A couple double wall construction Q's

[drfrankencopter]

Hi All,

I've got some of my outter walls completed in my double-wall construction (1 room plus iso booths studio), and have some questions about how to go ahead with the double wall construction.

1. Is it better to set up the studs of the inner wall so that they are staggared with respect to the studs of the outter wall? Most constructions that I've seen have all the studs in-line, but wouldn't it be best to have them staggared, as that way the drywall from one wall's antinode point (mid point between studs) would be at the other wall's node point.

2. What is the best way to get insulation inside the wall cavity while maintaining the air gap between the two walls? I assume that if the insulation from the inner and outter walls touch each other that there will be a reduction in performance owing to the direct conduction path. I plan on using Roxul (rockwool) inside the walls. Will this be rigid enough to just place inside the framing and will it stay in place while I affix the drywall to the inside wall?

Thanks,

Kris

[knightfly]

1 - Rod and I have discussed this very thing, and we both agree that staggering the studs of adjacent frames of double-framed walls (your scenario) SHOULD improve the wall's performance a bit, but to the best of my knowledge this experiment hasn't actually been performed - at least not with the level of anal retentiveness displayed by our Canadian friends in IR-761, etc -

2 - as to insulation "shorting out" a wall, this would depend on the type of insulation and its density. Standard house insulation can be compressed quite a bit and still not cause bridging; the preferred 2.5 to 3 PCF (40-48 kG/m^3) rigid fiberglass or rockwool should be inserted between studs snugly enough that it tends to bow, and placing the convex part of the bow TOWARD the wallboard that's attached to those studs; but it shouldn't be noticeably compressed. Just having it touch (lightly) won't cause a noticeable difference.

Those weights of the rigid batts are stiff enough that if you follow the above, they won't go anywhere...

In fact, one method of floating a heavy wooden floor (even concrete in some cases) is to do it on top of several inches of rockwool; may not get quite as good isolation as if the stuff weren't compressed, but still pretty good... Steve

[sharward]

Plus, if the batts are touching, you'll get the fire protection that the Roxul products feature.

[PhiloBeddoe]

1 - A small practical concern of staggering the studs is that you will not be able to walk between the rooms through the studs during the construciton. A benefit of staggering them is that you will not have any back to back electrical outlets.

2 - The USG manual recommends scoring the mineral wool insulation (e.g. Roxul) in the middle longways thus forcing the batt to bow outward and contact the drywall. I did this when insulating my rooms and it worked well. I've seen some people put strings through the studs to keep the insulation from falling in, but I found this to be unnecessary. The studs will be 16" on center and the Roxul is 16" in actual width so they fit pretty snug.