Secondary / Tertiary Glazing Question

Hello, First post here, I have been getting a lot of useful info from the forum whilst designing and building a studio over the past two years - thanks very much! This question isn't related to a studio but a not-for-profit music venue/social club, but I hope some people on here may be able to offer some advice. The venue is in an old Victorian factory building and has three large windows in the gig room. We are attempting to improve the glazing/reduce sound transmission through these windows as they are currently the most obvious weak spot in the room, the rest of the walls being thick red brick, around 400mm. The live music room also hosts non-music events in the daytime and these usually require natural light. We originally thought about building some mdf/fibreglass plugs that could be used to block the windows during live music events but decided that these would be very cumbersome and annoying to remove and re-install on a regular basis. We aren't kidding ourselves that our improvements to the windows will stop loud bass emanating from the building but just that they will be as effective as possible for the money we spend. I'm trying to help advise the venue owners on what the best solution might be. They are currently thinking of installing double glazing, which I know from reading here and elsewhere is fairly ineffective except for in blocking general street noise etc. I attach a picture of one of the windows (a bit of a mess at present). Please bear in mind that we will first remove the timber cladding you can see on the right hand side of the image. My First question is would keeping the existing windows (obviously ensuring that they are airtight) and installing secondary glazing internally be a good alternative to double glazing? I imagine we could have considerable air gap between the two panes (around 150mm - I will measure this accurately tonight). Given that existing windows are standard 6mm glass, what would be the advised airgap and thickness/material of the secondary layer? Secondly, would standard double glazing with an additional airgap and tertiary layer be any more effective than the first option? I have read that the triple leaf effect might mean that this is just a waste of money and could less effective than two layers? This is obviously a much more expensive option and would mean we also have to install fairly ugly aluminum frame double-glazed windows on the outside of the building instead of the current "period" windows. :) So I suppose the three options we are considering are: 1. Standard double-glazing 2. Using existing windows and installing secondary glazing 3. Standard double-glazing with an additional airgap and third layer of glass Any advice or other suggestions would be very much appreciated, and of course if you need more info/photos/measurements please just ask. Many thanks in advance!

Hi there "ouse", and welcome to the forum! :)

I'm trying to help advise the venue owners on what the best solution might be. They are currently thinking of installing double glazing, which I know from reading here and elsewhere is fairly ineffective except for in blocking general street noise etc.

Right. It works fine for that, but not so well for low frequencies. You can get much better "bang for the buck" with two panes of laminae glass, separated by a large air gap. The thicker the glass the better the isolation, and the larger the air gap the better the isolation.

My First question is would keeping the existing windows (obviously ensuring that they are airtight) and installing secondary glazing internally be a good alternative to double glazing?

It is a possibility, assuming that the existing glass is reasonably thick and well installed (hermit seals). The typical thin glass you find in ordinary house windows (3mm, 4mm, etc) would not be good, but you say it is 6mm, so that it is probably usable. So that would work, albeit not as well as if you were to replace it with laminated glass.

I imagine we could have considerable air gap between the two panes (around 150mm - I will measure this accurately tonight). Given that existing windows are standard 6mm glass, what would be the advised airgap and thickness/material of the secondary layer?

How long should a piece of string be? :) The answer to your question depends on how much isolation you need, and what frequency you need it at: The simple answer is "as thick as possible for the glass, and as large as possible for the gap". Assuming you need good isolation down to very low frequencies, I'd go with 6x6 laminated glass (ie, 12mm thick) plus a 20 cm gap. But you might not want to go that far, depending on what your real needs are, and your budget. You should set a goal, in terms of how much isolation you need (decibels) based on how much you are already getting from the wall itself, then work backwards from there to calculate the optimum glass thickness and air gap size.

Secondly, would standard double glazing with an additional airgap and tertiary layer be any more effective than the first option? I have read that the triple leaf effect might mean that this is just a waste of money and could less effective than two layers?

Right! :) Not to mention that it would be more expensive and less effective than just replacing the existing glass with 3+3 laminated and putting in a second pane of 4x4 several cm away. That's a LOT of glass you are talking about!

This is obviously a much more expensive option and would mean we also have to install fairly ugly aluminum frame double-glazed windows on the outside of the building instead of the current "period" windows.

Yup!

So I suppose the three options we are considering are:

And there's number 4 as well: replace the glass in the existing frame with 3+3 laminate (or even 4+4), then install another pane of 4+4 (or 5+5, or 6+6) as far away as you can get. But whatever you do, don't forget that you need to put silica gel in the gap between the two panes before you seal them in place! If not, the glass will fog up from the trapped moisture... - Stuart -

Thanks very much for your reply Stuart. Helps to clear up a few things we were confused about.

"as thick as possible for the glass, and as large as possible for the gap"

Good to know. I think the 4th option you suggested sounds like a goer, with as wider gap as the windows can accommodate. Thanks also for the silica gel tip! Cheers!