How far should secondary glazing be from original window?

[Applecakes98]

I've been speaking to different companies about secondary glazing. Most can't advise on how far it should be from the original window, but one company said that to provide good noise reduction it needs to be at least 6" from the original window.

The internal edge of the wall is 5" away from the original window, so to get 6" between the original window and the secondary glazing, I would need to have a wooden surround fitted around the window - a sort of box sticking into the room by just over an inch. This would obviously not be ideal in terms of aesthetics, and would add to the cost, but I don't think it would look too bad - the edges would be hidden by the curtains, and the bottom would just look like a window sill.

I'd be willing to do the wooden box thing if would make a big difference to the noise reduction. At the moment I get woken up at night by an aeroplane at about 4 or 5 am, and don't get back to sleep, so this is really affecting my energy levels and quality of life.

I'd be super grateful for any advice on how far secondary glazing should be from the original window.

That's not correct. We moved into an Edwardian recently and the secondary is only a couple of inches away.

Thank you for the reply. Do you find the secondary glazing makes a big difference in terms of noise? I guess some people might have it for warmth, and perhaps for that the distance doesn't matter?

Hmm that's true. I haven't compared noise levels with and without the secondary glazing, but even with it I can hear street noises at night. So maybe it wouldn't be good enough for your purposes. Have you also tried thick curtains (for nighttime)?

Yes, I do have thick curtains (made thicker by a blackout lining!).

I stand to be corrected, but I believe secondary glazing is more effective in terms of noise reduction with the greatest possible gap. Quite a lot of secondary glazing firms suggest gap sizes for maximum sound reduction on their websites.

We had secondary glazing in our last house, which was listed. We had it put in due to unexpectedly high levels of road noise ( rat run for container lorries). It looked great, virtually undetectable in fact, but its proximity to the original window ( to satisfy listed building/ conservation
Consent) made the impact on noise reduction disappointing.
Additionally, the glass was 4mm, the same as the glass in the original windows. I believe it’s more effective if there is a difference in the thicknesses of the glass.
Acoustic glass might have been more helpful but constraints due to it being listed and the size of the windows meant this wasn’t viable.
And wooden plantation shutters did nothing whatsoever to mitigate nouse!

it doesnt sound right that they cant give you figures. we were given noise levels dependent on 4mm, 6mm and 7mm glass. it has made a huge difference. The wider the gap the more effective up to about 6" when the improvement is marginal however with acoustic glass you will still see a huge improvement with just a few inches gap.

When you say you have thermal curtains - do you mean type that have interlining (like a "blanket") sewn between the outer and the lining. That is what you need - not something you can buy ready made.

Thank you for the replies!

My curtains are thick but not thermal. They were custom made with a blackout lining, but not an interlining.

The company that seems best so far can do 6.8mm acoustic glass - that's the thickest glass they can do, and is what I would go for.

The companies are able tell me how thick the glass in the secondary glazing would be, but not how far it would need to be from the original glazing.

Do you think a 5" gap would make a big difference with acoustic glass, or should I go for a 6" gap anyway?

Go for the largest convenient gap that looks good. No need to go overboard.

I have some with a 30mm gap, because it fits in the existing windowframe with a hardwood batten, and it makes a substantial improvement with 4mm glass.

My BiL fitted outer and inner French windows with an 8" gap and 6mm glass, and it is really good.

The glass is more effective if it is thick (heavy) and preferably a different thickness to the existing window, so it will have a different resonant frequency.

I had my patio doors reglazed in laminated glass for security, I think it was 5mm each pane, and was so heavy it took two men to carry it. The sound reduction is noticeable. You also need to seal any gaps or cracks on the inside frame, which will block draughts as well as preventing noise entry. The gap should have pinhole ventilation to the outside, not the inside. This prevents condensation.

You can find tables of gap size, glass thickness and sound reduction in building technology reference books and websites, but I haven't got them handy and haven't read one for years.

@PigletJohn Thank you - that’s really helpful.

p.s.

On checking, I think the thickness of mine would have been 6.4mm laminated each pane, which is two sheets of 3mm glass with a 0.4mm plastic sheet between them, forming a solid sheet. But my purpose was burglar resistance. Ordinary laminated is available from any glass merchant or glazier, often used for safety and security glass, and can be made into sealed DG units at greater cost.

I do not know the sound resisting glass, but it seems to be thicker so perhaps it has a thicker or different plastic layer. From what little I've read it is noticeably better at sound suppression and I expect the installers use extra fitting detail.

I found a site that says:

"Secondary Glazing Sound Insulation

The gap between the original window and the new secondary glazing is one of the most important things to consider regarding sound insulation. The most common size pane of glass in the original household glazing would typically be around 4mm, unless the property is pre 20th century, then it could be 3mm.

So if the original house glazing is 4mm and the secondary glazing is 4mm, the gap would need to be around 150mm between the original window and the new secondary glazing for optimal sound insulation.

With 6mm glass in the secondary glazing this gap could be reduced to 100mm.

If the reveal (or window cill depth) can't accommodate a 100mm gap then laminate or acoustic glass like Stadip should be considered for maximum sound insulation.

In general terms the larger the gap the better the sound insulation.""

there's more to read

I have no connection with this company

with 6.8mm glass I think a 4 " gap should be fine and i wouldnt increase from 5" to 6".

Worth getting interlined curtains in future especially as energy prices increase.

@PigletJohn This is super helpful. Thanks so much for taking the trouble to look that up and share it. Really kind of you!