Start with the thing that makes the rest of this page make sense: nothing sees mold through a wall. There is no meter, camera or device that detects mold inside a cavity. What we can measure is water — and mold cannot exist without it. So the whole job is finding the moisture, and the moisture tells you where the mold is.
Anyone offering you a device that finds mold through drywall is selling you something.
What the wall itself tells you, before anyone comes out
You can do this part yourself, with no tools.
Look low. Water runs down, so the evidence usually collects at the bottom — staining above the baseboard, a faint tide line in the paint, trim beginning to separate from the wall.
Look for the wall moving. Nail pops, a seam that has opened, paint that has lost its grip in one area, flooring cupping along one edge.
And use your nose at floor level and inside closets on that wall. A musty smell that gets stronger as you get lower, or that is much stronger behind a closed closet door, is telling you something a smell at standing height does not.
None of that is proof. All of it tells you where to point the first tool.
1. Moisture meter — is it wet?
This is the first instrument and it settles most jobs.
If the wall is wet, there is mold in it or there shortly will be. Twenty-four to forty-eight hours of sustained wet is enough to start growth on paper-faced drywall, and the paper facing is food.
The meter is not finding mold. It is finding the only thing mold cannot do without.
A dry reading is useful but not final — you may be reading the room side of a cavity that is wet on the other face, which happens constantly with exterior walls and with anything next to a shower.
2. Thermal — is it colder than it should be?
A thermal camera does not see water and it absolutely does not see mold. It sees temperature.
Here is why that finds water anyway: evaporation cools. Water leaving a material takes heat with it, so a wet patch inside a wall shows up as a cold patch on the surface — often several degrees below the wall around it, in a shape that does not match anything a wall should be doing.
Cold spots on an interior wall have a short list of causes, and water is near the top of it.
It is not conclusive on its own. Missing insulation, a duct behind the wall and a cold exterior surface can all read the same way. What it does brilliantly is narrow a whole wall down to one area worth checking properly.
3. Borescope — look inside
If the first two do not settle it, we stop guessing and look.
A borescope is a camera about the diameter of a pencil on a flexible cable with a light on the end. It goes through a quarter-inch hole. We can see the inside of the cavity — the back of the drywall, the stud faces, the insulation — and photograph it.
That is the difference between an opinion about your wall and a picture of the inside of it.
And it is worth being clear about the cost of that answer: one quarter-inch hole, filled afterwards with a dab of compound, versus a four-foot opening cut on a hunch.
When the wall does have to come open
Sometimes the answer is yes, and then how it is opened matters more than most people realise.
Mold sitting inside a sealed cavity is largely contained. Cutting into it without controls is the single most effective way to move a problem from one wall into the rest of the house — you are releasing it into the air of the room and the air of the room goes everywhere.
Done properly, the area is sealed off, put under negative pressure so air moves into the work area rather than out of it, and the cut is made outside the suspect zone rather than through it.
That is also why "I'll just open it up and have a look" is the most expensive free decision in this trade.
What it means when you find nothing
A wall that smells but reads dry everywhere, and shows nothing on a camera, is usually not the problem.
It is usually the air conditioning system. A unit running wet, moisture coming off the coil and into the supply side, growth inside the ductwork, or a system that is not pulling enough humidity out of the house. The air moves through every room, so the smell turns up in a room with nothing wrong with it.
The wall is where you noticed it. It is not where it started.
Mold in your air vents Musty smell and nothing to see
Common questions
Can you kill mold without removing the drywall?
Sometimes, and the test is whether it has penetrated. Growth sitting on painted drywall that has not gone through the paint film can often be cleaned. Once it is into the paper facing — and the paper is the food source — killing it changes nothing structural. The material is compromised and it comes out.
Does the drywall have to be replaced?
If it has penetrated the drywall layer, yes. Not the whole wall — the affected section, cut back past where the moisture readings and the cavity say it goes, which is usually further than where the staining stops.
Can mold inside a wall affect the room?
A wall cavity is not sealed. It connects to the room through outlets, switch boxes, gaps at the top plate and the space behind the baseboard, and air moves through those with every pressure change in the house.