Q: Hardwood floors sometimes shrink and show cracks between the pieces when the indoor air is dry. Should I insist that my flooring contractor let the flooring acclimate inside the house before nailing it down?

A: A WOOD FLOOR THAT'S INSTALLED properly (and under proper indoor conditions) should handle seasonal changes in atmospheric relative humidity without visible problems. Solid wood flooring systems are designed to let individual pieces move slightly as their moisture content rises and falls from season to season.

And while excessive moisture changes can sometimes open up large cracks between boards, the conventional wisdom that wood flooring should be left in the building to equilibrate with the ambient air, or “acclimate,” before the floor is installed may be oversimplified.

“Someone started this mantra of ‘acclimate the floor for two weeks,' or 10 days, or five days, or whatever it is,” says Howard Brickman of Brickman Consulting in Norwell, Mass. “But that assumes that the house is in the condition it should be, and it assumes that the flooring is not, and it assumes that 10 days or two weeks is actually going to make a difference in the flooring. And all three of those assumptions are incorrect.”

VAPOR RETARDER: A layer of #15 asphalt felt paper between the subfloor and the flooring  helps slow any moisture transfer.
VAPOR RETARDER: A layer of #15 asphalt felt paper between the subfloor and the flooring helps slow any moisture transfer.

A contractor and consultant with 30 years' experience in the flooring trade, Brickman studied wood science at the University of Massachusetts and conducts training seminars for organizations such as the Wood Floor Guild and Inspector Training Services for Floor Coverings (ITSFC). Acclimating the flooring on site, says Brickman, is “wrong-headed.” Instead, his crews keep their material in his climate-controlled warehouse and bring only as much wood to the jobsite each day as they can install that day.

QUALITY STANDARDS To begin with, Brickman observes, almost all wood flooring on the market is kiln-dried, shrink-wrapped, and delivered at 6 percent to 8 percent moisture content by weight (the industry standard). That's an ideal condition for service in the Midwest and Northeast markets where hardwood flooring is most common, and it's very close to the ideal condition for any climate in the United States.

“Most people only install in one region,” notes Brickman, “and they only have to get familiar with the correct moisture content for their own area.”

In very dry regions such as the high Western plains or the Southwestern deserts, wood flooring should be a little drier—no higher than 6 percent moisture content. And in the very humid South, from coastal Texas around the Gulf Coast and up to about North Carolina, 9 percent to 11 percent moisture content is correct. Smart floor installers in those regions, says Brickman, store their material in local warehouses until it has adjusted to the local climate.

But rules of thumb about how long to let wood acclimate make no sense, argues Brickman. “I tell people, ‘Lay your watch on a stack of lumber. Now what is the moisture content of that lumber?' Well, of course it's not related. Time is not part of the moisture thing. You measure moisture with a moisture meter, not a clock.”

ATMOSPHERIC VARIATIONS And anyway, Brickman goes on, stacking flooring in a building to adjust to indoor conditions is based on misconceptions. To begin with, indoor relative humidity fluctuates throughout the year, mostly in response to the outdoor climate. HVAC systems do affect indoor conditions, he acknowledges—“but let's face it, unless you're running the Museum of Fine Arts or the clean room at Intel or something, you don't have precise control of humidity.”