Window confusion

Hi, I'm unsure as to which window construction would be better for performance (and cost) in the following... Timber framing hasn't gone up yet, but I'd like to get this sorted beforehand. And thank you for your help. I've been speaking with local glaziers, and was intending on getting 1/2inch and 5/8inch glass for opposing windows. Going beyond 1/2inch has presently proved difficult, however more than one has come back with acoustic laminated glass. They suggest it's thinner but can offer comparable results. The second part of the issue is that one glazier has replied with:

(6.8mm Aco Lam = 35dB) or (6.8mm Aco Lam / 6-24mm Cavity / 6.8mm Aco Lam = 42dB) ...12.8 acoustic lam (40dB)...16.8 acoustic lam (42db) The rates we could offer you for cut-size stock is as follows: ü 6.8mm Acoustic Laminate @ £25.80m² ü 8.8mm Acoustic Laminate @ £37.50m² ü 12.8mm Acoustic Laminate @ £68.00m² ü 16.8mm Acoustic Laminate @ £96.00m²

They then went on to say:

Typically in recording studios, they use secondary glazing, which further increases this dB rating. When I’ve been in studios, they usually have an IGU with a 150mm Cavity and then a single skin of Aco Lam which increases the dB rating dramatically. But it depends also on the soundproofing of the wall itself as the glass is only as good as the frame its fitted into.

So my query is two-fold. Using two sheets of thinner glass, say 6.8, small airgap, 8.8, in one leaf would on paper offer a better dB value than one sheet of 12.8, while also being cheaper to boot. However using two sheets on each leaf would decrease the cavity between the two rooms at that point. I don't know where the priority should exist, cavity or dB. I could save money on the build, and increase dB reduction with two panes on each leaf it seems. I want to believe that, but my instinct says that a straight forward MSM would be better. I've attached a drawing further defining the two options, with cavity depth labelled. For your information, wall construction of each leaf is... 100mm cavity, acoustic roll insulation, 150mm stud work, 12mm OSB, 15mm soundbloc plasterboard, green glue, 15mm soundbloc plasterboard. While the floor is 200mm concrete slab, cut between rooms.

Sorry to bump this. Does anyone have any suggestions with this? We're getting on well but we're getting close to the glazing time now, so really keen to get an idea as we can put in an order for our windows soon. thanks for any help you can give. Sam / Blank

The rates we could offer you for cut-size stock is as follows: ü 6.8mm Acoustic Laminate @ £25.80m² ü 8.8mm Acoustic Laminate @ £37.50m² ü 12.8mm Acoustic Laminate @ £68.00m² ü 16.8mm Acoustic Laminate @ £96.00m²

Can you calrify what they are actaully talking about here? When they say "6.8mm Acoustic Laminate", are they talking about a pane of glass that is 6.8mm thick, but is made up from a sandwich of two thin panes that are 3.4mm thick, plus an acoustic PVB inter-layer that bonds them together? Is that it? If not, then what is it?

Typically in recording studios, they use secondary glazing, which further increases this dB rating. When I’ve been in studios, they usually have an IGU with a 150mm Cavity and then a single skin of Aco Lam which increases the dB rating dramatically. But it depends also on the soundproofing of the wall itself as the glass is only as good as the frame its fitted into.

Apparently, your glazier hasn't been in too many professionally designed studios! :) Because that is NOT the way most studio windows are built....

I want to believe that, but my instinct says that a straight forward MSM would be better

Exactly. Your instinct is correct here. Adding additional panes with tiny air gaps can potentially reduce your isolation, due to the four-leaf effect. The most effective method is to have two thick panes, one in each leaf, with as large a cavity as you can get in between them. Simple two-leaf MSM isolation remains the best and least expensive method for isolating. If you had four leaves in there, then you'd need roughly double the total mass of glass that you would have needed in a two-leaf, and also a much larger cavity depth. Your first drawing (on the left) is the correct one. But I have my doubts about those dB numbers being correct! There's no way a single pane of glass 16.8mm thick is going to get you 42 dB of isolation by itself. Mass law predicts about half of that, at roughly 36 dB. 6.8mm ain't gonna get you 36 dB either: more like 30. I'd suggest you ask your supplier for the acoustic lab test reports that show their glass really does get that amount of isolation.

100mm cavity, acoustic roll insulation, 150mm stud work, 12mm OSB, 15mm soundbloc plasterboard, green glue, 15mm soundbloc plasterboard.

The general rule of thumb is that the surface density of your glass needs to be at least the same as the surface density of your wall. One layer of 12mm OSB plus 2 layers of 15mm drywall works out to about (7+10.8+10.8) = 28.6 kg/m2. The density of glass is somewhere around 2400 kg/m3, so you would need each pane to be at least 12mm thick, absolute minimum. Therefore, your choice of 12.8 + 16.8 is borderline, and 16.8 mm on both sides would be better, given that the air cavity between the panes is thinner than the air cavity between the wall leaves, and obviously you cannot put insulation between the panes! You do seem to need a lot of isolation! That's a pretty hefty wall you are building there. What is your isolation goal, in decibels? - Stuart -

Hey Blankstudios and Stuart, I've been going through similar discussions with a few window companies for my studio build! When they write 6.8mm what they mean is a laminated glass that is 6mm+acoustic foil+8mm thick. So its actually 14mm thick glass with an acoustic foil in between. This way it might be possible to achieve those dB ratings. At least if its dB Rw, which seems to be the measurement at around 500Hz what i found out (which is only interesting for domestic areas). The windows i'm getting now will probably be 6.8mm - cavity - 4.4mm and have a rating of 48dB Rw. So this would essentially be 14mm - air gap - 8mm. They did give me more precise measurements and these windows would still have 34dB transmission loss at 100Hz! hope i could help!

Thanks for the advice both of you! I've spoken with the glazier we'll be working with. He says that with the 12.8, for example, it is indeed two pieces that amount to 12mm of glass and 0.8 for the foil (These composite sheets of glass would be say 8mm and 4mm). So a completed width of 12.8mm, unfortunately no bonus width in this instance. I have asked for paperwork regards to performance, so we shall see on that side of things. I've also requested a pricing on thicker units. I am maybe misunderstanding the laminate nature but my basic understanding is that two opposing sheets of matched thickness will have sympathetic resonance. As such I should aim for 16.8 (minimum) on one size and a thicker on the other leaf.... That said, if the 16.8 laminate is made up of, say, an 12mm and a 4mm to make the 16mm, does that mean that the resonances are reduced as it is in effect already two separate widths? Maybe I'm thinking too much into this..

Apparently, your glazier hasn't been in too many professionally designed studios! :) Because that is NOT the way most studio windows are built....

This may be the case! all the more reason for me to get the job done properly, and he'll be able to learn too for future clients!

You do seem to need a lot of isolation! That's a pretty hefty wall you are building there. What is your isolation goal, in decibels?

Our isolation goal is to use as much budget as possible to create as good a wall as possible! I appreciate that is vague, but we have no fixed target on reduction beyond making it as high as our spend could deliver (and building with as much attention to detail as our patience can handle!). One of the influencing factors is that there will be a 200cap music venue on the same site - though they have a good acoustician company who has specced their installation (it's a fairly big project) so we shouldn't hear it (supposedly).. We shall see. The other reason to want to achieve good reduction between spaces is better understood by looking at the studio layout below.. Here's a 3d render Our studio is ran by 3 day to day producer/engineers. In our current project space that we'll be moving from, we're pretty much always booked up and on each others toes so we need a space where we can get more than one job working at a time (usually one person record while another mix). In the existing space we have a main control room, small live room, then a second mix room (with inadequate sound proofing). So in this build we wanted to create a space with two control rooms, both looking in to 2 recording spaces (one small, one large). The idea here is that one person can be mixing - in either space - while another is tracking (using one or both of the tracking spaces). Or, if our walls and windows are good enough, both end control rooms can be recording in their respective recording spaces (dependant on instrumentation i expect and subsequent levels). We've designed it so line of site through all rooms is present, tried to keep good room ratios, and also had to work with building control to maintain a fire strategy that they're happy with (the long corridor to the front door exiting at the bottom left rated to 60minutes and available to those back rooms). Its possibly a bit of a clunky design, i'm sure some of the more experienced folk here will have some thoughts toward it. But it's built around the ergonomics of our desired mode of operation, so I'm excited to get it up and into action! I should start a build thread on the forum really, we're currently putting in insulation and the OSB in the live room. Thanks again for your help and advice!