Replace single pane in wood frame and add secondary pane

7 Mar 2010
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United Kingdom
Moved into an Edwardian house where the rear windows in one room have been glazed with thin single panes of obscure glass. (At the moment there is a leanto on the outside wall which we intend to demolish). None of the windows open. I want to replace the glass in these fixed frames with clear glass.

So far so good.

I'm also thinking of adding a removeable second panel on the inside of the frame leaving a gap of about 45mm between the two frames - a sort of DIY secondary glazing!

The frames are good and solid.

I'm attaching a cross sectional plan of the proposed idea because it may not be clear from the above description.


1. Does the secondary glazing idea make sense. The windows are fixed so there's no draught issue to deal with but would it help with heat loss through a single pane of glass to have the secondary pane.

2. If the secondary pane is a waste of time what thickness and type of glass would be best for the single exterior pane. From a security and insulation point of view should this be 4 or 6mm or thicker? Should it be laminated, toughened or something else. Should it be some sort of low emissivity glass.

3. If the secondary glazing idea makes sense what glass should be used for the external and internal panes. What should be the thickness of both. Should one or other or both be toughened or laminated? Should the interior panel be low emissivity or something else?

4. In the secondary glazing idea, should the secondary panel be attached in such a way as to create, as best as possible, an airtight seal (using for example a thin draught excluder all round) or should there be some sort of airflow between the two panes.

I hope this all makes sense and thank anyone who can understand my poorly explained concept and offer any help.
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can you ascertain why the glass is obscure and is the windows location (after the wall comes down) a security risk??
Now as for replacement glass and low E glass - Pilkington have a very thin D/G system called "Spacia" which allows vacuum D/G units of extremely thin thickness to replace ordinary glass inn astragal built window (which I'm assuming yours is?) just type in the search 'pilkington spacia system' to get to a pdf sheet on the product (it's about forth down on my Google search)

As to low emmision glass, they also provide a fact sheet - ...pinenot

I think the biggest issue you are likely to have with your proposed design is condensation, as you will be sealing the ambient air into your 'unit' and have no control over that environment. Also I believe that such a large gap between the panes would allow significant air circulation which will contribute to heat loss and therefore destroy any perceived gain in adding the second pane.

I have seen stepped double-glazed units made to replace single panes of glass in existing wooden frames, so as you are planning to re-glaze anyway, they might be a better option.
The glass is obscure because at some point in the distant past some poor sod had to use the leanto as a bedroom and this prevented people looking from the lounge straight into his room. Prior to the erection of the leanto they would have looked out over the garden. It's not really relevant to the issue.

The windows we want to deal with are in what was the external wall which is safe and secure as are the window frames themselves. Security isn't an issue.

As to spacia, I've looked at that along with slimlite and histoglass which I believe have both had a bad press (but everytime I read the alleged problems the advocate of normal double glazing seems to make wrong assertions regarding the absence of ventilation and/or the wrong glazing compound). I'm trying to do this relatively cheaply which is why I'd rather use single panes. I'm also, probably, going to stick lead strips on the exterior pane (front and back) to match the rest of the windows in the house which I think would rule out Spacia.
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Kbidy, I read somewhere that stepped units are vey prone to failure but thanks for your response. Crazy idea but what if the interior glazing panel was a double glazed unit. It would be openable/removeable to enable cleaning of the inside of the exterior pane. The window is 95 x 45cms so even a double glazed unit wouldn't be too expensive.
Crazy idea but what if the interior glazing panel was a double glazed unit. It would be openable/removeable to enable cleaning of the inside of the exterior pane. The window is 95 x 45cms so even a double glazed unit wouldn't be too expensive.

Probably not so crazy and might work if well fitted. Could be worth a go.

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