The last ten years in the wood flooring industry have seen an incredible growth in the diversity of wood species available for flooring. Because of this, consumers need to understand something important, flooring will behave (in terms of seasonal movement) many different ways depending on the specie you choose. It is also important that when making purchases, that the seller has a thorough understanding of the material and how it will behave in differing environments, such as radiant heating, concrete, or even different areas of the country.
Below I have placed a picture that diagrams what wood flooring cells look like up close. Wood is kind of like a sponge essentially and the area that gains and looses moisture is the lumen. Lumen thickness can vary quite a bit depending on the specie of wood. For example, many exotics have larger lumens than traditional domestic flooring like red oak. This means that exotics generally have more potential for shrinking and expanding in comparison.
So if you take an exotic wood such as Ipe (brazilian walnut) and assume that it’s acclimation will be as rapid as traditional rules for oak, then you are making a mistake. A wood like Ipe can take two to three times longer to acclimate than a domestic like red oak.
A second factor regarding wood floor stability is width. The formula for determining wood floor expansion or contraction for solid flooring is:
Percentage moisture change in flooring x dimensional change coefficient x width of board
Let’s do a problem for example. Let’s say that our 5″ wide hickory floor lost 3% moisture after installation. I chose hickory because it is generally less dimensionally stable than other flooring.
3 x .0041 x 5 = 0.0615 Doesn’t seem too bad, but in reality that’s about 1/16″ for each board. I can tell you reality is that the gaps may be even bigger depending on the other factors involved, that I will discuss in the next blog.
I have yet to meet anyone who pays for a 5″ hickory floor (or any wood floor) who will accept a uniform 1/16″ gapping across the whole floor. Looking at the example, you can see if we change certain variables, that we can improve things. There are only TWO variables in the equation that we can influence if you are set on a hickory floor: width of the board or the percentage in moisture change. Let’s investigate this further:
Width of the board– This is relatively simple because you can just order a narrower floor. However, the width of the boards in your floor has a big impact on the appearance and wider boards are in demand. To counteract the movement, a second option to consider is to buy an engineered floor in which the hickory is integrated with a cross-ply base layer. As you will see in the video I linked, it is certainly possible to buy a really high quality engineered floor that has a wear layer equivalent to traditional solid flooring.
Moisture change- If you have your heart set on a wide board and a solid flooring, then you need to understand proper acclimation is key as well as maintaining an optimal living condition for the floor. If you are installing a flooring with questionable stability that is solid and wider than 3″ I recommend that you allow closer to one month of proper acclimation. If you are remodeling or building a new house, the proper acclimation requires:
Fully operational heating or cooling system.
Windows installed, siding on and gutters operational
Insulation and drywall are completed and doors leading to the exterior are installed.
Once you have all of these factors, then the subfloor and the hardwood floor must be within 2% moisture content to one another for boards 3″ wide or wider. The rule of 4% difference only applies for narrow floors and this is often mis-referenced by those selling or installing wood floors.
After the floor is installed following proper acclimation then you should understand that maintaining a ideal interior heat and humidity balance will help prevent seasonal movement. The chart below shows what “ideal” is indicated by the yellow highlighting. On the chart you will see temperature and relative humidity (RH) are on each axis. Relative humidity is a very big deal for wider floors and I’ve written about it before. The chart below illustrates how the relationship works:
I realize that this post seems lengthy, but I felt it is important for consumers to understand that all the pretty samples on the rack at your local flooring retailer require a larger bank of knowledge than we tend to expect from sellers of flooring. Demanding knowledge from your wood floor provider means more than being upsold on warranties for finish. If you are about to embark on building a new house or doing a major remodel, then I urge you to possibly make sure that your well qualified flooring contractor, builder, and architect meet each other a whole lot earlier than you may have previously planned.