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windows on block/brick cavity wall - bridge the blocks, or dreaded 'triple leaf'??
Old 5th June 2020
  #1
windows on block/brick cavity wall - bridge the blocks, or dreaded 'triple leaf'??

Hi guys,

This is really me just thinking out loud, there's a very real chance that I'm overthinking this. Starting a new thread as I've been going DEEEP down a rabbit hole of complicated maths / opinions on 3 leaf designs, and I'm no nearer any sort of conclusion. Hoping someone out there might be able to shed some light.

I've put some notes on the wall construction on the drawings below in case my writing is too bad to read, from left (inside) to right, that's:
- a wall made of wooden studs, with (from L to R) two layers plasterboard, one layer 18mm osb.
- 140mm solid concrete blocks
- bricks

(The lines for the roof are just to show which way is inside, I didn't draw any of the detail, this Q isn't about the roof anyway)

So, I had planned to construct my windows as such:
link
That's using a wooden 'bridge' to join the block and brick together, so that in the space around the window there isn't a weak point for sound to escape through. The external window frame would then be a site made one. This design came from a designer, but it's only when I've actually started to think about the practicalities of constructing it that I've been put off a little bit. I do mostly trust my ability, but there would always be a bit of a worry that it wasn't totally weatherproof. Plus the wooden bridge is a little fiddly, with a cutout for a rubber seal, and the edge of it would be exposed to the elements too, etc.

I was talking about this in passing with my friend (who has a masters in acoustics, but openly admits he might be wrong, didn't do any calcs, and has never actually worked in the acoustics field professionally), and he said what about doing this:
link
This would be easier to install, and I'd be able to get an off the shelf external window frame, so I'd know it was weatherproof. BUT it's a three leaf system.

My friend said, if the inside (stud wall) window was laminate glass, the middle (block) window thicker laminate, and the outside (brick) window an off the shelf double glazing unit, then there should be enough mass to mitigate a lot of the bad points of a 3-leaf system.

Thoughts?

Thanks all
Old 5th June 2020
  #2
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We need information! How thick are the bricks? How large is the gap between the blocks and bricks? Is there any reason you can not use a ready maee window in the brick bring up a filler plate close but noy ouching the window?
Old 5th June 2020
  #3
Thanks, I'll gladly give more info!

There are 3 of these windows to the outside, and then a fourth which goes to the office / occasional control room.

Bricks are 100mm (photo)

Block / brick cavity is ~ 100mm (photo)

The airgap behind the internal leaf is ~140mm to the back of the OSB (photo)

I have also already fitted a window cill as I was thinking of modifying the bridge, and putting the bridge and then window above it.
(photo)
I filled the window cills with concrete, so they're very similar in density to the bricks etc. And they're fixed down with acoustic sealant/adhesive (this stuff). Happy to remove them if needed.

I had thought of a wooden bridge up to the back of a shop bought window, that would be ideal if it would work? I could jam tons of acoustic sealant alll over the join to the back of the PVC window frame, and then tidy it all up for aesthetics with acoustic tile / foam?

Thanks
Old 5th June 2020
  #4
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Great response! Either work. The weak point is the windows. Best response is 2 leaf with as massive as massive as possible with .060 (US design) laminate and at least 1" absorbent reveal treatment.

Enjoy!
Old 6th June 2020
  #5
Quote:
Originally Posted by avare View Post
Great response! Either work. The weak point is the windows. Best response is 2 leaf with as massive as massive as possible with .060 (US design) laminate and at least 1" absorbent reveal treatment.

Enjoy!
Thanks avare, that's very helpful.

The original design for the bridge had a rubber surround. The rubber was specced as Sorbothane rubber, which is very expensive when you factor in how much of it would be needed. How much of a difference would a rubber strip around the brick side of the bridge (ie directly in line with the external window) actually make?

If I just made sure to extensively seal the bridge with acoustic sealant, and mounted a manufactured window and frame directly onto the window cills that are already in place, would that still be effective?

Thanks
Old 6th June 2020
  #6
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Quote:
Originally Posted by Chris1234 View Post
Thanks avare, that's very helpful.

The original design for the bridge had a rubber surround. The rubber was specced as Sorbothane rubber, which is very expensive when you factor in how much of it would be needed. How much of a difference would a rubber strip around the brick side of the bridge (ie directly in line with the external window) actually make?

If I just made sure to extensively seal the bridge with acoustic sealant, and mounted a manufactured window and frame directly onto the window cills that are already in place, would that still be effective?

Thanks
Your descriptions are confusing. If you using a commercially made window you do not need any additional rubber. Get as thick as possiblr laminated (.060" 1.5 mm laminate) glass for both leafs.

Enjoy!
Old 6th June 2020
  #7
Quote:
Originally Posted by avare View Post
Your descriptions are confusing. If you using a commercially made window you do not need any additional rubber. Get as thick as possiblr laminated (.060" 1.5 mm laminate) glass for both leafs.
Apologies if confusing, I do find it hard to expain things.

Bridge design from designer here

There's the thick rubber strip on the outside of the wood in line with the window. I wasn't talking about the thinner rubber that seals the window to the frame, which in a manufactured unit would be part of the manufactured unit.

If modifying the window bridge as discussed it would look like this, which is obviously pretty different, but would it still work?

Basically, is that rubber doing much?

Does that make sense?
Old 6th June 2020
  #8
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I could not download the second image.

It d/led finally.. Yes the rubber is doing something. Have you considered using acoustic caulk? You will have already for the walls and ceiling edges.
.
Granted it is a conceptual drawing it does not show any absorbent on the reveal
Old 6th June 2020
  #9
Quote:
Originally Posted by avare View Post
I could not download the second image.

It d/led finally.. Yes the rubber is doing something. Have you considered using acoustic caulk? You will have already for the walls and ceiling edges.
.
Granted it is a conceptual drawing it does not show any absorbent on the reveal
When in doubt, caulk!

Yeah I had wondered if that would work. So a layer of acoustic sealant around the manufactured window frame would do the same sort of job?

Chris
Old 6th June 2020
  #10
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Quote:
Originally Posted by Chris1234 View Post
When in doubt, caulk!

Yeah I had wondered if that would work. So a layer of acoustic sealant around the manufactured window frame would do the same sort of job?

Chris
No. There is no need to isolate the frame from the brick.
Old 6th June 2020
  #11
great, thanks

One last (I think!) question - should I screw the bridge down to the block / brick as well as sealing, or just seal?

Am I overthinking?
Old 6th June 2020
  #12
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Quote:
Originally Posted by Chris1234 View Post
great, thanks

One last (I think!) question - should I screw the bridge down to the block / brick as well as sealing, or just seal?
You do not have a solid mechanical connection between the block and brick leafs. The bridge is cover up otherwise open paths for sound. Have you seen expansion joints in roadways? Sections of highway with caulking between roadway connections. Same idea, but for vibration isolation.

Enjoy!
Old 7th June 2020
  #13
Many thanks!
Old 7th June 2020
  #14
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Quote:
Originally Posted by Chris1234 View Post
Many thanks!
You are welcome. You want to avoid the Canadian saying of "I cut it twice and its still too short!"

Not too short, well on one foot,
Andre
Old 9th June 2020
  #15
Quote:
Originally Posted by avare View Post
Get as thick as possiblr laminated (.060" 1.5 mm laminate) glass for both leafs.
Hi again, the thickest laminate that I can find is 12.8mm, including the plastic interlayer. It doesn't seem thick enough to me? Will it be enough?

Thanks
Old 10th June 2020
  #16
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Quote:
Originally Posted by Chris1234 View Post
Hi again, the thickest laminate that I can find is 12.8mm, including the plastic interlayer. It doesn't seem thick enough to me? Will it be enough?

Thanks
It may a problem of language. I was referring to the thickness of of the plastic, or I think as you are referring to it, the the interlay er.or the polyvinyl butryl which is the common commercial formin North America.
Old 10th June 2020
  #17
Quite possibly a dodgy explanation by me again, but I was referring to the whole thickness of the pane.
All I can seem to find is 12.8mm total, so that's around half an inch total thickness for 2 layers of glass and one layer of plastic in between.

I was hoping to find at least double this thickness for my outside glass.

Is there anyone out there in the UK that can recommend any companies to call?
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