A wall is dry when moisture readings in its framing, plaster and cavity reach the dry standard set at the start of the job, not when the surface looks dry. Plaster and lath hold water differently from drywall, and a dry surface can hide wet framing behind it.
Key takeaways
- A dry-looking surface does not show whether the framing and cavity behind it are dry, because moisture moves slowly out of wood and plaster.
- Meters measure the material itself, so readings are taken in the plaster, the lath, the framing and the cavity, not only on the finish.
- The dry standard is set from starting readings in comparable dry material, and the job is complete when the wet readings reach it.
- Readings are taken at the same points on each visit, so the trend shows whether the wall is drying.
- A wall that is closed up before its readings reach the standard can trap moisture and cause odor, staining or mold later.
Why does a wall look dry when it is still wet?
Surface moisture evaporates faster than moisture held inside plaster, lath and framing. A painted wall can feel dry while its core is still wet, because water moves slowly from the center of a material to its surface.
Wood and plaster hold water in their structure. Once the surface has dried, evaporation at the surface slows, while the moisture inside the material keeps moving toward the surface at a slower pace. A hand on the wall reads only the surface.
That is why a crew reads the material with a meter rather than trusting touch or appearance. The meter tells the crew what is inside the wall, and it shows when the inside has caught up with the outside.
How do plaster and lath differ from drywall?
Plaster over wood lath holds moisture in its base coats and in the gaps behind the lath, and it can soften or crack when it stays wet. Drywall is a gypsum board with paper facings, which dries faster but can lose its shape when soaked.
Plaster walls in older houses often have several coats and a lath base. Water can reach the lath through a crack or a gap, and it can collect behind the plaster, where a surface check will not show it. The plaster may also loosen from the lath while it stays wet.
Drywall that is soaked at the bottom can swell or sag, and the paper can separate from the core. Each material needs its own readings and its own decision about whether it can be dried in place.
How is the dry standard set?
The dry standard is set by taking readings in comparable materials that are known to be dry, and by taking starting readings in the wet area. The job is complete when the wet readings reach the standard and stay there across visits.
A crew compares the wet material with dry reference material in the same house, or in a dry area nearby, so the comparison is fair. A reading that is high in one wall can be normal for a plaster wall, and the crew uses the reference to tell the difference.
The standard is written in the drying log at the start of the job, and the owner sees it. A target that changes during the job without explanation is a sign to ask why.
What do the readings show over several visits?
Readings taken at the same points on each visit show whether the wall is drying. A steady fall toward the standard is the sign the drying is working. A reading that stops falling, or that rises again, means something is still feeding the wall.
A single reading tells the crew very little. A set of readings taken over several days, at the same points, shows the trend. The log records each visit so the owner can see the change.
If the readings stop falling, the crew looks for the cause. It may be a wet cavity that was not opened, a leak that is still active, a closed room with poor air movement or a material that is wet underneath the surface.
Why should a wall not be closed up early?
A wall closed up before its readings reach the dry standard can trap moisture behind the finish. That moisture can cause odor, staining and, in time, mold growth, and the damage is then harder to find and repair.
Closing a wall is the last step of the job, not the first. Insulation and new drywall or plaster should go back only after the framing and cavity are confirmed dry. The crew will tell you when the wall is ready.
If a stain or a smell comes back after a wall is closed, the moisture has not gone. Call the crew to take new readings and to find where the water is coming from.
What do the readings look like in a cavity?
Cavity readings are taken through a small access point, placed where it can be patched later. The crew reads the framing at the top, middle and bottom of the cavity, and it records the access point's location on the map.
A cavity is hard to read from the outside, so the crew opens the smallest access that reaches the framing. The access point is placed where the repair can be made neatly, and its location is recorded for the repairer.
The readings in a cavity matter as much as the readings on the finish. The framing can stay wet long after the plaster looks dry, so the crew reads it directly before it calls the wall dry.
What is the difference between a dry wall and a safe wall?
A dry wall has readings at or below the dry standard. A safe wall has no hazards left in it: no wiring damaged by water, no gas line exposed and no active mold. A crew confirms the first and tells the owner when the second needs a licensed professional.
Dryness is what the meter measures, and a crew can confirm it. Safety includes the wiring, the gas lines and any growth that needs a licensed assessor. The crew checks what it can see and reports what it cannot judge.
Ask the crew to write down what it checked in the cavity, what it found and what it recommends. That record protects you if the wall is opened again later.
Why do older walls take longer to read and dry than newer ones?
Older walls often have several layers, a lath base and framing that has been in service for a long time. Each layer holds water differently and releases it at its own pace, so the readings and the drying take longer than they would in a single layer of drywall.
A plaster wall can have a finish coat, a base coat, a lath layer, a gap behind the lath and the framing itself. Water in each layer leaves at a different speed, and the outer coat is often the first to read dry while the inner layers remain wet.
The crew reads each layer where access allows, and it does not treat the wall as dry until the inner readings reach the standard. That takes longer, but it is the step that prevents the odor and the staining that come back weeks later.
Can a wall be wet in a way that a meter does not show?
Meters measure moisture in the material they touch, so a reading is only as good as its point. A wall can hold water in a spot the meter did not reach, or in a material that the meter reads differently. Mapping, multiple points and visual checks reduce that gap.
A single reading at one point can miss a wet pocket a foot away. Crews use a grid of points across the wall, and they read the top, middle and bottom of each cavity, so a dry reading in one place does not hide a wet one in another.
Visual checks add to the meter. A stain, a bulge, a soft spot or a line of efflorescence can mark where water has been, and a crew uses those signs to decide where to read next. The meter does the measuring, and the eye tells the crew where to look.
What should I ask the crew to show me?
Ask the crew to show the readings, the points where they were taken, the dry standard and the trend from one visit to the next. A crew that can show all four is working from a record, not an impression.
Ask to see the map of the points, with each point numbered and the reading written against it. A map lets you see where the wet areas are and whether the same points were read on each visit. It also shows the places that the crew did not reach, which matters for the later repair.
Ask for the dry standard in writing and the reference reading it was taken from. Then ask what would cause the crew to continue drying or to stop. The answer tells you how the job will end, and it lets you check the crew's work against the standard it set.
Frequently asked questions
Why does the wall feel dry but the meter reads high?
The surface can dry while the material behind it is still wet. The meter reads the material, so trust the reading and ask the crew to show you the points where it was taken.
Can I use a household moisture meter?
A household meter gives a rough reading and can help you see whether a wall changes over time. It does not replace the crew's readings, its reference points or its log.
How long will an old plaster wall take to dry?
It depends on how much water got in and how deep it reached. Plaster and lath can take longer than drywall. The job ends when the readings reach the dry standard, not after a fixed number of days.
Will the crew open every wall?
No. The crew opens only the walls and cavities that the readings show are wet and need access. Each opening is small and recorded.
What if the wall smells after it dries?
Call the crew to check the cavity and the readings. A persistent odor can mean moisture is still present somewhere, and a licensed assessor should look at any sign of growth.
Who decides when a wall can be closed?
The readings decide, and the crew confirms them with you. A wall should not be closed until it is confirmed dry and any needed repairs to the framing are done.
Should I open a wall to check it myself?
Not without a plan. A cut in the wrong place can expose wiring or a gas line, and an open cavity can let wet material fall. Call the crew, which will open the smallest area the readings support.
This guide is general information for Schenectady County homeowners, not a substitute for an on-site inspection. Prices are estimates. See our disclaimer.