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Discussion Starter · #1 ·
I have been reading a number of build threads lately. Generally, people talk of "loosely" packing this or that soffit with insulation. Almost never does anyone just say they put insulation in it, they make sure to note that it was loosely packed.


So what's the big deal? Why make sure it's loose? Why not pack it in tight? Why care either way?


I did some searching around - and maybe I'm not the best searcher - but didn't find a thread where this specific topic was discussed - loose vs tight fill vs who cares.


Thanks!
 

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It all has to do with air pockets in/around the insulation (for sound isolation, it is literally more on a molecular level). Packing the insulation simply removes that quality and increases the sound transference.
 

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It's also a general installation comment about fiberglass insulation. Tightly packing (squishing, crushing) fiberglass batts ruins the insulation "R value". Since that's somewhat counter-intuitive, you almost always see the phrase "loosely packed" in this context - even outside of the theater / soundproofing conversations...
 

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The best insulator is a vacuum. The next best insulator is dead air space. Anything that can be done to prevent air molecules from bumping into each other improves both thermal and acoustic insulation. Fiberglass works by creating small air pockets in which the air molecules are relatively unable to move within and against/through the glass fibers. Compressing the fiber glass reduces the number and size of dead air spaces, allowing the glass to transmit thermal and acoustic energy.
 

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Yep, it is all about air movement. While some of the foam and solid insulation is excellent for dealing with temperature issues, those same insulation are usually ineffective for sound isolation. The key are the tiny little air pockets made between the fibers of the loose insulation. Each time the sound crosses an air pocket, hits the next fiber, and then goes through that fiber to the next air pocket, the sound levels decrease. When you "eliminate" those air pockets with dense/rigid/solid insulation you actually increase the amount of sound that is transmitted through the insulation.
 

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Discussion Starter · #7 ·
I knew I'd get great answers out of everyone. Many thanks! The only place where I have packed my insulation has been where I needed to fireblock. Of course I haven't really put any other insulation in than that yet. But this helps me to know how to fill my soffits. I have to admit - it's hard not to cram too much insulation into a space, you kind of have to hold back.


Funny Johnsteph10 - I've been reading your build thread on and off the last week - about 300 posts in. Going back in time to June '09 - almost 4 years ago now!
 
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