We unfortunately lacked the multiple thousands of pounds required to replace the sash windows with double-glazed conservation-area-friendly equivalents
I have a number of the beefs with double glazing.
For us it would require ripping out perfectly user-serviceable and user-maintainable sash & case windows and binning the whole lot, to be faced with having to do the same thing in 15-20 years when the plastic and metal units have all failed and are out of warranty. Our windows have worked fine for 120 years and no reason why - properly maintained - they shouldn't continue for another.
Plenty of the sash window restoration companies claim that, properly draught-excluded, a sash window can be pretty close to a double glazing unit and further improved with either heat reflective glass (moderately expensive), conservation double glazing units for sash windows (hugely expensive, price on application, Grand Designs-spec stuff). Over the unit lifetime, double glazing is hugely more expensive than maintaining a wooden window (all parts replaceable and maintainable with some basic carpentry skills) and unlikely to ever recover its costs through the energy savings. Yes a professional sash window restoration is expensive, probably approaching the costs of double glazing, but you aren't having to do it every 15-20 years and once the window has been restored it's fairly easy to maintain it with a regime of painting and inspecting the putty and mastic. The energy footprint of the manufacture of a double glazing unit is probably never close to being recouped either.
No window can hermetically seal a house that was built not to be hermetically sealed, or the damp will have it in no time. Brick/stone/mortar/lime/render-built properties need ventilation and in the sandstone and lime mortar tenements this is naturally through the floorboards, the doors, chimneys and most importantly the windows. Sealing them up with DG units and silicon mastic isn't as smart an idea as it seems long-term; wood and linseed sand mastic is what naturally compliments the sandstone and lime-mortar construction of the tenements. DG units end up having to have trickle vents to allow this ventilation and therefore can't help but have to let draughts in to probably the same level that a draught-excluded sash window will. Add on good, thick curtains, shutters, drapes, blinds etc. and the difference is certainly closed right down.
If DG was such a great, money-saving idea, it's likely that the DG companies wouldn't (EDIT - terribly worded) it's unlikely that the DG companies would always be so keen to throw debt at people in order to be able to afford it.
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