Basement and crawl space mold
Below grade is where most Ann Arbor mold lives, and where the most money gets spent on the wrong repair. A wall that is wet from outside and a wall that is condensing look almost identical and want opposite work.
Three causes, one symptom
Telling them apart from inside
Nearly every damp basement in this city is one of three things, and the whole value of a survey is deciding which. The repairs diverge completely from that point: one means work outside the building, one means changing the air in it, and one means finding a single defect and sealing it.
Water arriving through the wall
Ground water under pressure moves through concrete block, and block is a generous host: hollow cores, mortar joints, and a lot of surface area. The giveaway is a gradient: worst near the floor where the head of water is greatest, fading upward. It tracks the weather with a lag of a day or two after heavy rain or a thaw, and it leaves a white bloom of salt on the inside face as the water evaporates and the minerals it carried do not.
The repair is usually outside the building and starts cheap: extend the downspouts, correct grade that falls toward the house, clear the window wells. Ann Arbor gets roughly 37 inches of precipitation a year plus around 56 inches of snow, and a downspout discharging at the foundation delivers a remarkable share of it straight to the wall you are trying to keep dry. Only when the cheap measures fail does anyone sensible start talking about excavation or interior drainage.
Condensation on a cold surface
Below-grade walls stay near ground temperature all summer while the air in the house does not. Warm humid air reaching a cool surface gives up its moisture on it, and the wall gets wet from the room side without a drop arriving from outside. The tell is that it is even, with no gradient toward the floor, and that it is worst in July and August, not after rain.
This one is the reason the humidity range matters. The target is 30% to 60% relative humidity; a Michigan basement in a humid August sits well above that without help. The fix is dehumidification, sorting out ventilation, and getting any indoor moisture generators out of the equation: a dryer venting indoors, laundry dried on a rack downstairs, an unvented bathroom directly above. Excavating the outside of this wall would achieve nothing at all.
A single point of entry
A crack from settlement, a form tie hole, a failed pipe penetration, or a window well that fills and overtops. This is the easiest of the three to identify because it is local: a narrow wet track with dry block either side of it, and it runs whatever the weather has been doing. It is also usually the cheapest to fix, which is a good reason to rule it in or out before agreeing to anything larger.
Finishing a basement that was never dry
A below-grade room that has been framed, insulated and boarded is the worst place in a house for a moisture problem to develop, because every material involved is on the county's list of things that have to be removed rather than cleaned: drywall, insulation, carpet and pad, and wood products other than solid wood.
It is also the place where the problem is least visible. Mold grows on the back of the board, in the cavity, against the cold wall, and the first sign upstairs is a smell that nobody can locate. By the time a stain shows on the painted face, the cavity behind it has usually been working on this for a while.
- Board that has been wet comes out. There is no method for cleaning mold off the back of installed drywall, and painting the front of it is a cosmetic operation on a material that has already failed.
- Fibrous insulation that has been wet comes out too. It holds water, it holds spores, and it does not dry usefully in place inside a sealed cavity.
- The cut is made above the wet line, not at it, because wicking carries moisture higher in the board than the visible tide mark suggests.
- Solid wood framing usually stays. It is non-porous enough to clean, scrubbed with hot water and a non-ammonia detergent, rinsed, then dried properly, provided the structure itself is sound.
- Nothing goes back until the wall is dry and staying dry. Rebuilding over an unresolved moisture source is how the same room gets done twice in five years.
Crawl spaces
The part of the house nobody visits
Crawl spaces get their own paragraph because they fail differently. A basement gets noticed, because people keep things down there. A crawl space can run wet for years with nobody in it, and what happens above it is that the floor of the house gets cold, the rooms smell faintly of earth, and joists start holding more moisture than they should.
An exposed dirt floor evaporates continuously into the space above it, which is why a ground vapor barrier is usually the first thing worth doing and often the cheapest. After that the questions are whether the space is vented or sealed and whether it is being treated consistently: a half-sealed crawl space with some vents open and some closed can end up wetter than either approach done properly.
The other thing worth checking is where the dryer and the bathroom fans go. Terminating either into a crawl space or an attic is a moisture problem with a simple fix, and it is common enough in older houses that it is worth looking at before anything more expensive is considered.
Straight answers
Basement questions
Will a dehumidifier fix this?
It will fix condensation, which is one of the three causes, and it will do nothing whatsoever about water arriving through the wall. In that case it is running continuously against a supply that does not stop, and the electricity bill is the only measurable result. This is exactly why the diagnosis comes before the remedy. If a dehumidifier holds the space at 50% and the wall dries out and stays dry, you had a condensation problem and it is solved.
Someone quoted me for interior drainage and a sump. Is that reasonable?
It can be entirely reasonable and it is also the most expensive answer on the list, so it is worth having ruled out the cheap ones first. Ask what was measured, whether the downspouts and grading were examined, and what evidence points at hydrostatic pressure and not condensation. A quote for a perimeter system on a wall that turns out to be condensing is a large amount of money spent on the wrong wall.
Is the musty smell itself harmful?
The smell is not the hazard, it is the indicator — it means active growth somewhere, usually on something you cannot see. The county's guidance is not to spend effort determining what species it is, because that does not change the response. The response is the same in either case: find the moisture, correct it, remove the porous material that has been growing on.
My crawl space has a vapor barrier already. Is that enough?
Often it is doing less than it appears to. A sheet that is torn, that stops short of the walls, or that is not lapped and sealed at the seams passes a great deal of vapor, and from a hatch it looks finished. Worth actually going in and looking at the edges before assuming it is handled.
Start with a survey, not a sales visit
Ask about a basement
The two details that help most before a visit are whether it is worse after heavy rain or worse in humid summer weather, and whether the affected wall is finished or bare block. Those two answers narrow it further than a photograph does.
Related work we arrange
What basement work connects to
Mold inspection and moisture diagnosis
Finding the water before pricing the repair. Washtenaw County’s own position is that in most cases sampling is not necessary, which makes the visual survey the thing you are actually buying.
Attic mold and roof ventilation
Usually a ventilation fault rather than a roof leak. A bath fan discharging into the attic puts warm wet air against a cold roof deck every night of a Michigan winter.
Water damage drying and material removal
The 48-hour window the EPA sets for drying wet building materials, and which porous materials have to come out rather than be dried in place.
Rental property mold and certificates of compliance
Ann Arbor inspects every rental unit every 30 months. Passing that inspection and fixing the moisture behind it are two different jobs, and only one of them holds until the next one.