Think of a set of brand new kitchen sponges. Each comes sealed in the packaging with equivalent moisture content. There IS some moisture in these items—we can see that over the life of the sponge and its behavior after several uses. The drying and wetting processes of normal usage expand and contract the sponge based on the equivalent moisture found in the item.
Consider the two pictures of the same sponge here. The heater is currently running, so the air inside this place is likely as dry as Death Valley (in terms of relative humidity). The heating system installed here has no humidifier, so the only source of additional moisture is a pot of simmering water that I occasionally keep on the stove.
It bears mention that other factors remain in play regarding the chemical composition of lumber. Thermal expansion and contraction is another force that should be considered when installing your product. Drive over a bridge of any length sometime. The expansion joints are necessary to keep the structure looking and performing more or less the same under a wide range of atmospheric conditions. On an atomic level, heat will generate greater elemental activity, making the matter expand. (Water is a funny element in this regard—Freezing this liquid actually causes an expansion of the matter at play.)
A person can get as scientific as they want when considering the affects of humidity on their installation, but the best policy remains the one that has been in play for hundreds of years: Allow ample time (we recommend a minimum of three weeks) to allow your product to adjust to its new environment. This isn’t perfect, but very few things ever are. This rule of thumb works very well for most exterior (and interior—the time may be sped a bit in this case) installations.
