Wood expands when it draws moisture, and contracts when it dries out. Boards expand and shrink more on their width, than on their length. The wider the board, the more it shrinks.
Minimizing Shrinkage
Our grandfathers discovered this shrinkage and expansion in wood many years ago, that is why when you see old hardwood flooring it is usually 1 1/2 to 2 inches wide. The narrow flooring will still expand and contract, but it is more spread out so you will have smaller gaps between boards.
Controlled Environment
In a home you have a controlled environment. In the winter when you heat your home the air dries out and therefore your hardwood floor dries out and it shrinks. In the spring and summer the air in your home is more like the air outside and the hardwood soaks up that moisture and expands. It is inevitable, wood will absorb the moisture in the air. No matter what you do, wood will always acclimate to the environment it is in.
Kiln Dried Vs. Air Dried
Some believe that the wood should be kiln dried. While others think that the air dried wood is far more stable than anything out of a kiln. It really depends on how quickly you want to bring your dried lumber to market. Air drying takes about a year or more, while kiln drying takes around 6-8 weeks.
Another factor is who you intend to market your lumber to. Musical instrument and gun makers prefer air dried wood, while furniture and cabinet makers often use kiln dried lumber. Instrument makers are looking for stability. The longer lumber has to dry the more evenly it dries making it more stable.
Kiln drying is also effective, if proper drying techniques are followed. You don’t want to dry the lumber to fast. The faster you dry wood the more likely it will shrink to fast, causing it to check, warp, or crack. Contrary to popular opinion, kiln dried wood has no advantage over air dried wood. At least as far as moisture content and stability is concerned. Kiln dried wood will gain moisture at the same rate as air dried wood.
Kiln Drying Pine
It is often necessary to kiln dry pine. Pine is heated to 160 - 180 degrees for at least 24 hours during the kiln drying process to set the pitch. This of course has to be done in a kiln. Doing this hardens the sap to prevent it from seeping out of the wood, it has nothing to do with moisture content or stability.
Learning how wood and water interact will make you better equipped to provide your customers with the best possible lumber.