I've seen numerous questions asking how dry wood should be for use in a smoker. I can't answer that because the answer varies. Some of the experts tell us to use wood that is very dry, others say to use wood that is slightly to very wet. But whatever wood you choose, here's some technical information on moisture content of wood. To better understand wood moisture content, there are two principles that need to be examined.
First Principle - Moisture Content
Green, fresh, uncured, live wood has a lot of water in it. Often there is more water (by weight) than there is dry wood. For example, a piece of wood might weigh ten pounds when wet, but only four pounds when thoroughly dry. Using the following formula to calculate wood moisture content, that piece of wood originally had a moisture content of 150%.
Moisture Content (mc) = (Initial Weight - Dry Weight) / Dry Weight * 100
Or
150% = (10-4)/4 *100
There were initially six pounds of water and four pounds of wood.
The mc of wood can be determined by two methods. The simplest way is to purchase and use a moisture meter. They are inexpensive, reasonably accurate, and are widely available in big box stores and online. Prices range from ~$15 to hundreds of $. Just turn it on, stick the prongs into the wood, and read the mc.
To absolutely determine wood mc, take a small to medium size piece of wood (green or partially cured) and weigh it accurately. I use a kitchen digital scale set to grams. After weighing the wood, place in it your kitchen oven in the convection (fan) mode set to about 200F. The oven fan circulates and vents the air, removing the moisture. After a period of time, weigh the wood again and put it back in the oven. Then weigh it again later and repeat the process until the wood reaches a constant weight. That is, no more moisture is being driven off. Then it's mc is assumed to be 0%. This may take several hours. Use the initial weight and the oven dry weight to determine what the mc of the wood WAS.
This method is impractical if not impossible to use for large pieces or quantities, so use the moisture meter for typical measurements. Oven drying wood is a good method to calibrate your hand-held moisture meter if you measured the mc prior to drying the wood. Compare the readings of the meter and the original mc of the wood. I use a piece that initially weighs about 100-200 grams. Large pieces may take a long time to dry, so it's not a practical bulk drying method. Air drying is the common way to dry wood.
Air-dried wood is usually drier on the surface than in the middle, so a meter reading of 15% on the outside may not reflect the mc of the middle. Only after a long time (months, a year) the wood should be equally dry all the way through.
So, how long does it take to dry (cure) wood? The answer depends. Doesn't everything? It depends on the temperature and humidity of the air and the size of the pieces. The drier and warmer the air, the faster the cure. Smaller pieces dry faster and air circulation helps speed the process. The general rule is that it takes hardwoods about a year per inch of wood thickness.
Second Principle - Equilibrium Moisture Content
The moisture content of wood depends on the relative humidity and temperature of the air surrounding it. If wood remains long enough in air where the relative humidity and temperature remain constant, the moisture content will reach constant mc at a value known as the
equilibrium moisture content (EMC). However, wood is like a sponge. If it reaches the EMC of, say, 12% under constant temperature and humidity and the conditions become more humid, the wood will absorb moisture and reach a higher EMC.
The EMC of wood can be calculated by using the information in this link:
http://www.csgnetwork.com/emctablecalc.html
Or you can get a general estimate for your area from the table in the link:
https://www.fpl.fs.fed.us/documnts/fplrn/fplrn268.pdf
These data assume that the wood is stored outside in ambient conditions.
Kiln dried lumber is usually dried to 6-8% moisture content. If the humidity is high and storage time is long enough, the wood will increase in moisture content, which can create problems in product manufacture or performance of an end product such as furniture and cabinet making. There are several ways to minimize this problem, including wrapping the lumber in a moisture barrier or store it in very dry conditions. For firewood, this is unimportant.
So if you live in hot, dry Las Vegas, the EMC of your wood can reach 4.0% during summer (from the table in the link). Conversely, if you live in cool, damp Seattle, the lowest EMC is about 12%. No further curing time will make it drier.
To check the validity of the calculator, I measured the temperature and relative humidity of my basement. It's pretty dry down there and the temperature is fairly constant and has been for years. I measured 60% humidity and 75F. That calculates to 10.9%. Then I measured the actual mc of several exposed studs with my moisture meter and came up with 9-11% mc. That's a reasonable reading under those conditions and represents what I can expect my drying firewood to be when fully cured (in the basement).
So when you're curing wood, don’t expect the mc to vary much from what the calculator or table says. After it reaches its EMC, it won't go much lower or higher.