How to Acclimate Flooring and Confirm It Is Ready

Why this matters

"Let it sit three days" is the most repeated sentence in flooring and the one that causes the most gapping, cupping and peaking. Flooring moves after install when it goes down at different conditions from the ones it will live at: wood takes on or gives up moisture and changes width, and laminate and rigid core grow and shrink with temperature and humidity. A day count does not measure either. A floor that sat three days in a cold garage, or in a house with the HVAC off and fresh drywall mud on the walls, is no closer to ready than the day it arrived.

This card owns acclimation for the shelf. Acclimation means the flooring has reached the conditions it will live at, with the HVAC running, confirmed with a meter. A day count applies only where the flooring maker states one, and even then it is a minimum, not the test.

The gate

Flooring is ready when all of these hold:

1. The space is at living conditions: permanent HVAC running at the temperature and humidity the occupants will keep, within the range the flooring maker states.
2. The flooring has been in the rooms it goes in under those conditions, stored the way its maker says.
3. The reading that matters for this product says it has arrived: for wood, moisture content that has stopped changing and meets the match to the subfloor; for laminate, rigid core and vinyl, the room and the material inside the maker's temperature (and, where stated, humidity) range for at least the maker's stated time.

Everything below is how to get each of the three and write it down.

Step 1: Get the house to living conditions before the flooring arrives

The permanent HVAC is on and set where the customer will keep it, and has been long enough for the room to settle. A space that has not reached living conditions cannot acclimate anything; it is just storing the flooring.

In new construction and remodels this is where it goes wrong. Fresh drywall compound, paint, plaster and a recently poured slab put a lot of water into the air, and the readings will be high until the building dries out with heat and air running. Portable propane and kerosene heaters make it worse: burning those fuels produces water vapor, and an unvented heater in a closed house is also a carbon monoxide source. If the only heat is a temporary unvented heater, the house is not at living conditions and the crew does not work in it with that heater burning.

Most flooring makers state an operating range for the room (commonly a temperature range around normal indoor comfort and an RH band); read it for the product and write it on the log.

Step 2: Deliver into the rooms it goes in

Stage the flooring in the room where it will be installed, or the nearest room at the same conditions, not in a garage, a basement or a porch. Keep it off a slab on spacers or on its pallet, and away from exterior doors, supply registers and windows with direct sun. Delivery and staging are run by 'The Flooring Delivery and Staging SOP'; this card picks up once it is in the room.

Step 3: Store it the way its maker says

This varies by product and the maker's word governs. Solid wood is commonly opened or cross-stacked with spacers so air reaches every board; some engineered and many laminate and rigid core makers say to leave cartons sealed and lay them flat, because the carton keeps the planks flat and clean while they come to temperature. Do not open what the maker says to keep closed.

Step 4: Record the room, every day it waits

Read and log room temperature and RH with a thermo-hygrometer at least once a day, at about the same time, until install. What breaks if you skip it: a floor that fails in its first season with no log cannot show it was installed at living conditions, and the warranty question usually goes against the installer.

Step 5: Read the material, not the calendar

Wood. Meter it, as 'How to Take and Log Wood Moisture Readings Before Hardwood' lays out, with the right species setting, on the same numbered boards each time. It is ready when its average has stopped changing and meets the match to the subfloor in 'Wood Moisture Content and the Match to the Subfloor'. Shop default for "stopped changing": two readings of the same boards at least a day apart differ by no more than 0.2 points of MC.

Engineered wood. Follow the maker. Many publish their own moisture window for the product and the subfloor and their own reading method, because a meter on an engineered board reads mostly the face layers and the plies below behave differently from solid wood. Where the maker is silent, meter it like solid wood on the species of the face and apply the same stability default.

Laminate, rigid core and vinyl. These are acclimating mostly to temperature. Use the maker's stated minimum time in the room, if it states one, and check that the room has stayed inside its range for that whole time. An infrared thermometer on a plank pulled from the middle of a stack confirms the material, not just the air, has come to room temperature.

Tile and stone. No moisture acclimation, but mortar, grout and adhesive all have application temperature ranges on their bags, and a pallet of thinset stored in a freezing garage is outside them.

One gate, two deliveries

Delivery A: rigid core plank in January

A 600 sq ft rigid core floor delivered off a cold truck. The maker's instructions, read for this product, state a minimum of 48 hours in the room, cartons flat and sealed, with the room between 65 F and 85 F during acclimation and after.

The room stayed between 69 F and 70 F, inside 65 to 85, for the whole 48 hours. The plank started 28 degrees below the room (69 minus 41) and finished 1 degree below it (70 minus 69). Gate 1, 2 and 3 all pass at 48 hours: the maker's stated time, the room inside its range throughout, the material at room temperature. Ready.

Had the crew installed at arrival, the planks would have gone down at 41 F and then warmed about 28 degrees. Rigid core grows as it warms, and a floor laid tight to its expansion space when cold closes that space as it warms up, which is how a floor peaks at a doorway in its first week.

Delivery B: solid red oak plank in a humid spring

800 sq ft of 3-1/4 inch solid red oak plank, plank width so the match is 2 points. The house runs at 70 F and about 40 percent RH with the HVAC on, which holds wood near 7.7 percent MC on the USDA Wood Handbook's table. The subfloor reads 8.4 on average. The flooring arrives from a humid warehouse reading 10.8 on average, ten numbered boards from five cartons, opened and cross-stacked.

Day 2 passes the match. 9.9 minus 8.4 leaves 1.5 points, inside 2. A crew working the match alone installs on day 2.

The gate says no. Between day 2 and day 4 the average fell 9.9 - 9.1 = 0.8 points, four times the 0.2 point stability default. The wood is still drying toward the house's 7.7. Installed on day 2, the boards would lose another 9.9 - 7.8 = 2.1 points in place. Using the Wood Handbook's dimensional change coefficient for flat-sawn red oak across the width, about 0.0037 per point of MC, each 3-1/4 inch board shrinks about 3.25 x 0.0037 x 2.1 = 0.025 inch, about 1/40 inch (between 1/64 and 1/32). Across a 12 foot room (144 inches / 3.25 = about 44 boards) that is about 44 x 0.025 = 1.1 inches of total shrinkage, showing as a gap at nearly every board.

Ready on day 12. From day 10 to day 12 the average moved 7.9 - 7.8 = 0.1 point, inside 0.2, and 7.8 sits beside the house's 7.7. The flooring is 0.6 points below the subfloor, inside 2. All three gates pass.

Why the two resolve oppositely. Delivery A was ready at its maker's day count because the measured material confirmed it. Delivery B passed the match on day 2 and was still not ready, because the reading that matters, MC that has stopped moving, said so. The calendar was never the test for either.

What changes the answer

• The house runs outside the maker's range (a home kept at 25 percent RH all winter, or a cabin with the HVAC off for months). No acclimation period fixes that; the customer's humidity control does, and the conversation happens before install, in writing.
• The subfloor is the outlier. If the flooring reaches the house and the subfloor reads high, the subfloor is out of step with the building; that is a subfloor problem, covered in the match card.
• Glue-down on a slab. The slab's own moisture test is separate and still required; acclimated flooring does not make a wet slab dry.

Finish test

The log shows the room at living conditions inside the maker's range for the whole acclimation period, the material reading that applies to this product (stable MC meeting the match for wood; maker's time with material at room temperature for rigid core and laminate), and the date and initials of the person who called it ready.

References

• See related: Wood Moisture Content and the Match to the Subfloor
• See related: How to Take and Log Wood Moisture Readings Before Hardwood
• See related: The Flooring Delivery and Staging SOP
• USDA Forest Products Laboratory, Wood Handbook, for equilibrium moisture content and dimensional change coefficients.
• Flooring maker's installation instructions for storage, minimum time and room conditions.