Even manufacturers and retailers don’t know the true purpose of sash locks. On one popular big box web site, the description reads, “keep your windows from rattling.” On the back of the package of reputable hardware company, it says, “Keep intruders out.” While these qualities are nice benefits, they are not the main purpose of a sash lock.
One of the ingenious aspects of the double-hung window is that when it is locked, it is sealed tightly against infiltration. This locking is facilitated by the wedge shaped upper and lower meeting rails. When the sash lock is engaged, the two wedges push against each other, causing the top rail to push against the head jamb and the bottom rail to push against the sill. In addition, the bottom sash pushes against the parting strip and inner stop, and the upper sash pushes against the parting strip and blind stop.
One of the flaws of the old double-hung window system is the blind stop is not adjustable, like the inner stop. However, this problem can be addressed with a high-quality storm window, or some other weather sealing method.