Would I have 2 leaves or 3 here.

jthomas1 · Post by **jthomas1** » Tue Oct 05, 2004 7:24 am

Got a Q. If I build like I have drawn here, am I going to create 3 leaves??? The walls of my garage don't seem to have a plywood layer on the outside. Just those foam insulation sheets and vinyl siding. Should I add the sheetrock on the inside of the outer wall or no?

thanks

-josh

dymaxian · Post by **dymaxian** » Tue Oct 05, 2004 7:34 am

The outside foam insulation sure won't be air-tight, and the vinyl siding itself isn't going to stop very much sound. I'd put 2 layers of drywall on the inside face of the outer wall (where you have one layer shown).

But the short answer is "yes".

jthomas1 · Post by **jthomas1** » Tue Oct 05, 2004 12:20 pm

Right on. Would it be better to insulate the inside face of the outer wall, like the inside out walls I've read about? Or, would I insulate the cavity created in the outside wall?

thanks

-josh

jthomas1 · Post by **jthomas1** » Tue Oct 05, 2004 12:34 pm

Just found this topic, http://www.johnlsayers.com/phpBB2/viewtopic.php?t=2366. Knightfly's answer was different, but this guy has wood siding. I would tend to think my setup wouldn't be considered to have a third leaf, but who knows.

What are the physics behind the 3 leaf problem anyhow?

-josh

knightfly · Post by **knightfly** » Tue Oct 05, 2004 8:34 pm

The third leaf (and yours is one, just not a massive one) makes the air spring between leaves stiffer, raising the resonant frequency - this worsens TL at lower frequencies, which are the hardest to isolate in the first place.

Insulation in the outer cavities would help some, Kase is right in suggesting more mass for your middle layer - in fact, I'd put at least three and preferably 4 layers of wallboard there with the outer vinyl causing triple leaf behavior - then, as much air space as you can allow between those layers and the inner leaf, and at least two more layers of wallboard.

This is NOT ideal, nor do I recommend it if there's a way to get rid of the extra leaf - but if you can't, then this is the best way to compensate. Don't ask me what kind of STC or TL you'll get though - I've no way to calculate that, other than to say it won't be nearly as good as a double leaf wall with heavy mass and wide air gap... Steve

jthomas1 · Post by **jthomas1** » Wed Oct 06, 2004 6:56 am

knightfly wrote:Don't ask me what kind of STC or TL you'll get

Heh, I wouldn't dare. I wish there were a way I could experiment though.

The garage walls are partially underground, so the bottom 2' is cinder block, it moves up to 4' of cinder block halfway to the back fo the garage. I'll have space to to add good 3" of mass before I'm flush with the face of the cinder block.

Thanks.

-josh