Structural drying for walls, framing and subfloors

Evaporation needs moving air, dry air and some warmth. Air movers break the damp layer on a surface, dehumidifiers remove what evaporated, and the building gradually gives up its moisture. Too few dehumidifiers simply move damp air around the house.

Dense materials dry slowly. Hardwood, plaster and concrete can take much longer than drywall, and cold or humid conditions slow things further. For realistic expectations, read how long drying takes.

Refrigerant dehumidifiers work well in warm, humid rooms, the conditions of most summer losses. Desiccant units are often chosen for cold spaces or dense materials, since they can reach very low humidity. Heat can speed things up in some jobs, but it must be used with care around older finishes.

In crawlspaces and wall cavities, air may need to be directed with ducting or injection systems instead of relying on room air.

Lath and plaster hold moisture and can lose their bond if dried too aggressively. Stone cellars keep absorbing moisture from the ground, so their readings may never match a modern wall. A good contractor sets a realistic target for materials like these. If your house was built before 1978, ask how they will handle lead paint when opening walls.

A complete drying log lists the date, location, material, reading and equipment used. Keep it with your claim file, as it may matter if a problem appears later or when you sell. For related decisions, read wet drywall: save or replace.

If the floor looks dry but a room smells damp, feels clammy or has swollen baseboards, drying may not be finished. Fans alone are the usual culprit. They dry surfaces while moisture remains in framing, subfloor and insulation.

A professional uses a dry part of the house as the reference, then measures wet materials until they match. That is the honest finish line, and you should see it in writing.

Read how to find hidden moisture and wet drywall: save or replace.

Drying is a contest between the water in a material and the air around it. Moisture leaves a surface when the air next to it holds less water than the surface does. So a good setup changes three things: it replaces the thin layer of damp air against the material, it keeps the room's air dry, and it keeps the material reasonably warm.

Take any one of these away and the job stalls. Air movers without dehumidifiers raise the humidity in a closed room until evaporation slows. A dehumidifier without airflow dries the middle of the room while the wall stays wet. A cold basement or a house in winter can stretch out a job until the temperature is managed.

Those conditions can be measured. Temperature and relative humidity combine into a figure often called grains per pound, or specific humidity, and comparing that figure in the wet room, in the unaffected part of the house and outside tells a technician whether the dehumidifiers are keeping up.

Most drying jobs keep windows and doors closed so dehumidifiers can do their work. Open windows in muggy weather simply feed the machines more moisture. The exception comes when outside air is truly drier than the air inside, which can happen on a dry winter day. Then a technician may vent the room for a while, on the basis of readings rather than guesswork.

Containment is a related choice. Plastic sheeting around the wet area can focus equipment on a smaller volume of air, which can be more efficient. It also helps protect the rest of the house when the water has been contaminated. Tell the crew if containment blocks a route you need, such as a stairway or a bedroom door, and they can plan around it.

Drying machines draw substantial current, and house circuits are not sized with them in mind. Expect the crew to distribute equipment over several circuits. If a breaker keeps tripping, do not reset it repeatedly, and tell the contractor. In a home that has had water around wiring, an electrician may need to clear circuits before they can be used at all.

Generators may be used if the power is off. CDC says never to run gasoline, propane, natural gas or charcoal devices inside a home, basement, garage or camper, or close to an open window, door or vent, because carbon monoxide is odorless and can kill suddenly. CDC also warns against connecting a generator to home circuits without an approved automatic-interrupt device.

Delays usually have a reason you can ask about. Hidden wet material found midway through is the most common one. Water that keeps arriving, from a slow leak or wet ground, can make readings level off. Humidity in the house may also stay too high if there are too few machines, or if machines keep getting moved or switched off. The EPA points out that materials wet for more than two days tend to grow mold, which can change the plan.

Look at the log for a trend. Readings should fall from one visit to the next, then flatten as they near the target. Flat readings early on, in an area that is still wet, suggest a problem to be solved rather than more waiting. Ask what the contractor plans to change. See how to find hidden moisture for where else water can be sitting.

Faster is not always better. Wood dried too rapidly can check, crack or twist. Plaster can lose its bond with the lath, and fine finishes on furniture and floors can craze. Musical instruments, antiques and art kept in the room may react badly to rapid changes in humidity, so move them out or ask how they will be protected.

Dry air can also pull moisture from materials that were never wet, leaving hairline cracks in paint or gaps in floors. A good contractor watches the unaffected areas as well as the wet ones, and adjusts settings as the job nears its finish. Tell them about anything fragile at the start.

Once readings hit the target, get the final numbers in writing before the repair crew starts. Let the repair contractor see them too, particularly where wood flooring is going in, since installers usually have their own requirements. Moisture in concrete deserves special attention before any glued-down floor is laid, as trapped moisture is a well-known cause of floor failure.

Keep an eye on the area for a few weeks after the repair. A change in smell, new staining or paint that bubbles may mean that something was missed. Keep the hygrometer running, with EPA's suggestion of below 60 percent humidity, and ideally between 30 and 50 percent, as your benchmark. For related advice, see mold prevention after water damage.

Bigger buildings raise issues that a house does not. Water may pass through several units, so each needs its own readings and access arrangements. Fire-rated walls and ceilings may have rules about how they are opened and how they are closed up, and building management should be told before anyone cuts into them. Shared mechanical systems can move moist air between floors, so ask whether a system should stay running.

Timing also has to work around residents and tenants. Noise and locked doors can slow things down, and a machine in a corridor may need clearance from the fire marshal. An agreed access schedule is usually the most useful thing a manager can provide.

A crawlspace dries badly because air barely moves and the ground beneath it can be damp. Standing water comes out first, then any wet insulation under the floor is usually removed. Next comes a ground cover, along with ducted air and a dehumidifier sized to the space. Fix the drainage if water came from outside, or the crawlspace will get wet again.

This is tight, dirty work, so leave it to people with the right protection. Look for damp joists and subfloor from below during the process. Since there may be pests, old insulation or exposed wiring in there, ask for photographs showing what was done. The emergency crawlspace page has immediate steps.

Where you live changes structural drying. See Oregon, Connecticut, Delaware, Kansas, Louisiana, New Hampshire, Chicago, Dallas, Colorado Springs, Cedar Hill, Lake Stevens, Norcross, Port Charlotte, Prosper, Atlanta, Indianapolis.

Go straight to the first steps: emergency structural drying.