I just wondered if there was a product that could be affixed externally that incorporated a form of insulation for timber framed windows.

What I was wondering was whether an existing U=3ish 2G window could be improved by adding external insulation on the frames and maybe adding a third glass/polycarbonate pane internally with clips.

Not sure if this would require consent for external changes... my building is 1950s and isn't listed or in a conservation area, so shouldn't be too much trouble.

Secondary glazing isn't unheard of and was featured in a brief segment of a Sarah Beenie programme, think it was "double your house for half the money". The focus there was more about sound reduction to road noise and in their case, the secondary glazing appeared to be more effective than triple glazing.

With the vast array of profiles, frame materials and opening types I would say insulating the frame retrospectively would not be all that feasible. If your Uw (whole window U value) is ~3W/m2K then you would probably see a better improvement by changing the glass packet to a top spec 2G unit with a layer of low e glass, assuming the frame had a deep enough rebate to accommodate it. Normally the glazed area will be larger than the frame area and even in new windows the frame will underperform compared to the glazing unit in most cases.

Draft sealing would be another cost effective measure, both at the window seals and the junction between the frame and the wall.

You might find that replacement windows are a more cost effective approach, assuming no third party objections and you can tolerate the disruption.

Shutters?

Does it have to be outside?

I ask because I fitted some internal secondary glazing to a clients 2G sash window. I squared up the opening inside the reveal with a simple rebated wood frame, which I sealed and painted into the wall. Once I had this "ground" to work to, I then made a pair of hinged, inward opening lightweight glazed doors. The client wanted it as a permanent feature hence the doors. You could easily use the same technique with a seasonal removable glazed panel, leaving the frame in situ.

<blockquote><cite>Posted By: gravelld</cite>..................Not sure how well this really affects U though...</blockquote>

No, neither am I, but I'm sure there are others on here willing to apply science to that end much better than me. I'd guess it will improve to some extent but maybe it will just "feel" warmer, a bit like the other thread about curtains, and will most likely cut down noise, if that's an issue.