I have been considering installing secondary double glazing units in my home in an attempt to turn my double glazed windows into triple glazed windows.

I understand the [law of diminishing returns](http://en.wikipedia.org/wiki/Diminishing_returns), and the fact that sealed units are inherently more efficient, but is this a practical solution to reducing energy bills?

Amongst my concerns are:

* How I avoid condensation between the secondary glazing and the existing double glazing.
* Whether I can find secondary glazing units which won't prevent the existing windows being opened and or cleaned easily.
* Whether I should try to fit the secondary glazing unit as close as possible to the current double glazing unit, or whether a larger air gap would be preferable.
* If close placement is better, whether I can fit a secondary pane to the existing frame (i.e. fitting it so that it opens with the existing glass).
* If frame placement is possible, whether inside or out would be preferable or even allowed†.

If it makes any difference, my current double glazed windows have [Trickle Vents](http://en.wikipedia.org/wiki/Trickle_vent)‡, so the windows are not as sealed as one might hope.

† I could see that bolting an extra pane of glass onto the outside of an existing double glazing unit may pose a risk of the pane falling out and thus might this be banned under planning regulations.

‡ These appear to be required in my block of flats, though I'm not sure whether this is due to local building codes or something specified in the lease for the block, and I'm not sure why we seem to like them so much in the UK.