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I have deep windows frames like this:

enter image description here

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.

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  • A) I'm not sure what you mean by "deep" window frames. B) A picture of your actual windows will be much more helpful to us and, frankly, would have probably been easier for you, than a random copyright image from the internet. A tight shot like you posted won't give away any more identifying details about you/your house than your profile picture does.
    – FreeMan
    Dec 3, 2020 at 16:42
  • I meant that the distance from the pane of glass to the edge of the frame is approx ~30mm - the depth of the bottom rail on one side. That gives enough space for a spacer (~10mm), a pane of glass (4mm) and a space for a moulding or putty (~16mm). I would take a photo but I'm not at the property. Thanks.
    – Simon
    Dec 3, 2020 at 16:56
  • Ed Beal's answer addresses the primary problem with this--moisture entrapment. Modern double-glazed windows are made with sealed units. Without reliable seals you're destined for disappointment.
    – isherwood
    Dec 3, 2020 at 17:01

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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.

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  • Thank you for your thorough answer. Did your second solution of the copper plate last for 20 years? I was thinking I could line/wrap the batten with self-amalgamating/fusing/vulcanising tape to ensure if there is moisture trapped in the panes it only compromises the battening, not the frame. Or did the moisture come from the environment?
    – Simon
    Dec 3, 2020 at 21:08
  • Yes the second one lasted for 25 maybe 27 years. I believe the moisture came from the environment, I had sealed both pieces from the outside and I installed the second inside ones during a hot week in the 90’s both times but the second time I put the weep hole in it had “smiles” of condensation but not all the time usually after a warm day following a day where the fog had been thick (San Francisco Bay Area) heavy fog is common there. Smiles is what a window guy called the condensation on double and triple Payne windows and that stuck with me.
    – Ed Beal
    Dec 3, 2020 at 22:01
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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.

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  • And as an added benefit to doing this the critters like spiders will thank you for the sheltered home when they crawl into the vent and spin a web in between the panes of glass. (I've got 2 windows like this but aluminum not glass. The panes must be removable to clean in between)
    – Ted Mittelstaedt
    Dec 3, 2020 at 17:48

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