Why Michigan Basements Need a Vapor Barrier
Concrete block walls breathe. Water vapor from the soil migrates through the block constantly, not just when it rains. In older Detroit-area homes where the block was laid without a proper waterproof coating on the exterior, this vapor migration is significant year-round.
When you finish the basement and put a stud wall against the block, you create a cavity between the block and the drywall. In winter, the block face is cold. Warm, humid indoor air moves into that cavity and condenses when it hits the cold surface. Over time, you get standing moisture in the cavity, and mold follows.
The mistake I made, and that a lot of people make, is thinking that moisture-resistant drywall addresses this problem. It doesn't. Moisture-resistant drywall is designed to handle occasional exposure to humidity, not sustained contact with condensation. It holds its structural integrity better than standard drywall in wet conditions, but the paper and organic components can still support mold growth when they stay damp for weeks at a time.
What I Found When I Opened the Wall
The section I opened was in the corner of the basement near the furnace, which I mention because that corner had slightly more warm air circulation than the rest of the space. The mold was worse in the cooler parts of the room.
On the block side of the stud cavity, there was a line of efflorescence about eight inches up from the floor. That's the white chalky deposit that forms when water moves through masonry and deposits minerals on the surface. It told me that water had been wicking up from the footing or through the lower block courses and evaporating inside the wall cavity. Above the efflorescence line, the block face was dry but there was mold on the back of the drywall and on the bottom foot of each stud.
I called my friend Carl, who does renovation work in Detroit, and he came over and looked at it. His first question was whether I had a dehumidifier running in the basement. I did not. His second question was whether I had put any sealant on the block before framing. I had not. He shook his head in the particular way that knowledgeable people shake their heads when someone shows them a predictable problem.
The Remediation and Redo
Remediation first. I removed all the drywall and insulation from the affected area. Drywall with visible mold growth goes in a contractor bag and out of the house. The studs I could keep if the mold was surface level and hadn't penetrated the wood, which I tested by scraping a pocketknife lightly across the surface. Studs where the mold went deep got replaced.
For the block wall, I used DRYLOK Extreme masonry waterproofer applied with a stiff brush to force it into the block pores. A gallon costs about $35 and covers 75 to 100 square feet. The block wall in my affected area was about 120 square feet, so I used a gallon and a half and went over the lower section twice. DRYLOK isn't a permanent solution if you have active water intrusion from outside, but for the vapor migration and minor seepage I was dealing with, it's been effective.
After the DRYLOK cured for 24 hours, I installed a continuous polyethylene vapor barrier against the block before framing. Six-mil poly, taped at all seams with acoustic sealant tape, run up the block to the rim joist and over to overlap the floor poly. The framing went up in front of the vapor barrier, not against the block directly. The cavity between the vapor barrier and the drywall has air circulation at the top because I kept a gap at the rim joist rather than sealing it off completely.
Two years since the redo and no recurrence. The Department of Energy's basement insulation guide covers the moisture physics behind this in more detail if you want to understand why this assembly works better than what I originally built.
What Detroit-Specific Basements Need
The Craftsman bungalows and brick colonials that make up a lot of the older Detroit housing stock often have rubble stone foundations or early poured concrete rather than the concrete block that was standard by the 1950s. Stone foundations need different treatment. You can't apply DRYLOK effectively to rubble stone because there's no continuous surface. Those foundations typically need interior drainage systems or exterior excavation and waterproofing, which is a contractor job.
For block and poured concrete, the DIY approach I described works reasonably well when the moisture source is vapor migration rather than active water intrusion. If water is coming in during rainstorms or snowmelt, no amount of interior treatment will fully address the problem. The water is finding a path through the wall or floor and needs to be redirected, either at the exterior or with an interior drainage system like a French drain around the perimeter.
One thing I'd add for anyone in Detroit working on an older home: check the original basement floor before putting any new flooring down. A lot of those old basements have poured concrete floors with no vapor barrier underneath. Ground moisture wicks up through the slab constantly. If you're finishing the floor, you need either a dimple mat and floating floor system that allows the slab to breathe, or a vapor barrier under the flooring. Same principle as the walls.