Hi Richard,
My kitchen double glazing is fine at 120x with a good 100mm scope, O.K. at 190x also, viewing birds on chimney pots at 124 metres. At a horizontal angle of 30 degrees. At least terrestrially on a bright day.
For planetary observations about 100x is the limit for the south facing window.
The other windows are not so good, but absolutely fine with binoculars up to 30x and beautiful with a PST H alpha telescope at 80x. Only 40mm aperture scope. Usually used at 32x. With good Seeing the resolution is amazing, so long as Sun is at around 20 degree elevation early morning or late afternoon.
Resolution deteriorates at above 30 degree elevation.
The double glazing is 1970s float glass uncoated and simple. I did not change it when advised to do so for more modern glass.
Heating must be off, and definitely better images than outside at medium magnifications during the day.
It depends on the actual glass. Some old windows are poor even with unaided eyes.
I note that modern double glazing is awful on reflection giving terribly distorted images, but in transmission maybe O.K. with binoculars.
With any window there are sweet spots where the resolution is better.
However, one must always beware of ghost images.
I once 'discovered' a faint diffuse comet with a 12x50 binocular. I had to make a round trip of ten miles to get my 20x80 binocular. I eventually realised it was a ghost image of Jupiter way off axis.
My previous 1960s plate glass single pane window was fine with any binocular, but I opened the window for 3 a.m. observations terrestrially over houses and parkland at 4.7 miles using a fine 120mm refractor. Resolution around 1 arcsecond on good nights. Magnification 250x.
It needed about 1 to 2 hours with the window open and I dressed carefully with an anorak to avoid my body heat affecting the view.
Ideally at 15C/16C or 60F ground and air temperature.
Viewing Mars in Horace Dall's attic high up on a hill in Luton with his 8 inch superb Maksutov at 400x was just mind blowing. If I hadn't seen it with my own eyes I wouldn't believe it. As good as my 12.5 inch Dall Kirkham outside.
This was through his selected and probably worked single plate glass window.
His 108mm f/30 Camera obscura was also world class, corrected for 4 colours.
Wall tested his 30 inch apertures optics at ten miles from I think his bathroom.
So it depends on the glass.
My friend, and I hope he doesn't mind my mentioning this, just bought a wall clock for about £6. He discarded the clock and uses the glass front window and its mount on his 8 inch Newtonian. He reports excellent star images at 150x with no sign of astigmatism etc. He will report further.
The optical window of my 12.5 inch Dall Kirkham, though, cost more than the main mirror.