add insulation to an already existing wall + hempwool ?

[niko]

Hello there,
This is a great forum! I just wish I had found it before.
I'm building a recording studio, in a old house (built in 1680) in the countryside just outside Paris, France. We intend to use the studio to record very different kind of material, lots of acoustic stuff but sometimes also the loudest rock music. This will be half personal/ half commercial.
It's going to be basically two rooms, the first one (the live room) is about 22 * 17 (feet), the other one (control room) is 14*17 and height for both rooms is 11 feet (before insulation)
The existing wall between these two rooms is about 5 inches, made out of plain old red bricks and plaster (the house is more than three centuries old, it's structurally in a really good state though, constructions were solid at the time!)
I was thinking of doubling it with : a gap of air (one inch), 5 inches of rockwool or fiberglass, and 3 layers of drywall (with steel framing)
I'm wondering if that's a good setup or if I should put some rockwool on the old wall, then a gap of air then rockwool again, then layers of drywall.
For that purpose is it better to have very dense rockwool, semirigid rockwool, or softer fiberglass (less dense) to obtain the best STC possible.
I have the choice between rockmur and rockplus (both from ROCKWOOL)
I couldn't find to find density or absorption specs on Rockwool's site
but I finally found these densities (in kg/m3) on some contractor site
Rockmur : 30 semirigid slab
Rockplus : 50 rigid
Isover fiberglass 22 roll
I also found some hemp wool, if i want to go 'green'
which they dont say much about STC, but apparently the density is 35 kg/m3, would that be a viable alternative? here's the link (in french, sorry about that)
http://www.isover.ch/jahia/page754.html


I'm also wondering what would be the best way to decouplate the metal framing from the floor/ceiling/adjacent walls, I guess I can put some resilient foam but there still will be contacts through the screws? Is that ok?
I've been looking for posts about this but it seems that most of you use wood framing (steel is cheaper than wood here) and I dont think the way John Gardner did it would work for me.
http://www.johnlsayers.com/phpBB2/viewt ... c&start=30
A mason friend told me I should put 'tar rings' but I'm not sure about that.

One last question: I'm looking for acoustic caulk here in Europe, but all
I can find is just basic acrylic caulk, would that be good enough ? (I know they're not that many
of you located in Europe, but maybe somebody knows a good equivalent here)

I will soon finish my plan/layout of the place and post it with some pictures and all the details, and include them in a a thread about the whole project ...when the work will advance a bit.
I understand that some of these may seems elementary, but any help would be great thanks! , It's pretty hard to find good information on this subject where I leave.
PS Sorry about the possible language mistakes, my english got better thanks to my wife (she's anglophone from Vancouver) but I'm not quite bilingual yet...

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[knightfly]

You should make sure that your plaster over the bricks is intact, or have it re-plastered before doing anything else; brick is actually kind of porous, so a half-inch or so of plaster makes a noticeable difference if it's unbroken.

For really loud stuff you may want to leave more of a gap between brick and the second wall - up to about 12" total distance between the two masses (insulation counts as "air", so I'm talking about from brick to first drywall layer.

If you use steel studs, I'd recommend getting the "structural" weight, they won't tend to twist as much with drywall only on one side (see the REFERENCE section for more on this, if you haven't already) - non-load bearing steel studs are typically 25 gauge, and these are what's given steel the reputation of having better isolation; but that's ONLY when using drywall on BOTH sides of the frame, which you do NOT want to do (3 leaf problems) - the "structural", or "load-bearing" steel studs are typically 20 gauge, so they'll hold 3 layers of drywall better with less chance of screws popping.

Insulation - the lightest one you mentioned will give better LF isolation, which is always the hardest part. Anything from about 1 to 3 PCF, or 16 to 48 kG/m^3 - the lower end of that range gives best bass attenuation inside a wall. (Another of those non-intuitive parts of acoustics)

Caulk - no, acrylic won't do; you need something that will retain its flexibility without cracking. Other than acoustic-rated, the only other (very expensive) alternative would be Butyl-based; but that's about the same cost for a 10 oz tube as the acoustic stuff is for a 29 oz tube.

Be sure to check out the couple of links in the REFERENCE section on caulking as well.

Sources - not getting anything in the way of European ones yet. I have a request in to OSI to see if they have distribution there, will let you know. If all else fails, a top grade (50-year life) silicone caulk can work, or butyl based. Either will cost you 3 times what the acoustic stuff will, if you have the choice.

Frame isolation - if you use screws, there's little point (other than rattle elimination) of putting foam between members. Ask your local supplier about Resilient Channel as a means of mounting inner drywall leaves on ceilings/walls. You've not yet said anything about outside noise, separate inner ceiling frames, etc, so I can't advise on that.

The concepts of isolation aren't different with steel compared to wood framing, it's just finding the right steel shapes to support long spans when ceilings are framed in steel - other than that, the first 4-5 threads in the REFERENCE section may help with concepts, etc... Steve