Drying Oak Lumber: Step 7, Storage

The Wood Doctor discusses how to protect dried lumber from damage during storage. August 27, 2012

From Professor Gene Wengert, Sawing and Drying Forum technical advisor:

How to store oak lumber without risk
Itís taken a while, but you now open the dry kiln door and look at the wonderful, valuable, flat, bright kiln-dried lumber. In fact, if you look again and look hard, you will likely see a deposit slip for your local bank with a large dollar value going to your account. But wake up - it is not a dream, but you do still have to treat this valuable wood product with care to avoid losing the quality you now have.

We have several risks that we need to be aware of when we store kiln-dried lumber. These risks include fire, theft, flood, change in moisture content and insect damage. Kiln-dried lumber (under 15% MC) cannot check or split (it is too strong), cannot warp much anymore (it is too strong; any warp will occur only if the MC changes), cannot stain (stain needs moisture) and cannot mold, mildew or decay (these fungal activities need moisture).

In this article, we will discuss protection from moisture content changes and insects. Fire, theft and flood protection are left to others.

What is the correct MC?
If we want to protect our investment, the first step after opening the kiln door is to double check the final moisture, usually checking about 30 pieces from various locations in the kiln. We usually use an electric moisture meter with a correction for species and wood temperature, as appropriate.

The correct MC for kiln drying is what your customer wants. But most of the time, that decision is left to you as a kiln operator. The customer does not know what is best.

So, as a basic guideline, oak lumber is most useful to most people if the lumber is between 6.0% MC to 7.5% MC. At this moisture level, there are a few times and cases where the lumber might dry a bit further and shrink a little bit (but not enough to cause any problems), and there are a few cases where the lumber might be used in a more humid situation and gain moisture. In most of North America, hardwoods in use range between 6 to 9% MC. As gaining moisture is less of a problem in most cases than losing moisture, we shoot for the drier end of this range for the final MC when kiln drying.

Maintaining the correct MC
The correct moisture content level is maintained in kiln-dried lumber by storing the lumber at the appropriate relative humidity. Temperature has no bearing on the moisture level, although at cold temperatures any moisture changes do occur more slowly.

The following chart provides the appropriate relative humidity level for a desired moisture content. Actually, to help us, the moisture condition of the air is called the equilibrium moisture content (EMC). This means that at 6% EMC, lumber will maintain itself at 6% MC forever.

0% 0%
30% 6%
50% 9%
65% 12%
80% 16%
99% 28%

It is simple to maintain the desired moisture content simply by storing the lumber in a room that has the same EMC. This EMC could be obtained by having an electric humidification system to add moisture to the air or an electric dehumidification system to remove excess moisture from the air. If the room is well sealed, and if the lumber is at the correct MC initially, the size of a humidifier or dehumidifier will be quite small as there is little work to be done.

A simple system can be used by using only heat. The basic principle for these systems is that when air in a closed room is heated the relative humidity and the EMC will be lowered. When the air is allowed to cool , the RH and EMC will increase. In fact, for most locations in North America, the outside air is around 12 % EMC, so it will be necessary to add no more than 15 degrees F to the air to reduce the moisture level to 7% EMC.

Even though this is a small amount of heat, using insulation for the walls, floor and roof is a good idea. In fact, with a black-colored, south-facing roof, it is possible to gain enough solar heat during the day (probably do not insulate the roof and also do not use a metal roof as sometimes condensation that forms on the cold roof at night could drip on the lumber) that supplemental heat needs will be minimal. (A small fan running during daylight hours to blow the heat from the ceiling and circulate this air throughout the room is probably necessary.) Such a solar-heated storage building may even qualify for government subsidies or tax savings.

Protection from insects
If the kiln was operated at temperatures over 130 degrees F for any length of time, the lumber coming out of it is sterilized and free of insects and their eggs. If the lumber is under 15% MC, there are only a few insects that will find such kiln-dried wood tasty and will infect it after the wood leaves the kiln.

For hardwoods, the threat is from the lyctid powderpost beetle (PPB). This insect makes 1/32" to 1/16" diameter holes in the wood. Unfortunately, once the insect infects wood, it can be a year or more before the insect and its holes appear on the outside of the wood. To reduce the risk of getting this insect in kiln-dried wood, the storage area should be kept clean and free of any sawdust or wood debris. This area must also not have any other wood in it, other than your own kiln-dried wood. Wetter wood and wood from foreign countries are often carrying the insect, so this type of wood should never be in contact with your kiln-dried wood. (Be sure that any handling equipment is also free of sawdust and wood debris.) The walls of a storage building are usually softwoods, and softwoods are not attractive to this insect. Hardwood walls in the storage room would not be a good idea.

For softwoods, the major threat is from the old house borer. Again, following the guidelines for the powderpost beetle will essentially eliminate this risk.

Both hardwoods and softwoods are subject to attack from termites. If there is a risk in your area that these insects are active, a commercial pest control company should be contacted. Your local county extension office can often help with information about termite control.

In spite of your best efforts, you may find you encounter the PPB. Fumigation, using a commercial service, or heating to above 130 degrees F are the two options to eliminate the insect. The old house borer is usually restricted to one or two pieces of wood, so removing and destroying any infected wood is often all that is required. If insects are discovered, a thorough cleaning of the storage area is also essential.