DEAR TIM: Our full-size divided basement has mold growing on the west side of the wall, as well as the south side on the other end of the basement. The basement has two sump pumps, one on each side of the basement. The basement does not leak water. We’ve scrubbed the mold away with bleach/water and painted the walls with Concrobium Mold Control. We leave the basement windows opened during dry warm periods to help with air circulation and at times will use two dehumidifiers, but to no avail. We even had the soil around our house re-graded, and that was a tremendous help but not enough to stop the mold completely from growing back. The basement can never be finished unless the mold is eliminated. What do you think the problem is, and how would you fix it? – Vic G., Delaware, Ohio
DEAR VIC: You and millions of other homeowners suffer from mold growth in basements, crawl spaces, closets, attics, bathrooms, exterior surfaces and so on. It’s an unending battle. The good news is mold can be prevented, once you understand how it grows.
Mold growth is very similar to fire. It takes just three things to create a fire: fuel, oxygen and heat. When you have enough of each, a fire starts. It’s called the fire triangle.
Mold is similar. To have mold grow, you just need three things: mold spores, food, and water. That’s the mold triangle.
Here’s the bad news. Until such time as you convert your home’s basement into something similar to the Center for Disease Control – where they have advanced air filtration that blocks just about every mold spore, among other contaminants – you’re going to have thousands or millions of mold spores constantly floating around your house or basement. It’s unrealistic to ever think you’ll stop mold spores from landing on surfaces. You may clean a surface, but an hour later mold spores can attach to this clean surface. Strike one.
Different molds in your home eat different things. Some eat dust, some love food, some love the paper backing on drywall. In other words, it’s nearly impossible to cut off the food supply of mold around your home. That’s strike two.
Your last chance to avoid a strikeout is water. You must stop water from touching the mold and its food source. This is easier said than done, especially on the cool walls of a basement.
I’m sure you’ve seen what happens when you take a can of soda from the refrigerator. Within minutes, even in the winter, water droplets form on the outer surface of the cold can. This is just condensation. The water vapor in the air is just transforming to a liquid because the surface of the can is at or below the dew point of the air in the room where you have the can.
Exterior walls, basement walls, windows, attic surfaces, roof sheathing and closed closet walls on exterior walls are all prone to mold growth because these surfaces get cold or cool. Water vapor can turn to liquid water on these surfaces and because the surfaces are not like a mirror, you can’t see the liquid water. This water sparks the growth of the mold.
To stop the mold, you need to stop the condensation. You mentioned you open the windows of your basement to allow outside air into the space in the summer. This is a bad thing because that air is warm and often has a very high moisture content or relative humidity. When this air floods into your basement and touches the cool walls below grade, the water fog forms on the walls.
You can stop liquid water from forming on the walls by making them warmer than the dew point of the air in the basement. This is not easy and with you trying to finish the basement, it’s nearly impossible. My guess is you want to erect walls and insulate them. This will make the basement walls even cooler than they are now. You can increase the air flow over cool surfaces to immediately evaporate the water vapor that turns to liquid. Both of these things are unrealistic in your situation.
I feel the only way to stop the mold from growing is to clean the walls really well, get them dry and then immediately frame the new walls. Create a 1-inch air space between the back of the wall and the basement wall. Caulk the bottom wall plate so no air can get behind the wall. Insulate the wall and install a continuous cross-laminated vapor barrier on the wall before you install the drywall.
You then need to seal the gap between the top plate of the wall and the top of the basement wall. You may discover expanding closed cell foam does a good job. Your goal is to isolate this space between the basement wall and the back of the wall so that no humid air can get to the basement wall where it can transform to liquid water.
Think of your new walls in your basement as the sides of a swimming pool. Anyplace that water would leak through you need to seal. Just imagine the air in your basement is the same as water in a swimming pool. Stop that water vapor from getting through to the basement walls and you’ve got a very good chance of arresting mold growth.
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