sources: the leak piece in this section, the drainage piece, and any crawlspace with bare earth, which puts gallons of water into the house every day until it gets a plastic vapor barrier.
• Do not store cardboard, fabric or paper on the basement floor against the wall. They are food, on the surface that is coldest.

When you find it

A small patch of surface mold on a hard surface, under about ten square feet, is a homeowner's job: detergent and water, dry it, fix the moisture. Bleach is not needed and does not help on porous material. Wear gloves and a mask.

Anything larger, anything inside a wall or under a floor, anything that keeps coming back, and anything after a flood or a long leak is a restoration Pro's job. They find the extent with meters, contain the area so the cleanup does not spread it through the house, remove what cannot be cleaned, and dry the structure. A company that only sprays and paints over it is selling the smell going away for a season.

Testing

Mold testing is mostly unnecessary. If you can see it or smell it, it is there, and the type does not change the fix. Air tests are useful for a dispute with a landlord or an insurer and for little else. Spend the money on the dehumidifier.

The attic and the roof deck

Black staining on the underside of the roof sheathing, usually on the north side and over the bathrooms, is mold from moisture that came up through the ceiling and could not vent. See the ice dam and attic heat pieces in this section; it is the same fix, air sealing and ventilation, and a bath fan that goes outside.

The rule

Every musty smell has a water source. Find it and take it away, and the mold takes care of itself. Cover it up and it comes back in a year, behind the paint.