Windows and energy: secondary glazing with curtains is more efficient than double-glazing

In November's issue of 'Period Living'
Period Living November 2013 | Period Living
a piece looked at the alternatives to replacing wooden windows with uPVC:

Worth saving 
Old single-glazed windows with original plain glass can contribute immeasurably to the special interest of a building. Their retention also saves energy that would otherwise be consumed in the manufacture of new windows; the singleglazing doesn’t rely on seals that can quickly fail, causing misting up; and in many cases older windows are more readily repaired. What’s more, the timber used for old frames is often superior to that available today. The use of uPVC as a substitute material is rarely as low-maintenance as supposed – testified, for instance, by the availability of purpose-made paints for improving its appearance as it deteriorates with age. 

Old windows deserve a reprieve | Period Living

Secondary glazing is cheaper than double-glazing:
The case for secondary double glazing | YouGen Blog | YouGen, Renewable Energy Made Easy
Ecoease Secondary Double Glazing Systems
Double Glazing Vs. Secondary Glazing | TheGreenAge

And much more flexible and versatile:
Secondary glazing | Centre for Sustainable Energy

Double-glazing is not necessarily more efficient:

The research shows that while installing double-glazing reduces heat loss by 55 per cent, other coverings did better. These were: secondary glazing combined with insulated shutters (77 per cent); secondary glazing with curtains (66 per cent); a Victorian blind with shutters (58 per cent); and insulated shutters on their own (60 per cent).

Alternatives to double glazing - Telegraph

And if one of the points of going for double-glazing is that it will reduce our carbon footprint...
Discussions: Homes, double glazing - WWF Footprint Calculator

... the issue of reducing embodied carbon should also be considered:
Double glazing or triple glazing? All pane and no carbon gain -
Embodied C02 in float glass
Choosing low-carbon windows | Magazine Features | Building
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