Adding more drywall may or may not impact the R value in the cold room. I honestly don't know the answer to that, and would only be speculating. I would *think* that it would help, but not sure how much as the cold would still transfer through the drywall due to direct contact (that is where air gaps and "airy" insulation helps out).
I don't think that it would be worth the effort just for temperature control to add the drywall and have to finish it (mud/tape/paint/etc). I don't think that it would hurt anything from an R value perspective, but again only talking about temps, it won't help much/any. Sound, different story.
EDIT: I had to look it up. From the National Gypsum website.
Drywall thickness and R Value:
1/2" -> R0.45
5/6" -> R0.56
Not going to make much difference at all for R value help.
EDIT #2: OSB is a little better at R0.51 for 1/2", but still nothing great.
FYI that you could use OSB for your first layer if you wanted to. It weighs almost the exact same as drywall so you have good mass, and you can then screw the drywall anywhere you want because it won't have to hit a stud, but it costs about 50% more than the same essential thickness of drywall and OSB has to be cut with a power saw, vs a box cutter and straight edge.
Oh, and just remember that when you do put up your two layers, do one layer horizontal and the other layer vertical so that you have a more rigid, and less common "edge points" where the horizontal edges of one layer overlap the vertical edges of the other layer. You can use regular clear (not colored) long life 100% silicone caulk on the edges and seams of the bottom layer to get you REALLY close to the more expensive acoustical sealant. Don't use it on the top layer as 100% silicone doesn't allow mud/tape/paint/primer/anything to stick to it. Or just use it if you have any gaps and call it pretty darn good.
EDIT #3: Don't get the lightweight drywall either. Go for the heavy stuff (like 54 pounds for a 1/2" thick 4'x8' sheet and 70 pounds for a 5/8" thick 4'x8' sheet). You want mass, and now you can also see why that little extra 1/8" thicket 5/8" drywall works better than the 1/2" at 15 pounds more per 4'x8' sheet.
Don't be afraid of the weight. I used two layers of the 5/8" drywall but in 4' x 12' lengths. So each sheet was about 100 pounds and really big to work with, but it also meant fewer seams, and each sheet that went up covered more area faster.
EDIT #4: Check out local drywall supply shops before going to a home improvement store. I had all of the drywall for our entire basement delivered exactly where I wanted it INTO my house for a LOT less than buying them, even at the "bulk" rate at the cheapest home improvement store and I would have had to haul them home myself and unload them myself too.
I saved time, money, my arms/back/legs, and I got "fresh" drywall that wasn't brittle from sitting in a home improvement store for a long time.
Last edited by nickbuol; 01/21/14 05:19 PM. Reason: I kept adding more.
