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Recently there were comments on a WFB Facebook post with one of my favorite sayings, and those comments inspired me to write once again on a favorite topic: acclimation.
Because wood shrinks and swells when it changes moisture content, we must manage the MC of wood flooring before, during and after we install it.
BUT FIRST WE HAVE TO MANAGE THE MC OF THE JOB SITE ITSELF. Please get that building dry first. We manage MC by measuring it with a professional moisture meter.
With new construction, our greatest concern is to be certain that excessive moisture is removed before we even deliver the wood flooring. You can't know the MC just by looking or touching the wood framing and subfloor. And MC in the subfloor is the most important since any excessive moisture will be directly absorbed by the bottom of the flooring. Underlayment papers (and that’s another topic layered with a great deal of Bravo Sierra) may slow the MC down a little but won't stop it.
For optimum results, the max subfloor MC should be as follows:
- Eastern U.S. away from damp, humid coastal areas south of the Mason-Dixon Line: 11%
- Southeastern U.S. damp coastal regions: 13%
- Western U.S. arid regions: 9%
- Western coastal regions: vary from 11–14% depending on local micro-climates.
For optimum results, the wood flooring MC should be less than the max subfloor MC:
- Eastern U.S. away from damp, humid coastal areas south of the Mason-Dixon Line: 7.5%
- Southeastern U.S. damp coastal regions: 10%
- Western U.S. arid regions: 5.5%
- Western coastal regions: 7.5% to 10% depending on local micro-climates.
You can verify your local values by measuring MC in existing buildings that have wood floors at least three years old that look good throughout the entire year. Here's a map from the Wood Handbook that gives you a general guide of wood MC throughout the U.S.:
Figure 13-1 from Chapter 13 of the Wood Handbook shows recommended average moisture content for interior use of wood products in various areas of the United States.Wood Handbook
One of the mythologies of the wood floor business is that you can prevent problems if you just let the flooring acclimate for [fill in the blank] days before installing. Since most solid wood flooring is manufactured at 7% MC, the only areas that would really require any acclimation are the damp coastal and arid desert regions … and then only if the conditions on the site are properly managed.
If the MC of wood were related to a required amount of time, you should be able to measure it with a clock or a calendar. Not even the 1971 Playmate calendar will help you determine the MC of wood flooring. Once you master the fundamentals of psychrometrics, wood and building science, it is not that complicated.
Remember, with wood floors it's not a time thing. It’s a moisture content thing.
