Installing window unit

My old house has a picture window unit: 2 doublehung windows border a 54x60 inch plate glass window. The center glass is not insulated and 50 years of condensation has rotted the sill and underlying framing such that the glass in sagging. I plan to remove the center glass panel, replace any rotted framing studs, cripples, etc., and install a new insulated glass panel.

How to I install this unit? Does it sit on some sort on cushioned sill?. We get temp ranges of -15 to +105 degrees so it needs to be framed right!

Thanks for your advice.

 

Usually with the arrangement you have, the center glass is still mounted in a wood sash, though sometimes it IS just a pc of plate glass held in by some 1/4 round or equivilent stop material..... but on most, the sill extends all the way from one end to the other, under both the double hungs that flank the sides of the picture......IF the sill is rotted as bad as you say, I personally would vote for taking the entire window out and replacing it.....because trying to pc in just the center would be fairly hard, plus if you have that much rot in the center, the side windows PROBABLY ain't in that good a shape either.

IF, when you get the center glass out, you find it IS mounted in a sash frame, and THAT is what has mostly rotted, you could go a couple routes:

Get a wood replacement sash of the same size with double pane glass already mounted in it.

Get a vinyl sash and rest same as above

Get a vinyl picture unit, which will have a mounting frame + the glass in a sash.

Get just a 60x54 ( or probably 53 5/8"ish, as the measurement at the top of the sloped sill is less than 54 usually) double glazed unit and mount it in the hole using wood stop inside and out to hold it. It can sit right on the wood, though I would use some clear silicone to seal it against the stop on one side.