Anyone ever use sill seal as a barrier between drywall and floor joists/wall studs? I was recently told that is can help with acoustic deadening (and even thermal bridging). Figured I would check with some experts.

I like your thinking, but I would have concerns about the flammability of the materials (never researched it, just a thought).

In the stick built areas of my house, I used either 1/2" or 1" rigid foil faced foam between the studs and the drywall.  Did the same for the ceiling of the under house garage.

I recently had a chance to check my house with a thermal imaging camera.  Attached is a thermal image of one of those walls.  The only thing you can see (colder) on these walls are the screw heads (and the window of course).  Neat.

Good luck,
Ed 

Sill gasket is flammable, but 1/2" of gypsum board is deemed an adequate thermal barrier for fire safety for such materials in the IBC.  By virtue of it's being under the wallboard, it's safe.

The effect on thermal bridging is very slight with then materials like sill gasket. Closed-cell foams of that ilk run about R6 per inch of thickness, but when you're talking only 1/8" (less, when compressed), figure on getting about R0.5 out of it, best-case.

As for acoustic isolation with sill gasket, it won't buy you much- it'll be measurable with instrumentation, but not so noticable with your ear.  Double layering of the gypsum, glued together with acoustic dampening adhesive buys you much more.  Blowing cellulose into the cavity to a density of at least 3lbs/ft^3 will also have a large effect- far more than standard density fiberglass batting, and more than lower density 2-hole method blown fiber.

Acoustiblok/Thermablok strips between the studs and gypsum will give you both significant acoustic isolation AND thermal break (at similar cost to double gypsum, IIRC).

Dolphin, there's a product called green glue that would probably get you better sound dampening.

Egouin, did you experience any nail pops on your drywall using the foam board between the drywall and studs? What did you use to fasten the drywall to the studs through the foam?

I have one screw pop that is noticeable.  I am not sure if it occurred post construction, or if we simply didn't screw it in properly in the first place.  I simply haven't had the motivation to fix it. 

I used longer sheetrock screws to fasten the wallboard through the foam.  Let's say 2" screws.

Thinking.. One caveat here... these are 2x6" stud walls with 4-5" of closed cell spray foam.  These walls are likely much more rigid than a standard stick built wall.  A less rigid wall system might result in more nail pops.  Just something to keep in mind.

Ed

Dana1, do you know exactly what acoustic dampening adhesive (green glue) is made of? I wonder how it differs from standard (cheaper) construction adhesive?

Don't know for sure what it is, but it's substantially more flexible than most construction adhesives- as flexible as RTV silicones or butyl caulking also used in soundproofing. 

See:

http://www.quietsolution.com/html/quietseal.html

http://www.soundisolationcompany.com/sound-solutions/accessories/silenseal-acoustical-sealant/