Shrinking Away Cellular PVC trim has a big “coefficient of thermal expansion”—meaning that annual temperature swings can make long boards grow and shrink an eighth of an inch or more, opening up unsightly gaps. In extreme cases, the shrinking plastic exerts enough force to break a glued joint or even snap a board.
Filling the Gaps If you’re trying to fix an existing problem, a very flexible and resilient caulk can be used to fill gaps that open up. Painting to mask the spot can help (PVC trim holds paint beautifully). But it’s only a partial fix: expansion and contraction will squeeze and stretch the caulk.
Controlling the Action As with masonry expansion and contraction, the best solution to PVC trim movement is to allow it, but control it. Use glued half-lap joints at the seams where you don’t want movement; leave the joints unglued to allow movement where you choose to allow it (such as behind corner boards).