Adding custom secondary glazing to existing frames

Question

I have deep windows frames like this:

Rather that adding a whole secondary glazing frame on the inside. Could I add a spacer/battening to the inside of the existing frame and second pane of glass? I could hold the glass in place with either more battening or glaziers putty. So if the window has 3 seperate panes, I would do this for each.

Sounds like it would work to me. What are your concerns? Poor insulation, condensation? The spacer will be visible from the outside and make the window slightly smaller but I think I can live with that.

Answer 1

I actually did a similar thing custom plant room I built with 12” wide 4’ tall windows on 3 side (about 20 total). Standard glazing but thought I should make these double pane so I cleaned the inside and added the inside glass well sealed and it looked wonderful, made the room much warmer. The first year I noticed some slight condensation smiles on the windows, by the 3rd year mold growing between the glass (the frame work was all cedar so a natural inhibitor) I pulled the inside glass on the 4th year and was going to add desiccant blocks to absorb the moisture but in that short time the cedar had rotted out the mold had covered it so it was not obvious the extent of the damage. I replaced the bottom sill and added a thin copper plate and sealed everything up but left a weep hole. That lasted until a few years ago we decided to remove the room because the package heater had died and it was going to need a new roof. The bottom spacer between most of the windows was spongy but it was not bad for 25years, make sure to allow moisture to escape and I would suggest a metal sill or covering so it won’t won’t rot out as quickly as my first attempt did.

Answer 2

If you are in a "mostly heating" climate you need the outside pane (or panes) vented, and the inside one sealed to prevent moisture build-up in the space between window panes. If adding panes inside this means you will need to deliberately alter the (normally sealed) single pane you have in place now. Exterior "storm windows" are or were commonly used which are not tightly sealed, and being on the outside require no alteration to the original window, as well as being removable in summer.

If in a mostly cooling climate, outside sealed, inside vented.

Do appreciate that each pane of glass reduces your light transmission as well as heat loss - it normally is of adequate benefit to be worthwhile at "single-to-double pane" but it's not a no-consequence change.