First of all, why are they called “sash” windows? It’s probably a shortened form of double hung, sliding sash window. A sash, when you’re not wearing it around your waist, is simply the frame that holds the pane of glass in a window. A casement window – one that is hinged – has a sash, but we don’t call it a sash window.
(Even more puzzling is when we call a mortice lock a sashlock (as opposed to a deadlock). Whereas a deadlock like a 3G114 just has a deadbolt operated by a key, a “sashlock” like a 3K74 has a latchbolt as well as the deadblolt, operated by a handle. If you have wooden front and back doors, chances are that you’ll have a deadlock (and latch) on the front door, and a sashlock on the back door.)
It seems to me that double hung, sliding sash windows have no redeeming features. They are complicated in that the frame has to have boxes with weights and pulleys to compensate for the weight of the sashes. These ropes break; or they get painted over by unversed decorators and become inflexible. When a rope breaks the weight falls off, disappears inside the box along with the rope, and the sash won’t stay up.
And they seem to go out of alignment more than casement windows do. I guess that’s because they tend to be bigger. This is particularly a problem where houses move a lot – like London. Any two part window locks that you fit will soon become misaligned and cease working.
For this reason we usually fit sash stops. These are simple little posts, removeable with a key, that screw into the sides of one sash and prevent the other sash sliding past. Usually they are fitted so that an opening of up to 10cm is possible for ventilation. If you are going for these, you just need to check that your insurer is happy with them. They should be. My insurer even has a sash stop pictured in the explanatory security booklet.
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