Double up on the double pane glass = savings
 

I know this is unconventional, perhaps unpleasing to the eye? but I have thought about doubling up my big picture windows, by adding a 2nd double pane window to the window frame, from the inside. To make it money wise I would buy & install used windows.
I can buy 5ft x 4ft double pane windows for $80 from the recycled building supply store or from the news paper/online buy n sell from $20 & up.
The payback time would be a few years as compared to a few decades or more with bran new triple pane Picture windows.

It would certainly benefit most any houses energy consumption, Insulation for the windows,in a sense.
You could say good-by to your window sills, which is a plus in my opinion.

MetroMPG is thinking about doing this as well. My main concern would be moisture build up between the two windows. You'd ideally want to vacuum the space between the windows or something to get the moisture out of there. There is also the fact that doing this will reduce the VT (visual transmittance - the amount of light you get) through the opening. Otherwise, it should work rather well. Its basically the principle of the super high performance windows like Serious windows where they have 2 panes of glass and multiple sheets of plastic between the glass to create more chambers in the window to reduce heat loss.

That thought had crossed my mind, moisture/ fog between the two windows.
My immediate thought/remedy was to have a vent below the the window sill and one above the top of the window opening.
So on occasion i could slide the vents open and clear the air.
It may well be that fogging between windows may not occur, the vent could elevate it if it did, on occasion fog up.

This might be cheaper and would have a similar result, I think. It worked for Guy Mardsen. This is what I want to do.

Building interior window insulation panels

My neighbor has a 100 year old house with big pitcher windows and he has double pain windows installed on the outside as if they were storm windows, they look great and work great, not sure if they have drains on the bottom or not.

moisture drainage is an issue, I would have the drains open to the outside, that way any fresh air that gets in there will be dryer in those seasons when water could condense and the warm humid air from inside the house never gets in that space, to that point you also want to make sure that the windows are sealed well from warm humid air from the inside, making sure that the trim around the window is even caulked tight, everything is painted well and that the sill is not rotting.

Other option that might work is a tiny solid state dehumidifier in each space, you can buy a whole unit for $40 or buy the parts for about $15.

That would work well, The costs to do each of my three 4x5 windows would be $25 @ $1.25 sq.ft
The construction would be simple enough.

The double pane window would be easy and have a higher R value then the plastic window build. It would be a two man job, windows towed behind my Car on the utility trailer. They would fit in my window openings with no modification needed(standard size opening) just require proper sealing of the windows frame with the houses window opening.
It is superior ,IMO. But I still like your idea, it could be done by someone without a vehicle / spare cash or anyone to help move/install the windows.
A good option.

Unless he put the windows in upside down they would have the vents on the bottom( my neighbor hired a guy who installed his bathroom window upside down). Which has giving me good ? idea .
With the 2nd window mounted on the inside the vents would drain inside the house, if it was to condensate in between the two windows.
But if i mounted it upside down it may allow the windows to breath a tiny bit. From the outside bottom vents threw to the inside reversed drain holes
at the top, and prevent condensation. My theory, it would have to be actually done to know.

I have just remembered I have two twin 4x5ft full double pane windows giving to me by a neighbor who did not use them, instead opting for windows that could open. I could remove one window from the living room, put it on the inside of the other matching window and install the two free matching windows together. So i could have two 4x5 windows double double paned for free, or next to it.

Consider rebuilding the actual window itself, sealing the glass in place in the frames, and using something like schrader valves at each end, flushing the area between the glass with CO2 or Nitro. Inert gasses help prohibit heat transfer and will also help with moisture accumulation, etc.

I do like your tip Christ, or should i call you Jesus ?.
All kidding aside that may well work for in between the two double windows mounted together.

The windows could be glued in place sealing the inner frame work between the two windows. Then just fill the big space with Co2, threw the schrader valve attached via a hose into the top of the window sill.

I don't think it would be that more difficult to do then just installing the 2nd window.
Having windows with good seals to start with would help. A older window could be touched up with a sealer and filled as you described.
Just the double double panes would be killer helpful, and no fuss.
Simply place it in the window sill on top of a Silicon or rubber compound and its good to go.

The thing is, moisture will work it's way threw wood in to the dry space between the windows unless that space is 100% sealed.
Have you ever seen double pane windows that have a fog inside? that's from the seal going bad and moisture getting in between them.