You forgot the option of adding storm windows too.

Depends entirely on the condition of the existing window, the look that is tolerable to you, etc.

In an older wood sliding type window with single pane glass it is generally not possible to add insulated type glass as the sash will usually not allow it, and it would also more than likely be cost prohibitive. Storms or replace the windows would be the most effective way to go.

Keep in mind that if the weather is mild cost savings from better windows go down in value.

I've since learned from a repair specialist the glass can't be replaced in an old wood window...
My options are I can keep the single pane and add weather stripping, maybe storm windows, but not sure I have room for those definitely�"not at the patio door....
Another option is fiberglass which is supposed to look more like wood, and since my frame and sill and window is all white it should blend in...
I just hate throwing out windows, made of old wood, etc.
The vinyl I hear turns yellow over time, and the fiberglass is better, unless it is hit....

High quality vinyl windows do not turn yellow as they age. Their warranties specifically cover yellowing, splitting cracking, warping etc.

Fiberglass is a good alternative to wood and requires much less maintenence concerns.

What was your "repair specialist" indicating when he said the glass can be replaced? All single pane glass can be replaced, however, as Millworkman said, most older wood windows are not well suited to accommodate a dual pane IGU. The added thickness and required material removal from the wood sash often weaken the window to a point that is not acceptable to anyone else but the person selling the service.

Fiberglass is a fine option but will come at a premium but may give you the look you are looking for.

Good vinyl does not yellow so put that out of your mind for consideration.

Even the windows we sold in the early 80�s that I view as junk today never turned yellow. The only thing that turned yellow were the small plastic corners they used on the screen frames

thanks for all the input... a historical repair co. told me the vinyl will yellow but have heard differently from others who have it as you say.
Ican replace the glass with a 1/4 inch laminated one that will fit I'm told... fiberglass is recommended if I want the look of wood.... but I like the windows as they are... so perhaps I will keep and just weatherstrip and if that isn't enough add the lami glass later and if that still doesn't work, switch w/figerglass. All it is is money, time, work and energy!!?

Monolithic laminated glass will give you the same energy performance as a single pane of regular glass at the same thickness and with the same LowE / tinting options. There is no inherent energy advantage to using laminated glass.

"I've since learned from a repair specialist the glass can't be replaced in an old wood window..."

Very unlikely.

It can be a pain to remove old glazing, but very few windows have ever been made that cannot have the glass replaced.

What do you mean by "sliding single pane windows"?

Horizontal sliders or vertical?

There are plenty of weatherstripping options that can be added to wood windows to tighten them up.

Check Resource Conservation (Baltimre, MD) for their selection of 'flipper' seals, felt seals, etc.

yes, I've learned the glass can be replaced, not sure it's worth it; will possibly try the weather stripping first, they are horizontal sliding windows,

Are you confusing laminated glass (think car windshield) with insulating glass (think dual pane with an airspace between the lites)?

If you really are thinking laminated and not dual pane IG then why do you want to change the glass in the windows?