Want to know the secret for wood to last forever in an exterior application? Many years ago, a colleague told me (tongue in cheek) that there are two ways to achieve this – (1) keep the wood dry and (2) don’t let the wood get wet. However, since that is not possible in exterior applications, and only a small handful of wood species have natural decay resistance, we need to use preservative-treated wood in exterior applications. It is important to realize the treated wood is decay and insect resistant and not decay and insect proof. By the way, the same can be said of fire retardant treated wood. The key for longer lasting treated wood is inspection and remedial treatments. This is true for the homeowner with treated decks or the utility company with treated poles. The inspection will identify the problem in its early stages and the remedial treatment will introduce an on-site preservative to stop the problem. The inspection must be performed by trained personnel to identify early stages of decay, if present, and then assess if the damage has compromised the structural integrity if advanced decay is present. Remedial treatments are excellent means of extending the service life of industrial products such as poles, piles, and cross ties. Homeowners can also use them for millwork, columns, decks, etc. A homeowner that is interested in remedial treatments should consider BoraCare (boron-based and very safe) or copper napthenate (very effective but has a strong odor). Homeowners can also purchase Bor8 Rods and Cobra rods. To use these products, you must first drill a hole in your wood, insert the rod, plug the hole. Bor8 rods contain boron and Cobra rods contain boron and copper. All of these products are non-restricted used pesticides available to the general public over the internet.
A common question among folks that have small portable sawmills is – how can I dry small loads of lumber? Kiln drying lumber is an excellent means to add value to the lumber and make a small sawmill more profitable. Many people that are interested in lumber drying don’t want to buy and operate a boiler and don’t feel that their operation is large enough to justify a dehumidification kiln. Dehumidification drying is an excellent method of drying small to medium quantities of lumber.
Another alternative is solar drying. A solar dry kiln is simple to construct and operate. The USDA Forest Products Lab has already done all the research needed on how to build and operate the kiln. The operator just needs to be aware of a few drawbacks. First, there will be little to no drying on cloudy days so your total drying time may take longer than other drying methods. Second, on sunny days you may have excessive drying so it is important to monitor the drying rate and use your fans and vents appropriately. Solar lumber drying is no different than any other method in that you will need (1) moisture meter, (2) sample boards to monitor moisture drying, (3) scale or balance to obtain weights of your sample boards during drying, (4) a drying schedule. Below is an excellent publication that summarizes how to build and operate a solar dry kiln. Let me leave you with a few final thoughts on solar drying. It is very important to understand that lumber can dry very fast or very slow in a solar kiln. It can dry so fast that drying defects such as warp and case hardening develop. Green wood that is put into a solar kiln has the potential to develop surface mold if the initial drying rate is too slow.
The moisture content of wood is one of the most important properties of wood along with density. To determine moisture content, we take the ((original weight – oven dry weight) / oven dry weight) x 100. Moisture content affects many wood properties such as strength, dimensional stability, acoustical properties, conductivity, decay resistance, and weight. A couple of topics to emphasize here are (1) dimensional stability and (2) decay resistance. Wood will shrink and swell as it gains and loses moisture from 0-30%. The length of the board will remain virtually unchanged but the other dimensions can change considerably depending on the species – more on that in another blog. Wood that is going to be used for interior uses needs to be dried to the equilibrium moisture content (EMC) of the area that it will be used. The EMC is a function of temperature and relative humidity. In most houses, it is 6-8%. So, if a piece of wood is placed in the house at 1% and another at 99% MC, they both will eventually reach 6-8% MC. The piece at 99% MC will not experience any shrinkage until the MC drops below 30% (fiber saturation point for most species). The moisture content can be determined directly, and more accurately, by using an oven and determining initial and ovendry weights or indirectly by using a moisture meter. Most moisture meters are much more accurate below 30% than above. Moisture content is also important for wood durability. Most, but not all, insects and fungi that attack wood prefer to attack wood that is above 30%. Wood that is kiln dried to 6-8% for interior use or 19% for studs has two advantages: (1) the kiln drying temperature has killed any insects or fungi in or on the wood and (2) the moisture content of the wood is now sufficiently low to discourage most insects and fungi.