Are these cracks in crown molding being caused by foundation movement?
Many foundation repair contractor salesman will point to crown moldings and wood casing cracks and separations and say that they indicate foundation movement or floor sag, and structural repairs are needed. There is nothing farther from the truth. So, why do cracks and separations appear in crown moldings?
It's pretty simple, wood is hygroscopic and every piece of sawn or milled molding has a unique and specific direction or orientation of wood grain. The physical characteristic "hygroscopic" means that wood will always try to reach moisture equilibrium with the surrounding atmosphere. It will either absorb moisture (from the air) during periods of high humidity or off-gas moisture into the air during periods of low humidity. The wood moisture content is constantly changing throughout the year due to the changing seasonal weather conditions. And depending on where the molding was cut out of the tree or limbs, the molding has a specific wood grain pattern (the pattern of the tree's annual growth rings). The two most common grain patterns that arise from the production of sawn lumber and milled moldings are flat-sawn and cross-grain. Wood expands and contracts with changes in wood moisture content, swelling when wetted and shrinking when dried. These movements in wood take place tangentially and radially to the wood grain. This results in the distorted shapes shown in the attached figure. Flat grain lumber tends to warp and twist, while cross-grain lumber tends to maintain a stable profile. Hence, long, relatively thin pieces of crown molding with flat sawn grain are always wanting to distort according to their grain patterns as the moldings dry during the winter time (heating season) and take on moisture during the spring through fall (cooling season) and when windows and doors are often left open causing the interior humidity levels to rise along with the outdoor conditions.
After multiple attempts or years of caulking and painting wood moldings, the paint/caulk usually form a flexible covering which is strong enough to withstand the movements in the crown moldings due to humidity changes inside the home. This is because the crown moldings will eventually reach a stable or "mean" (average) moisture content throughout the year, based on each homeowner's unique living style, such that the ensuing wood movements that do occur, due to humidity changes, results in such little movement, that the caulked joints do not tear open or crack.