How to acclimate flooring properly, and the meter reading that says you can start

The instruction is usually given as a number of days: leave the boxes in the room for three days, or five, or a week. That framing causes more failures than it prevents.

Acclimation is not elapsed time. It is a moisture condition, and the only way to know it has been reached is to measure.

What is actually happening

Wood exchanges moisture with the air until it reaches equilibrium with it. The moisture content it settles at depends on the temperature and relative humidity around it.

Flooring arrives at whatever moisture content the mill and the supply chain left it at. The subfloor is at whatever the house has produced. If those two differ, the flooring will move after installation until they agree, and that movement shows up as gaps in one direction or cupping and buckling in the other.

So the goal is not to wait. It is to get the flooring, the subfloor and the room into the same condition, and then install.

The condition that has to exist first

Acclimating flooring in a room that is not at its normal living conditions achieves nothing, because the material equilibrates to conditions that will not persist.

Before boxes come into the space:

• Wet work is finished and dry. Concrete, plaster, drywall compound, paint and tile mortar all release large amounts of moisture, and a room that smells damp is a room that will change.
• The building is closed in, with windows and exterior doors installed.
• Heating or cooling is running at the temperature the household will actually keep, not at a construction setting.
• Humidity is in the range the manufacturer states in its instructions, which is usually a band rather than a single figure.

In an occupied house this is easy. In new construction or a renovation it is the step that gets skipped because the schedule is tight.

How to stack the material

Boxes stacked tight and sealed do not acclimate. Air has to reach the boards.

1. Bring boxes into the room where the flooring will be installed, not into a garage or a basement.
2. Open the boxes or cut the ends so air can circulate. Some manufacturers direct you to leave them sealed, and where they do, follow the instruction because their number assumes it.
3. Cross-stack with spacers between layers where the material is loose.
4. Keep the stacks off exterior walls and away from vents and direct sun.
5. Leave room to move around them, since you will be taking readings.

Taking the readings that matter

A pin meter or a pinless meter, set for the species, and a note pad.

Measure two things:

• The flooring. Take readings from several boards in different boxes and from different parts of the stack. Boards at the outside of a stack reach equilibrium before boards in the middle, and it is the middle that tells you whether the material is ready.
• The subfloor. Several locations, including near exterior walls and any plumbing.

The number you are looking for is not an absolute value. It is the difference between the two. Manufacturers publish a maximum acceptable spread between flooring and subfloor moisture content, and it is a small number, usually stated in percentage points and tighter for wide boards than for narrow ones.

Take readings on arrival and again every day or two. When the flooring readings stop changing and the spread against the subfloor is inside the manufacturer's limit, acclimation is complete. That may be two days or it may be three weeks.

The three ways this goes wrong

• Installing on the calendar rather than the meter. The most common. A crew arrives on the scheduled day and installs whatever the readings are.
• Acclimating in the wrong place. A garage or an unconditioned basement puts material into equilibrium with conditions the finished room will never have.
• Over-acclimating in the wrong season. Material left for weeks in a house that is unusually humid picks up moisture it will later lose. The band matters as much as the wait.

What to put in writing before the job

Ask the installer three questions and record the answers.

1. What moisture readings will you take, and will you record them?
2. What is the manufacturer's acceptable spread for this product?
3. What happens to the schedule if the readings are not there on the install date?

A contractor who answers those readily is one who does this work. Recorded readings also matter later, because a manufacturer's warranty claim on a moisture-related failure will ask for exactly that documentation, and a job with it is in a much stronger position than one without.