Glass Sliding Door Question

Single pane, 25 years old, bunged up sill? You've answered your own question. It's not insulated.

Generally the side jambs are on top of the sill and screwed into the studs, with interior and exterior finishes attached. I don't know of a real simple easy way to replace the sill without disturbing the sides. All things considered, I'd weigh out the labor involved if it is worth the effort save that single pane settup as compared to leaving it alone or installing a better unit, be it glass, french doors, etc.

If it is a single sheet it is not insulated. If it is two sheets is was indulated, but 25 years later, the seal may be gone. If you see any fogging, it is gone for sure.

I used to have one of those. I bought a new Pella door and got rid of the aluminum cheapo from the builder. Best move I ever made in that area.

A door like that likely is 1 R at best and wind infiltration is very high. A new door lowE Argon will be apx R 3.5. I relpaced my aluminum single glass and noticed a big improvement in heating - cooling costs and comfort. All are different on ratings Air infiltration, solar heat gain, condensing factor, R value etc, compare specs before you buy, Anderson has great specifications.

light a match.... hold up in front of glass

Lenny