A window is a controlled gap in an otherwise sealed exterior wall, which means its performance depends entirely on how well that gap is sealed rather than on the glass alone. Most homeowners focus on the glazing when they think about window performance, but the seal between the window frame and the rough opening in the wall does just as much work, and it is the part that is invisible once trim is installed. That seal is asked to hold up against the same freeze thaw cycling and wind pressure that affects roofing and siding, except at a much smaller scale where even a minor gap allows a disproportionate amount of air movement. Over years of that cycling, caulking dries out, weatherstripping compresses, and gas fills between double pane glass slowly leak out, all of which happen gradually enough that homeowners often do not notice until a draft or a heating bill makes it obvious. Because window openings sit inside the same weather barrier system as the siding around them, a window that was sealed poorly during a previous installation can also be a hidden source of water intrusion into the wall, not just an energy loss. That is why window replacement is treated as an extension of the same exterior envelope work as roofing and siding rather than as an isolated glass swap.