Floor Show

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.

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.

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.”

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.”

In the high Western plains or desert, home interiors may be dry year-round. Homes in predominantly heating climates such as the upper Midwest or the Northeast experience their driest times during winter, when the heat dries out the house. In the South, the driest indoor conditions come in mid-summer when the air conditioners are cranking. But flooring installers work year-round—so even if floors are installed when the HVAC is operating, only through coincidence would indoor conditions bring the wood to the desired moisture content. And, says Brickman, wood flooring is typically installed in an unconditioned house, exposed to the same humidity as the outdoors.

More to the point, says Brickman, wood flooring absorbs moisture very slowly from ambient air: “In my personal experience it takes three to four weeks for a totally cross-stacked material to reach equilibrium with a stable environment.”

On the other hand, wood flooring will gain moisture quite rapidly if it's placed in contact with a damp subfloor—an all-too-common occurrence, says Brickman. In fact, he maintains, almost all problems with wood flooring result from the flooring being installed in a house built with wet materials—and in that case, “acclimating” the material can even hurt.

Framing lumber is typically shipped at about 17 percent moisture content, notes Brickman. “And then as we are building, the house gets rained on, and all the components in the rough structure get saturated. Then we take this beautiful wood flooring, which is kiln dried to between 6 percent and 8 percent, and we nail it to a plywood subfloor that was soaking wet a few weeks earlier.”

That moisture load from the subfloor typically dwarfs any vapor pressure from the house air, says Brickman. “Just look at the physical quantities of moisture,” he explains. “Say we've got a room 10 feet by 10 feet, with a 10-foot ceiling. That is a thousand cubic feet of air. And let's say its relative humidity is 75 percent—a reasonable summertime number. If you run the numbers, there is a little less than a pound of moisture in that room air—in terms of volume, a little less than a pint of water.

“Now compare that with the moisture in the hardwood flooring,” Brickman continues. “Red oak flooring weighs about 3 pounds a square foot, oven dry weight. That 10-by-10 hardwood floor, bone dry, would weigh 300 pounds. Wood moisture content is expressed as a percent of oven dry weight—so at say, 7 percent moisture content, that wood floor would contain 21 pounds of moisture. To raise it by another one percentage point—which would still be within the standards—would require adding 3 pounds of moisture. And there's less than 1 pound of moisture in that room air. Even if every bit of the moisture from the room air entered the wood, you would raise the wood moisture content by less than a third of a percentage point.”

Now, says Brickman, suppose the subfloor is at 14 percent moisture content—not unlikely. “That means it contains not 21, but 42 pounds of moisture—more than 40 times the moisture in the room air. And the moisture will always travel from wet to dry—if you nail strip flooring down and bring it into direct contact with that subfloor, moisture is going to move into the flooring at a very rapid rate.”

The moisture will cause the wood flooring to swell asymmetrically, as the floor's underside gains moisture faster than its upper face. The floor may lie down flat again during the heating season, or it may remain partly cupped, says Brickman. “Sometimes you get some creep, where the boards set in a cupped shape and you can't do anything about it.” On the other hand, sometimes the installer may sand a cupped floor down flat, only to have the wood return to its original shape when the floor dries out—leaving the floorboards with a convex, “humped-up” shape—a “crowned” surface.

Even if they don't cup severely, boards that take up moisture from the subfloor may swell significantly. So flooring that is spread out on the floor to acclimate when the subfloor is damp, then nailed in place, can shrink drastically in a later season and show excessive gapping between boards. In some cases, floors may “panelize”—several courses of flooring may stick together and shrink as a unit, with extra-wide cracks opening between sets of boards. That's caused by insufficient nailing, says Brickman: With close enough nail spacing, floors are unlikely to panelize, regardless of what finish is used.

Brickman offers this formula for success with strip floors: First, use a moisture meter (Brickman recommends Delmhorst meters) to make sure the flooring is at the proper moisture content (6 percent or below in the dry West, 6 percent to 8 percent in the temperate Midwest and Northeast, or 9 percent to 11 percent in the very humid South). Also use your moisture meter on the subfloor: It should be no higher than 8 percent in the dry region, 11 percent in the temperate region, and up to 13 percent in the humid region. If the subfloor is too wet, don't install the flooring.

In any case, installers should place #15 asphalt felt paper on the subfloor as a vapor retarder before nailing down flooring. The felt paper slows any intrusion of moisture from below and gives the subfloor a chance to dry downward rather than up. Red rosin paper is not a substitute for asphalt felt, says Brickman: “It doesn't do diddly-squat.”

Finally, Brickman adds, be sure to use enough nails. Serrated power-grip nails tend to hold better than staples, he believes, and 3 to 4 inches between nails is a good distance (although industry standards allow spacing as great as 8 to 10 inches). “As long as you aren't splitting the tongue,” he says, “you aren't spacing them too close.”

Brickman's prescription may confuse installers who have followed the acclimation rule. But contrarian as it sounds, Brickman's advice is actually not far from that of the industry mainstream. Timm Locke, executive vice president of the National Oak Flooring Manufacturers Association (NOFMA), agrees that the moisture content of the flooring and the site are the critical concerns, and that the recommended values vary by region. Installers need to acclimate their material to average conditions for their area, says Locke, but “whether this occurs off site in a controlled environment or on site once the site conditions are appropriate really doesn't matter.”

And Locke says NOFMA's manual is being revised to call for flooring to be acclimated to regional climate conditions—but not necessarily on site. “We recognize that in the real world of new construction it's often not the case that site conditions are ready for acclimation to occur five or seven days before the builder wants the floor installed,” Locke says.

But in remodel situations, Locke maintains, five to seven days of on-site storage before installation is usually beneficial. “In these instances, site conditions typically represent a controlled environment where acclimation on site can be achieved and will result in proper moisture content for the occupied condition.”

HOWARD BRICKMAN Owner, Brickman Consulting Norwell, Mass. hbrickman@earthlink.net