Let’s walk through what is actually happening, from most common to least.

Your window frame is typically made of wood or vinyl. The wall around it is drywall, wood studs, and caulk. Wood expands when humid and shrinks when dry. Your house’s framing settles slightly as lumber loses its initial moisture content. These tiny, natural movements create stress at the weakest point: the corners of the window opening. A crack running diagonally from the upper or lower corner of the sill is almost always just truss uplift or seasonal shrinkage . It’s cosmetic.

Here is the proper story.

Most homeowners notice them eventually: fine lines spreading from the corners of a window sill, or a vertical crack splitting the drywall just beneath the frame. The immediate fear is always the same: “Is my foundation failing?”