there are several options, i would need to see the location for the most suitable, how big is the window/s 2 x 2 or 10 x 20 feet?.

food for thought, if has to be internal due to the listed building restrictions, a short rang ir ray may be the most suitable, but it would depend on things like curtains or ornaments, as they might be blown into it by wind etc. a narrow beam or 'curtain' pir/microwave 'duel tec', but need to be of good quality or again air movement could be an issue.

not keen on just a vibration sensor on the frame, as if the window is already open it is possible for an intruder to enter without causing enough vibration to trip it, also the window rattling on its stay if open, could cause false alarms in windier weather.

if you can go external, even using seperate polls away from the main structure if needed, the there are several dual beam rays or Gardtec's 'WatchOut' external detectors will likely do the job.

regs

alan

Not as a 24 hour/audible, if he normally leaves the window open then is it 100mm past the "normal position" hmmmm!

put yer tongue away - its 100mm for a contact LOL

(but not if the O/P is diy`ing - other disclaimers are available LOL)

you sure you can`t fit grills on the inside of a listed building? we did