Ceiling question - Rockwool thickness and fixing

I am building a studio at the bottom of my garden - a long cherished dream. :yahoo: The construction of the studio is a stand alone, timber frame affair at the bottom of my garden. It will have oak cladding over a 25mm airgap on batons with a vapour barrier and 6mm of external grade ply. The roof will be slate over a 6mm external grade ply, vapour barrier, 25mm airgap, heavy roof felt and slates. It is 4Mx3M with a pitched roof 4M high at the apex and 3M high at the eaves. I don't play guitar loud and won't track drums. But I want a reasonable level of sound attenuation. The internal construction I was going for is a resilient strip, walls and ceiling, into the Apex (i.e. not flat) and then two layers of gypsum plasterboard, maybe one layer will be high density sound board with something like green glue in between the layers. The timbers in the walls are 100mmx50mm (4"x2") and 16" (400mm) apart. The roof timbers are 5"x2" (125mmx50mm). In the walls, I will use 100mm thick Rockwool insulation. But in the ceiling what would you guys recommend? Do I fill the space by using two layers of Rockwool, say, 3" (75mm) and a 2" (50mm) to make up the 5" (125mm) - or do I use 4" Rockwool and leave a 1" gap? If I understand correctly, mass is what you need to absorb sound so my inkling is to fill the space. One further question, when I use the Rockwool in the ceiling which is at 45 deg, should I use some kind of netting to a) keep it in place and b) stop the Rockwool from falling if there is a fire?

Here's a recent picture of the studio under construction - I was fixing the 6mm External Ply sheets to the roof timbers. They look white on the picture but its a trick of the light - and poor photography :?

use the 100mm and leave the gap. the air space is your friend although in an enclosed space the light (15kg/m3) insulation can be more effective. for mass - the insulation isn't. 5/8" (16mm) type "X" firewall is around 700kg/m3 versus rigid insulation which maxes out at around 120kg/m3 (although you would want to use something around 48kg/m3 for acoustic treatment). so go with multiple layers of 5/8" drywall on the inside. if you could use isolation clips and hat channel off the frame you will increase your isolation as the inner mass will be decoupled from the frame and outer shell. you can also beef up inside the plywood using multiple layers of 5/8 drywall then the iso/channel and inner drywall to achieve a much higher level of isolation than simply the drywall attached to the frame and no additional mass. per xSpace's question - is your vapor barrier on the correct side of the wall? also, what are you planning for HVAC? fresh air in particular? for the treatments - hang clouds off the peak (say 1m down) to trap the peak and still leave enough reverberant (close enough) space.

Thanks Glenn; I got the bit about using 4" Rockwall insulation leaving 1" air gap between the Rockwool and the inside of the ply outer covering. I am using 65Kg sq M Rockwool in between the roof joists and then 15mm 12.6Kg SqM SoundBoard and then a 12mm 10Kg Sq M plasterboard layer on top with a Green Glue layer in between - but I could be persuaded to use two 15mm layers of SoundBoard. What I think you are suggesting is that in the ceiling instead of using resilent strip which increases the stress on the strip because of the angle of fitting and use isolation clips and top hat instead? Is that correct? And if I did that would your advice still be to leave 1" space or fill it with Rockwool? So my question about netting stopping the Rockwool dropping down onto the back of the roof plasterboard still stands - good idea or not? The vapour barrier has to be on the outside to stop any rain or condensation that might form on the underside of the slates coming through to the inside! Even though on top of this VB we have 50mm airspace, heavy duty roof felt and slates. I won't be fitting A/c as it only gets hot enough in the UK for about two weeks a year so isn't worth the hassle and cost. If I need fresh air I'll just open the quadruple glazed and isolated door for a minute -we have lots of fresh air where I live! :lol:

you can use the iso clips/channel and also allow the rockwool to contact the plasterboard. leave the air gap.