Secondary double "glazing"

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Dorset
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Our house is Listed, so we can't change five of the windows. They mist up in winter - have to be wiped every day - and obviously also allow the heat to escape. The opening lights are not very draught-proof, either.

Upstairs
are three three-light casements, the centre light is hinged. Each of the three windows is about 1350 x 1000 high between reveals. One room has another (double glazed!) window on the opposite wall, so may be completely sealed; the other two need to have the opening light usable as a fire escape.

Downstairs
are two sash windows, each about 1380 x 1550 high between timber reveals. Both may be sealed.

What I am thinking of, for the three windows I can seal, is to just fix a sheet of clear acrylic, with draught-proofing strip round the edge of the face, and held in place by a small bead panel-pinned to the timber reveal, then painted.

Or maybe even, instead of that bead, a 10 x 20 strip all round, then another sheet of acrylic, THEN the bead, so it's triple glazed with a 20mm gap.

For the other two, that have to have the opening light usable, I would do the same, but add a vertical mullion on each side of the opening light, and then make a hinged frame for the acrylic sheet for the opening light.

Anyway... my question is - whaddya think ? Has anyone tried this ?

*I* am not too bothered about the look of the thing - the edges will be behind the curtains anyway - but Senior Management is not very happy with the idea.

AND.. crucially - what thickness of acrylic should I use ? The price varies with thickness, so 4mm would be twice the price of 2mm.

Or should I use glass ? It would have to be toughened, as the sills are quite low. Or should I use sealed double-glazed units - MUCH more expensive ? Should I really install frames ?

And finally ;) what's the best way to set about fixing the sheet so that the trapped air is as dry as possible, to prevent misting on the now unreachable glass ?
 
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I have seen this done before.... using 6mm plate poly that is UV stable and a magnetic tape . 6mm due to the flex you will get on 2mm or 4mm , Uv stable for reasons that if it isn't it will go yellow in the sunlight and in a few years look horrible , and the magnetic tape , as if you put a small handle on it , you can the remove the panel to a/ escape if needed , and b/ clean it!

You can also get very good proper secondary glazing, that could be liftouts, horizontal sliders or even balanced vertical sliders ...and in this type of system you could incorporate a Low e pane of glass ( Pilkington K ) to help with heat retention.
your biggest problem is always going to be that if the existing windows are not sealing very well, then you WILL get moisture and creepy crawlies ( spiders mainly ) between whatever you put in place and the original windows .... I would always advise something removeable because of cleaning ...and also the need to get out in an emergency
 
Thank you, Ronnie. Yes, I know you can get "proper" secondary glazing - but it's expensive... :(
But thanks for the rest of your advice.
 
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