Damp Basement Fix

DEAR TIM: We have a L-shaped basement storage area under our front porch. There's a metal ceiling in this space that supports the concrete porch slab above. There are no cracks in the concrete porch. This space has no ventilation and everything stored in this area becomes damp and then moldy. We installed a small tube vent, but it proved to be useless. Why is this happening, how could it have been prevented and is there anything I can do now to transform this nice area into a mold-free zone? Sharon G., Pittsburgh, PA

DEAR SHARON: I grew up in a house that had this exact same under-porch storage area. My dad had a small workbench in there and he put up some metal shelves too. We experienced the same problems with dampness and mold in this space and other parts of our basement too. Fortunately, this under-porch space at my childhood home had a large tilt-in steel window with no weatherstripping that did allow some air to leak in so the mold and mildew was not too bad.

I'm not like some super-hero that has x-ray vision and as such can only speculate what's on the other side of the foundation walls and under your basement slab. What I can tell you, and many people find this astonishing, is that concrete and concrete block allow vast amounts of water vapor to pass through them. Liquid water will also leak through tiny cracks or the contact point between the concrete block and mortar joints.

This basement under-porch area is damp and things stored here get moldy fast. Note the rusty metal ceiling. Photo credit: Sharon Geibel

The temperature in your storage area is cool enough that it's below the dew point of the air that's in the space. When this happens, condensation can begin to form on the walls, floor, and any stored object. This condensation doesn't have to be large drops of water. It always starts out as a faint fog on the surfaces. You can see this happen if you breath on a clear glass or mirror. That fog you see is water condensation from your breath.

You can get large droplets forming on the steel ceiling in your confined space if the outdoor temperature really drops making the concrete slab quite cold. That cold is transferred to the steel and any humidity in the air in the space will rapidly create drops of water that can fall on your possessions or drip down on the floor.

Much of the water vapor could have been stopped completely by doing a few simple things. A 6-mil plastic vapor retarder should have been placed under the concrete slab is this storage area. This plastic is readily available at any home center and if you want fantastic performance, you use a cross-laminated vapor barrier product that's commonly used in commercial construction. These products stop virtually all of the water vapor that's streaming up from the soil under the slab into your space.

The outside foundation walls of the storage space should have had an asphaltic damp-proofing spray applied at the very least. The better choice would have been to apply a true waterproofing compound on the walls to ensure both liquid and water vapor never get through the walls.

I'd also have sprayed a silane-siloxane clear water repellent on the concrete porch slab. This would repel most liquid water that would normally soak into the porch slab. The metal ceiling under the slab is stopping water vapor, but some could be getting through around the edges.

To transform this storage area into the mold-free zone you desire, you've got several options. The immediate challenge is to lower the humidity in the space because mold needs water to grow. You can lower the humidity with a great dehumidifier.

You need to place the machine in the space and monitor it. Purchase a decent hygrometer that measures relative humidity. These are common and fairly inexpensive. Try to operate the dehumidifier to get the humidity below 40 percent.

You might also contact a local business that sells specialty concrete products to commercial contractors. Believe me, these businesses exist in every large city. These companies have amazing products. I don't know if this product exists, but I've begged chemical companies for years to make it.

What you want to ask these specialty businesses is if they have a clear non-toxic spray-on product you can apply to the inside of your foundation walls and on your concrete slab in the space. This product needs to penetrate, and not be a film on the concrete or concrete block.

The product needs to block water vapor just as if a sheet of plastic had been put on the other side of the concrete slab or foundation wall. You can see how amazing this product would be if available. There are hundreds of thousands of older homes in the USA that would benefit from a product like this.

The final thing you can do, and this is somewhat expensive, is to raise the temperature in the space. If you get the temperature of the walls and items you're storing in the space above the dew point of the air in the space, condensation will not form and there will be no mold.

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