Insulation in a double stud wall

I was wondering to myself last night: Is it necessary for a double stud wall to have insulation in each stud wall? What will one lose if putting insulation in only one side? The reason I ask is because when ever I see cutaways of a staggered stud wall, there is one piece of insulation, but the double stud wall always has two (one in each stud wall). I've tried searching for answers on this site, but I couldn't really find any. If any one has any data or practical experience, I would love to hear about it.

Although it is not much mass, it is still mass.. I use insulation in both walls inner and outer. The key here is that you want to match the mass for both walls.

There s a pdf here sorry in french but dB is dB: http://www.placo.fr/bpb_fr/BaseInfos/do ... laco%20MEO look for CLOISONS SEPARATIVES PLACOSTIL SAA & SAD Pdf on page 4

garlo wrote:

There s a pdf here sorry in french but dB is dB: http://www.placo.fr/bpb_fr/BaseInfos/do ... laco%20MEO look for CLOISONS SEPARATIVES PLACOSTIL SAA & SAD Pdf on page 4

I could not find it; could you post a direct link to the PDF? My wife can translate for me :)

Although it is not much mass, it is still mass.. I use insulation in both walls inner and outer.

Actually, the way I understand it, insulation in the wall isn't really about mass: That's what the gypsum board is there for. Gypsum board is for mass. The purpose of the insulation is to damp the cavity. It's part of the "spring" in the "Mass-Spring-Mass" concept. It damps the acoustic energy in the cavity between the leaves of the wall, turning the sound wave energy into heat. You should put as much insulation as you can possibly afford in the gap. More is better. It doesn't really matter where you put it in the cavity (between studs on one side, between studs on the other side, in the air gap, all over). In fact, you can cram the entire air gap full of insulation for the best possible effect, (as long as you don't cram it in so tightly that it acts as a flanking path!).

Caught me again! but it is still mass at some level no?

Soundman2020 wrote:

Although it is not much mass, it is still mass.. I use insulation in both walls inner and outer.

Actually, the way I understand it, insulation in the wall isn't really about mass: That's what the gypsum board is there for. Gypsum board is for mass. The purpose of the insulation is to damp the cavity. It's part of the "spring" in the "Mass-Spring-Mass" concept. It damps the acoustic energy in the cavity between the leaves of the wall, turning the sound wave energy into heat. You should put as much insulation as you can possibly afford in the gap. More is better. It doesn't really matter where you put it in the cavity (between studs on one side, between studs on the other side, in the air gap, all over). In fact, you can cram the entire air gap full of insulation for the best possible effect, (as long as you don't cram it in so tightly that it acts as a flanking path!).

OK, sounds reasonable. With that said, is the effect of insulation linear? i.e., will one row of insulation at 7.0" perform the same as two rows of 3.5"? I can cover the same amount of Sq. Ft. with one row of R-30 (9.5") for the same price as I can with two rows of R-13 (3.5").

From what I've learned here, its the total thickness that counts, and it doesn't matter much how you make up that thickness (within reason!). So two layers of 4" thick each should give you the same effect as one layer of 8" thick (is there any such thing?), or four layers of 2". I think the cumulative effect is linear, but I'm not sure. But the basic rule is "thicker is better", so if you can get the overall thickness higher for less money somehow, then I reckon that sounds like a good idea! One thing that you should be aware of that I just found out on re-reading a good paper on the subject of walls: If you have a very deep air gap, and you can't fill it to the full thickness with insulation, then you should try to "angle" the insulation across the space, kind of zig-zag, both vertically and horizontally, to maximize absorption. This is because part of the resonance in the cavity will be parallel to the walls, and if you leave a totally open airspace in that direction, with no insulation, then you will not be damping that resonance. So if you have, for example, a ten-inch air gap and you can only afford to put in six inches of insulation, then you need to make sure that you place the insulation in such a manner as to completely "bridge" across the entire ten inches every now and then, by angling the insulation back and forth diagonally across the gap. Not sure if the explanation makes sense. If not, I'll do a diagram.

Caught me again! but it is still mass at some level no?

Well, yeah, it is mass, but it's not really acting as mass, since it isn't very dense, and isn't rigid, and has very low flow resistance, and is limp and fluffy. The exact opposite of a sheet of gypsum board. On the other hand, I read one comment somewhere that if it is very dense, and is directly attached to one of the leaves, then it can actually add slightly to the mass of that leaf, but not very much. However, if it is just loosely placed in the gap then it doesn't really perform as "mass" in the sense of the "M-A-M" concept. To block sound totally, you actually want an infinitely large, infinitely dense, infinitely massive, infinitely stiff, infinitely impermeable panel that is hermetically sealed around your sound source. If you could do that, you wouldn't need insulation and another panel! But of course there ain't no such thing in reality. (Pity!) But insulation isn't stiff, impermeable, dense, or hermetically sealed: sound pretty passes right through it, especially at low frequencies. The fibers only vibrate to absorb sound by converting it to heat: they don't reflect much sound, nor do they diffuse it, so they don't act like the mass in the gypsum board. Maybe Andre or Ro or Ethan can go more into the acoustic theory of insulation. That's about as far as my knowledge takes me!

kstreetrecorders wrote:

I was wondering to myself last night: Is it necessary for a double stud wall to have insulation in each stud wall? What will one lose if putting insulation in only one side.

It is an easy question in a pragmatic way. :) Double stud walls are tested with insulation on both sides. If you change to one side you are getting into constructions that are not tested. Acoustics professionals are loath to suggest anything that is either not tested or physics are not clearly understood. In short, it is unknown and do not do it expecting predictable results. Look through NRC IR 761 for a wall assembly that fits your requirements and budget. Andre

AVare wrote:

kstreetrecorders wrote:

I was wondering to myself last night: Is it necessary for a double stud wall to have insulation in each stud wall? What will one lose if putting insulation in only one side.

It is an easy question in a pragmatic way. :) Double stud walls are tested with insulation on both sides. If you change to one side you are getting into constructions that are not tested. Acoustics professionals are loath to suggest anything that is either not tested or physics are not clearly understood. In short, it is unknown and do not do it expecting predictable results. Look through NRC IR 761 for a wall assembly that fits your requirements and budget. Andre

Why do you have to bring logic into this :) I'm reading the document now...will check in later.

Move volume of wool is better absorption. And placing it against the mass (GB) will also dempen the fibrations of the boards a bit. There you go, better results with more wool :)

From Placoplatre tests 3X13mmGB+2X85mminsulation+ 3X13mm GB =dB 69 3X13mmGB+1X85mminsulation+ 3X13mm GB =dB 62

OK, I found the place where I had read about this in the 1973 Wyle paper: