Q. We buy lumber from a reputable dealer, store it and then make various solid wood panel products and ship them to the customers, all over the United States. All too often for us, the customer reports in the wintertime some end cracks and some upward cupping, which we have to have repaired. Expensive! What is causing this and what can we do? Virtually all complaints are within the first six months.
A. Good question. We are 100 percent certain that the defect of splitting you mention is the result of drying after the product is made. (Oftentimes the splits at a glue joint are due to a weak glue joint due to a manufacturing error. Splits within the wood are actually pre-existing, but undetectable, splits in the lumber that reopen and worsen after manufacturing. Dry wood is too strong to split from a small moisture change. Better joints and better defect detection when cutting the lumber can indeed help, but still the issue is drying of the wood.)
Likewise, we are about 99 percent certain that cupping is because the wood is not dry enough, so it dries after installation. We might argue that the customer’s home is too dry for the wood’s moisture content (MC), but this is so rare, that we know the wood is too wet for the customer’s home.
Basics of wood
To solve this problem, let’s go over so basic knowledge about wood.
1. A little swelling (which is a gain in moisture) creates minor problems in most cases. A little (or even more than a little) shrinking (which is a loss in moisture) creates big problems with cracking and cupping. Further, if the moisture change is rapid, the problems are worse than a moisture change that takes six months (winter to summer or summer to winter).
2. Air at 30 percent RH will create 6 percent MC in wood, so we call the air as having 6 percent EMC conditions (equilibrium moisture content). A 6 percent increase in RH to 36 percent RH means that the EMC increases by 1 percent to 7 percent EMC; a 6 percent drop in RH means the EMC drops by 1 percent to 5 percent EMC.
3a. When outside air is brought into a manufacturing plant and the air is heated to comfortable levels for workers, its relative humidity drops to very low levels (under 6 percent MC in wood), even when the outside is near 100 percent RH or foggy. Oftentimes, the outside humidity is already quite dry. Check your own humidity outside by watching the local weather at noontime on TV. (Heated air around the finishing booth is extremely dry.)
3b. Note that this same affect occurs in homes that are heated. So, the EMC in homes in the wintertime runs about 30 percent RH average, or 6 percent EMC. If the lumber is not 6 percent MC, then it will change its MC when it reaches the plant or home.
4. For each change in EMC, the wood tries to shrink or swell by 1/4 percent, although when going from drying to an increase in MC, there is an almost 1 percent EMC "dead band." This means that wood that is fairly dry can easily tolerate a 1 or 2 percent increase in MC (12 percent RH increase), while any more than a 1 percent drop in MC (6 percent RH drop) creates shrinkage that can easily result in cracking. Further, cupping is the result of drying during or after manufacturing.
5. Wood will naturally cup toward the bark side of the lumber. Sometimes we hear that alternating the grain of the individual staves up and down will lessen the cup, which is partly true, but this approach fails to address the main issue. The main issue is that cupping means that the wood has dried further…that is, the MC was wrong. (See more below.) Having said this, another cause of cupping is that the face side is drier than the back side, or vice versa. An excellent finish, face and bottom, will prevent this.
Conclusions and what to do
So, the issues you mention are the result of having the wood dry further after manufacturing.
Step 1. Require your supplier to provide wood that is no higher than 7.0 percent MC. Likewise, wood no drier than 5.0 percent MC. Use a moisture meter (costing more than $250 and made in USA) to double check the incoming MC.
Step 2. You can have the correct MC in lumber coming into your facility, but the MC can change if you do not store the lumber at humidities in the low 30 percent RH range for lumber that you will using in the wintertime. If you cannot store lumber in a dry humidity-controlled environment, wrap incoming lumber in plastic when you get it. The plastic assures that there can be no loss or gain of moisture during any reasonable storage conditions.
Step 3. Check the moisture again when you begin to use it.
Step 4. Check the RH in your plant. Small battery power hygrometers are available that will do the job, but their life may be shorten by dust in the air.
Step 5. Wrap the fished product in plastic so that its moisture cannot change at all before it is installed.
Step 6. Include a note (large print on red paper) that states that your products are designed to be exposed to and used in 30 percent RH to 50 percent RH conditions. If the homeowner has drier or wetter conditions, there is no warranty. Further, you can make products for use outside these humidity levels if so indicated when the order is placed with you.
Step 7. To make higher MC products, make them in the summertime and then store them, wrapped in plastic for shipment at any time of the year to humid locations. Similarly, for very dry locations, even in the summertime, make dry wintertime products and then wrap them and store them until needed.
Step 8. Review your gluing procedures, including the length of time betweenez ripping and gluing of strips in a panel which should be around 15 minutes maximum for the strongest glue joint.
Step 9. If the incoming lumber has end splits that you cut off, make sure that you cut back far enough. The end of a split is often another inch or two beyond what can be seen.
Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.
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