Five Situations, in Order of Seriousness
Bathroom mold covers everything from a cosmetic nuisance to a structural problem behind the tile. Working out which of the five you have decides whether this is a Saturday morning or a contractor.
- Grout, caulk and sealant — cosmeticSurface growth on the silicone line and in grout joints. Clean it, and if the caulk is colonized through, cut it out and replace it. No specialist needed.
- Ceiling and the tops of walls — a ventilation faultSpeckled growth above the shower means the moisture is not leaving fast enough. The fan is absent, undersized, blocked, or running for two minutes when it needs twenty.
- Behind the shower surround — a failed sealWater passing the tile or the surround into the cavity. Usually found because the adjacent room's wall or the ceiling below is damp.
- Under the floor — a slow leakToilet wax ring, supply connection, or a shower pan. The signs are a soft spot, a stain on the ceiling below, or movement in the floor at the fixture.
- Inside the wall — supply or drainConstant, no seasonal pattern, and frequently discovered from the other side of the wall rather than in the bathroom itself.
On the surface, or behind the tile?
Tell us whether it is on the grout and caulk or whether the wall feels soft when you press it. The first is an afternoon and a better fan; the second is a different job with a different price.
(608) 724-1919Free phone consultation.
The Ventilation Arithmetic
Most bathroom mold in Rock County is a ventilation problem, and the numbers are simple.
| Item | What is needed | What is usually there |
|---|---|---|
| Fan capacity | Roughly 1 CFM per square foot of floor, minimum 50 CFM | Undersized, or original to a 1960s build |
| Run time after a shower | 20 minutes | Until the person leaves the room |
| Where it discharges | Through the roof or a wall, to outside | Into the attic, or into a soffit that feeds it straight back in |
| Duct | Short, smooth, insulated through cold space, sloped to drain outward | Long flexible duct, sagging, uninsulated, full of condensate |
| Make-up air | Gap under the door | Sealed threshold, so the fan cannot move air |
A timer switch is the single best thirty dollars anybody spends on this problem. It removes the only variable that actually matters — whether the fan runs long enough — and it does it without asking anyone to remember.
It is a device for moving your shower into the roof space, where in a Wisconsin winter it condenses on cold sheathing. This is one of the most common faults in postwar housing here and one of the cheapest to correct. Go up with a flashlight and physically follow the duct to where it terminates. See the fifteen-minute check.
Behind the Tile: What Is Actually Back There
When growth appears at a tiled wall rather than on the sealant, the question is what the tile is fixed to. That determines whether this is a cleaning job or a rebuild.
| Substrate | Era | What happens when it gets wet |
|---|---|---|
| Paper-faced drywall (“greenboard” included) | 1970s onward, still common | Moisture-resistant is not waterproof. Once water is behind the tile the paper facing feeds growth and the board loses strength. Removal. |
| Cement backer board | 1980s onward, correct practice | Inorganic, so it does not feed growth. Usually cleanable and reusable if sound. |
| Mortar bed on lath | Pre-1960 | Very durable. Growth is on the surface, the structure is inorganic, and it is usually worth keeping. |
| Fiberglass or acrylic surround | Any | The panel is fine; the problem is the framing and the board behind it at the flange and the corners. |
The tell that water is behind the tile rather than on it: loose or hollow-sounding tiles when tapped, grout that stays dark for hours after use, a soft spot in the adjacent room's wall, or a stain on the ceiling below.
How to test a shower without opening it
- Dry the enclosure completelyThen do not use it for 24 hours.
- Flood test the panPlug the drain, fill the base to just below the curb, mark the level, leave for 24 hours. A drop means the pan or the drain connection is leaking, which is a rebuild rather than a repair.
- Then test the walls separatelyRun the shower against the walls only for 15 minutes, with the drain open. Check the adjacent room and the ceiling below afterwards.
- Check the valveMany leaks are at the mixing valve body rather than at the tile, and those show up behind the wall with no surface sign at all.
Redoing it so it does not happen again
- Cement backer board or a foam waterproofing board, never paper-faced drywall in a wet area
- A continuous waterproof membrane behind the tile, lapped into the pan — this is the layer that actually keeps water out; tile and grout do not
- Silicone at every change of plane (corners, wall-to-pan), grout only on flat surfaces
- A pre-formed or properly sloped pan with a bonded drain
- Exhaust sized and ducted outside, on a timer
Ceiling paint is not a fix
Mold-resistant paint contains a fungistat that protects the paint film. It does not protect the plaster behind it, and it does nothing about why the surface is wet. On a bathroom ceiling that gets speckled every winter, the answer is extract that runs long enough and terminates outside — paint buys you a season.
What You Can Do Yourself
Surface growth on tile, grout and sealant under about ten square feet is well within homeowner territory, and the EPA says so. Detergent and water, a stiff brush, gloves and an N95. Dry the surface thoroughly afterwards. Do not use bleach on grout or caulk that is colonized through — the chlorine bleaches the surface white while the water feeds what is underneath, which is why it comes back looking clean and then dark again in two weeks. Silicone that has growth inside it is replaced rather than cleaned.
When It Is Not a Bathroom Problem
Growth in a bathroom on an exterior wall, particularly in a corner or behind a fitting, may be a thermal bridge rather than a plumbing or ventilation issue — the surface is simply colder than the dew point of a humid room. That one is solved with insulation and air sealing, not with a bigger fan.
Frequently Asked Questions
Because cleaning removes the growth and changes nothing about the conditions. If the surface is wet for long enough, often enough, something will grow on it again — that is not a failure of the cleaning product. The three things that actually change the outcome are running the fan for twenty minutes after every shower, making sure that fan discharges outside, and replacing sealant that is colonized rather than scrubbing it. Do those and the cleaning holds.
Roughly one CFM per square foot of bathroom floor, with 50 CFM as a practical minimum, and more if the ceiling is high or there is a separate enclosed shower. Capacity is only half of it: a 110 CFM fan on twenty feet of sagging flexible duct moves considerably less air than its label. Short, smooth, insulated duct with a proper exterior termination matters as much as the rating.
If it is genuinely surface growth on grout and caulk, effectively nothing beyond your Saturday and a tube of silicone. A ventilation correction — a properly sized fan, ducted correctly through the roof, with a timer — typically runs a few hundred dollars. Opening a wall behind a shower to deal with a failed seal moves it into the $500 to $1,500 small-contained-area range, and more if the framing or subfloor is affected.
More urgent than most bathroom questions, because it means water has already passed through the floor assembly. The usual culprits are the toilet wax ring, a supply connection, or a shower pan. Turn off the supply to the fixture if you can identify it, stop using the shower, and get it looked at. Water moving through a floor structure wets framing and subfloor, both of which are considerably more expensive than a wax ring.