Wood doesn't shrink or swell unless its moisture content changes.
Here are nine key points.
First and foremost, wood does not shrink or swell in use unless if its MC changes. Read this again, as it is so importnat. Wood’s MC changes when the RH around the wood changes. Of course, if the wood is at the wrong MC when first put into use, it will adjust to achieve equilibrium with its environment and therefore may shrink or swell initially quite a bit.
Second, another key point is that the MC of lumber, as well as the shop conditions, should be such that the wood, from lumber to finished goods, will change less than 2 percent MC after drying, in storage, in manufacturing and when put into service.
Third, temperature alone does not cause any significant size change in wood. Heat, however, does causes moisture changes to occur faster. Heat will also lower the humidity of the air and this drier condition can be an issue.
Fourth, the basic relationship between MC and RH is given below, with a third column for the equilibrium moisture content (EMC) which is a property of the air.
0% RH = 0% MC = 0% EMC
30% RH = 6% MC = 6% EMC
50% RH = 9% MC = 9% EMC
65% RH = 12% MC = 12% EMC
80% RH = 16% MC = 16% EMC
99% RH = 28% MC (approx.) = 28% EMC (approx.)
Fifth, most heated homes and offices will run 6% EMC in the wintertime or a bit lower. In the summertime, 9% EMC is common. In most of North America, the outside humidity averages 65% RH which is 12% EMC, summer and winter. In coastal locations, 16% EMC outside is common.
Sixth, when air is heated, its RH drops unless moisture is added (that is, the air is humidified). For example, if it is 30 degrees F outside and 100% RH = 28% EMC; snowing or foggy perhaps) and that outside air is brought into a home or office and heated, but not humidified, the following will be seen:
Heated to 40 F will result in 68% RH and 13% EMC.
Heated to 50 F will result in 47% RH and 9% EMC.
Heated to 60 F will result in 34% RH and 7% EMC.
Heated to 70 F will result in 24% RH and 5% EMC.
Note in the above numbers that a home with plants, cooking, bathroom showers and so on will add moisture, increasing the RH and EMC, so these values might be a bit low, but not much. Of course, using a humidifier will increase these values; offices are not humidified too often, although such higher humidity would help keep static off of paper used in high speed copy machines, etc.
Seventh, because a few pieces, or sometimes more, of purchased wood is often around 8 and 9 percent MC (I suggest 7.5 percent maximum MC) and a shop can run around 5 percent EMC during the wintertime heating system, some folks are tempted to increase the shop air to about 50 percent RH or 9 percent EMC. Indeed this will eliminate most or all moisture related problems in manufacturing, but it actually only postpones the problem until the customer gets the wood in his/her dry home or office. The bottom line: Do not over-humidify a shop. Run the shop at the same humidity that the customer will have.
Eighth, checking and recording on paper the MC of wood you are using and the RH of your shop using fairly inexpensive instruments is a prudent thing to do. It provides documentation to prove that you did the right thing regarding MC so that subsequent MC problems are not your fault--assuming you would not use wood that had incorrect MC readings and that you would not keep a shop at the wrong RH or EMC. (Sometimes this approach is called CYA.)
Ninth, once a piece of wood or a wood product is put into a fairly tight container, including wrapped in plastic or into a closed truck or trailer van (that is, liquid water cannot get in and there is little RH exchange with the outside), the MC will not change in transit or storage, regardless of the temperature. For 100 pounds of wood (perhaps 100 BF) to change by 2 percent MC will require the addition or loss of one quart of water...that is a lot of water to be brought into a container by merely exchanging a small amount of air with the outside. It would involve bringing in more than 1,000 cubic feet of humid air and this is just for one 100 pound piece of cabinetry, furniture or flooring.