Many customers ask us the deference between kiln dried and green material, and if kiln dried material is worth the added time and cost to get. Here is some information to help you make that decision for your project.

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What Kiln Drying Does:

The Kiln drying process is used to quickly reduce the moisture content of lumber in a controlled environment to reduce warping. This is most useful in the wood furniture and flooring industries. Although, any size lumber can warp, if green material is secured in place, as it naturally dries, it can prevent warpage.

What Kiln Drying Does not Do:

'KD stamped' lumber only indicates the moisture content of the the lumber at the time of manufaturing. Wood is a hygroscopic material. It constantly interacts with moisture in its environment. The cells of the living tree will still absorb or release moisture as necessary to find a balance with its environment.

If the surrounding air is higher in moisture content (relative humidity), wood will absorb some of that moisture. If the air has lower moisture levels, the wood will release moisture. This process will continue back and forth until the wood has reached a point of balance with the surrounding air.

Why is that so important?

Wood may have most of its moisture removed during the kiln drying process, but it does not lose the ability to reabsorb moisture from its environment. Whether from a direct water source, moisture in an adjacent material or even humidity in the air, by its very nature, wood will be ready to absorb any moisture it contacts. Kiln drying does not “fix” the moisture content level of a wood product; it merely reduces the amount of moisture to a generally workable range. Wood’s moisture content will always be a variable, not a constant. In other words, paying for kiln drying your material to install it over a bridge or a pier would be redundant, as the natural composition of the marital would cause it to absorb the surrounding moisture and put it back into its warp-able state.

At any step of the process after the kiln, transportation, manufacturing, storage, or installation, it is possible for wood, even dried manufactured wood flooring, to lose or re-absorb ambient moisture and have its moisture content level, and possibly even its dimensions, change again. This can present some very real challenges for you as the end user, particularly if you assume that kiln-drying alone is sufficient to protect from moisture-related problems.

This is why your packaged wood flooring manufacturers tell you to leave your new flooring in the room it is to be installed, for a specific length of time, prior to instillation. So it can 'acclimate' to its final location. Otherwise, the material can expand or shrink causing buckling or gaps in your new floor. This is related to the moisture content and temperature of the room. (Humidity)

Think of a sponge. When a sponge is wet, it holds moisture in each hole or cell. As it dries, it not only releases that moisture, it shrinks in size, and may twist or curl. It does not, though, lose its capacity to reabsorb water when it is present and allow the sponge’s physical dimensions to change each time it absorbs and releases moisture.

Types Of Wood Warping

Different forms of warping can include cupping, twisting, bowing, kink and crooking.

• Cupping occurs when the edges of the board rise up while the center is concave in shape.
• Twisting takes place when the board’s length causes it to turn or rotate.
• Bowing is a condition along the width of the board which causes its curves.
• Solid wood boards can be affected by kinked warping, which produces a non-straight shape. This occurs when the fibers along the length of the board twist, bend, or kink.
• Crooking is a similar bending of the panel but it is along its diagonal axis.

Timber grades and their effect on warpage.

Now days a lot of timber is grown for volume, harvested early, and visually graded as sawn lumber. This can increase the chances of warping. New lumber is kiln-dried at the sawmill to 19% moisture content (moisture content), but it still takes about 120 days for the lumber to dry down to an “equilibrium moisture content” of 7-10%. As it dries, some of that lumber will still warp, bow, or cup. But, in a wet environment it can revert back to a state that may allow warping.

So, what can you do?

The secret on how to dry wood without warping is to monitor moisture, ensure ventilation, store wood horizontally, add weight or secure it in place with anchors, dry slowly, use stickers, and seal end grains. These tips will increase your chances of a straight, stable, and beautiful finished piece of wood for your projects.

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