No matter how well we plan things, it always seems that at times we have a little too much lumber that we cannot use promptly. (Or perhaps we should say that sales often do not exceed planned production?) As a result, we end up storing lumber for a day, several days, or even months. Let's look at the risks involved and also the best storage procedures.
All fungi that cause problems with wood require moisture. Therefore, kiln-dried lumber that is under 22 percent moisture content has no risk of having any active fungi, including rot or decay fungi (there is no such thing as dry rotting of wood; the wood must be wet to rot), blue stain or sap stain fungi, mold fungi, and mildew fungi. Keeping KD lumber dry will eliminate any fungi risk.
Temperatures above 130 F kill all insects and their eggs. As almost all kiln drying operations dry at temperatures above 150 F, the wood is sterile when it leaves the dry kiln. Insects do not like wood much under 10 percent MC, so very dry lumber is well protected. Kiln-dried lumber over 8 percent MC has a risk of being infected by just a few insects the ones you may encounter are the lyctid powderpost beetle (1/16 inch diameter holes, sometimes seen as long as two years after drying, mainly in grainy hardwoods like oak), termites (rarely a problem in North America), and the old house borer (softwoods only, mainly pine).
These insects can get into stored KD lumber when the lumber is in close proximity to previously infected wood. This means that if the storage facility is kept clean and free of wood debris and the incoming lumber is freshly kiln dried, the risk of insect infestation is nil. Be very cautious of bringing lumber or wood products into the dry storage area that have not been kiln dried, or whose storage history after kiln drying is not known. Be especially careful with tropical species; they may have gotten infected in transport.
Fumigation to control the powder-post beetle and the old house borer is an expensive but totally effective treatment; this must be done by commercially licensed people. Reheating the wood for several hours so that internal temperatures exceed 130 F is also effective and can be fairly inexpensive. Both of these treatments stop existing activity within the wood.
Chemicals put on the outside of the KD lumber are not effective in controlling continuing internal damage, may not last long enough to affect the establishment of new infestations, and may create environmental hazards when the lumber is planed, sanded, and handled.
Although these insects, especially the lyctid powderpost beetle, can be devastating to a particular operation, they are not that common.
Moisture content change
It is likely that the damage and cost of storing kiln-dried lumber in conditions that result in moisture content change far exceeds all other storage losses.
Most often, the MC change is a gain of moisture in properly dried lumber. Wood that is dried to a given MC must be stored at a relative humidity (RH) that will maintain the MC at the correct level; temperature is not important. For example, at 30 percent RH, wood will achieve 6 percent MC. Therefore, at 30 percent RH, we say that the air is 6 percent EMC equilibrium moisture content. Here are a few key values for most of North America:
30 percent RH = 6 percent EMC (typical wintertime interior condition)
50 percent RH = 9 percent EMC (typical maximum summertime condition)
65 percent RH = 12 percent EMC (typical outside condition, summer and winter)
80 percent RH = 16 percent EMC (typical outside coastal condition).
Anyone who stores kiln-dried lumber should have an inexpensive, electronic RH sensor in the same room in order to monitor the room conditions. Record this data and keep it on file for at least a year so that you can prove you stored the wood correctly.
The biggest problem with changes in MC is that wood shrinks and swells. As a rough rule of thumb, wood shrinks or swells 1 percent in thickness or width when the wood changes by 4 percent MC. Some species, like oak, shrink a little more; some, like teak, shrink less. Another problem with changes in MC is that gluing and machining are likely to be more difficult, especially if the wood is drier than desired. Further, if the MC is wrong, then it is likely that the finished product may shrink in use, resulting in cracking, finish failure and warping.
In short, it is critical to store dried lumber at the correct EMC at all times. Short deviations of a day or two are permitted, but then the wood must be returned to the correct EMC. What is the correct EMC? It is the same EMC that your customer will have in their home or office. For most of North America, this is an EMC of 6 percent in the wintertime and 8 percent EMC in the summertime. Coastal climates will have higher EMCs and dry desert or mountain climates will have lower values.
For anyone purchasing lumber, always measure the MC when the lumber arrives. (See this column in Feb 2005 CabinetMaker for moisture measuring techniques.) Checking the MC and rejecting any wet lumber will eliminate the chance that you might purchase lumber that is actually at the wrong MC. If it is at the correct MC when it arrives, and you store it at the correct EMC, you will have perfect lumber when you are ready to use it, and you will make wood products that will have almost no risk of shrinking, swelling or warping in use. The correct MC is the MC that the customer will have in the finished product. In other words, the ideal incoming hardwood lumber is 6.0 percent MC to 7.5 percent MC in most cases. For softwoods, we require slightly higher values to ensure good machining 8.5 percent MC to 10.0 percent MC.
The RH can be controlled in various ways. It certainly helps if the storage area is fairly well enclosed; a simple room with plastic sheets for walls and ceiling is fine.
In most cases we are concerned about storage conditions that are too humid. An electric dehumidifier can be used to lower the RH of the storage. Perhaps the easiest way to lower the humidity in a storage facility is to use heat. When the conditions are too humid, increase the temperature a few degrees and that will quickly lower the RH; a rough rule of thumb is 2 degrees will result in 5 percent RH decrease.
When storage conditions are too dry, moisture can be added to the air using steam injection or water mist spray. Commercial units are widely available. However, make sure that you find someone else using the same equipment and that they are satisfied with its performance before you invest in humidifying equipment. Not all equipment works well.
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