Insulation between walls

Does it matter if the insulation between the walls touches? Since there is only 2 inches air space betwween the two leaves, and the insulation is kinda thick, it will be touching, which will not really leave an air gap per say. Is it important that there be a clear air gap with nothing at all touching, not even the insulation?

My instincts say no, it shouldn't matter. If the insulated space between the studs is considered as airspace in the M-A-M formula, it should make no difference whether the insulation is touching or not. Then again, it seems that much of this acoustics field is counter-intuitive. I guess i'll wait and see what the experts have to say.

Hi, AFAIK it must be clear air gap. If it’s not than you have one leaf with different materials.

groff wrote:

Hi, AFAIK it must be clear air gap. If it’s not than you have one leaf with different materials.

Actually i disagree, fiberglass is considered the "air" part of the MAM formula as it can pass air through it. Whether it is touching or has a gap of air between it, it doesn't make a difference to the sound wave. Only if you overc ompress the fiberglass to the point it acts as a "mass" partition, will you reduce your leaf isolation. Code in certain areas dictates that some builds must use a full fill of fiberglass for fire resistance, and that means no open space at all between the inner and outer leaves.

Hmmm... I put some up today and it IS touching in many places. Not compressed , but definitely touching. :?

Recommendation is to use something to hold the insulation in place. Can glue the insulation in (depending on type), tie/rope it in OR the most recommended thing is to use packing straps with staples. I think that's in Rod's book? I know Steve (Knightfly) wrote about on some post. There are pics of this somewhere on this site. Key thing Steve said was to install the straps before putting the wall frame up, BUT I can imagine putting straps in when walls are up but before insulation. IN the thread where Steve wrote that (maybe more then one thread) another poster questioned whether the straps could act like a guitar string and somehow decrease ISO via exciting a frequency and the answer was something like better to have straps then not... really should not make a difference. Over all though we'll never no if insulation will fall out of place over time. I can only imagine the 2 leaf wall system will only slightly be coupled together if insulation from 2 leafs/double frame touch. For sure it won't be a hard connection like a single frame wall, single frame with RC or other hard attached decoupling methods like wall stabilizers. It won't be the end of the world BUT if it can be helped YES try to keep the leafs not touch via loosely placed insulation. Marc

guitardad72 wrote:

Recommendation is to use something to hold the insulation in place. Can glue the insulation in (depending on type), tie/rope it in OR the most recommended thing is to use packing straps with staples. I think that's in Rod's book? I know Steve (Knightfly) wrote about on some post. There are pics of this somewhere on this site.

Yeah. I put lillte nails with a big head on them nailed inside the back of the stud and tied fishing string to hold the insulation.

eah. I put lillte nails with a big head on them nailed inside the back of the stud and tied fishing string to hold the insulation

AND it's all tight and still not holding up the insulation? How far apart did you space the fish string? Does it look like adding in more might work? Marc

guitardad72 wrote:

Does it look like adding in more might work? Marc

No.

You also need vertical and horizontal draft stops, almost certainly required by code in your jurisdiction, which can entail complete fill of insulation every ten feet vertically and horizontally. For example, on 16" o.c. stud spacing along a long wall, every eighth stud bay would be completely filled with insulation without a gap between the walls. This helps to prevent a fire that breaks out in a wall from engulfing the entire area. --Keith :mrgreen:

An air gap is unnecessary IMO...and, as Sharward says, there needs to be fire-stops every so often, which mean either a complete fill with insulation or wood cross braces. The reason why an air gap is suggested is that it does not allow for sound to mass from one leaf to another via mechanical conduction (i.e. vibration). But, isulation is not a material that supports mechanical vibration, so it's not a problem....EXCEPT if you stuff it too tightly. Too much insulalation density can support vibration. Cheers, Kris