Quick question......exterior roof insulation??

hi i've searched and have come up empty. i'm attempting to salvage an old church in austin tx and make it into a recording space. my builder loves the look of the old timber rafters and wants to leave them exposed. he says we can insulate the roof from the outside.......... he suggests putting two layers of 3/4" plywood decking, then batts, then another layer of decking before the shingles are applied. can anyone point me to any information on this? this would still be a 2-leaf system right? would radiant barrier plywood on one of the layers help? (which layer?) which insulation would you suggest? i would love to make this work, but the texas summers are a KILLER and it would have to make sense financially as far as how much isolation this could provide. thanks

Let me try to tackle this 8) Yes TX is hotter than a bank robbers pistol. Im NOOOOO expert but the picture in my head sounds like your working a 2 leaf system. My sugestion: Instead of 2 layers of plywood, why not replace 1 layer with 5/8 dry wall? It has more mass (stopage) than wood and up north here its cheaper than plywood. 5/8 drywall is 9.99 per sheet and plywood is running about 12.00 per sheet. If the roof your discribing is the only barrier between the sound and the outside, you have to get very attached to a caulking gun too......... :shock: its not fun at all, but a very very important step or so Ive learned.

Oh yeah from what I understand the insulation in a sealed cavity deosnt have to be the real hard to get stuff (rockwool or 703), so as long as its sealed good Id go for the weather proofing over the sound stopping on insulation. If I understand your situation and this cieling is also your sound barrier, you probbably should put 2 layers of decking on the top layer too before the shingles. The shingle nails will punch 100's of holes in your sealed space and Im pretty sure thats not good?

Can you post a sketch showing the roof beams, plus whatever else you're proposing to add on top of them? As to radiant barrier ply, here's what one manufacturer says - Radiant Ply FAQ From that, it sounds like you'd need to put the radiant ply on the INNER layer of the outer leaf, with the silver side facing inward. They also say you should have at least an inch of air space next to the silver side, so you can't do a complete fill of insulation to improve damping of your roof/ceiling - this will cost you at least a couple of dB, possibly more. In addition, with solid coupling between inner mass and outer mass, you'll not have as good mid and high frequency isolation as you would with either resilient couplings or separate frames, but the lows will be about the same because they are primarily controlled by the air gap between inner and outer mass. For this reason, if the existing construction can handle the weight it would help you to use wider boards as spacers/rafters between inner and outer mass - Also, because the foil side of the radiant barrier needs to be to the inside on roofs any extra outer leaf mass would need to be plywood, which isn't as dense as drywall or mdf but is more durable. Something else that can help this outer mass is to use TWO layers of 30# felt under the shingles instead of ONE layer of 15# felt like most roofers tend to use - you'd need to pay a bit more for this, but it would help the outer mass layer somewhat as well as "self-seal" better around all those shingle nails. HTH... Steve (Oh, and yes this would be a 2-leaf barrier, unless you're adding another ceiling you didn't mention)