We have a very old property that has secondary glazing as it is close to the M6.We want to have new double glazed windows fitted. Can they be fitted from the outside and retain the secondary glazing with no problems? Can you please advise?
Yes, it can be done.
The fitters might need to temporarily remove the secondary glazing in order to get sufficient room to manouevre when installing the double glazing. So you will have to check with the fitters before placing the order. The sales people won't know, and will just say whatever it takes to get the order, so do get a fitter round. Local suppliers tend to be better at this: the national double-glazing companies in Britain are pretty much the worst for shady sales practices (but some of the local ones are sharks too).
There'd be no point in putting in trickle ventilation in the double-glazing (that's assuming your secondary glazing has no trickle ventilation - it usually doesn't), so make sure it comes without that. After all, the trickle vents would just be another route for the noise to get in.
You will need someone to check compliance with Part F (the Building Regulations regarding ventilation). The trickle vents have been used as a lazy way to ensure compliance, even when they haven;t been necessary. Sometimes, when air-tightening is done, you can make a place so airtight that it becomes unhealthy. This is not often a problem in Britain, because our buildings are so leaky, but check it anyway, otherwise you could get problems with mould, and (if you're in a radon area) with trapped radon or other pollutants - I've seen it happen. We have another question that deals with ventilation strategies - check out the answers, as well as the answers to this other question about using secondary glazing with double-glazing.