Why Insulation in Inside Wall?

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I'm about to hang one of the LED big screen TVs on my wall. To tidy it up I decided to run the cables through the wall. When I cut a hole in the wall I discovered it was filled with insulation. It's an inside wall. The master bath is on the other side of the wall. Why is an inside wall insulated?
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jim evans wrote:

Minimal amount of noise reduction
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A lot of noise reduction.
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Ron wrote:

Not really only from fiberglass insulation unless also did something about separating studs, etc., ...
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It is a damper to prevent the drywall from acting as a soundboard.
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snipped-for-privacy@dog.com wrote:

But if the rock is mounted on the the opposite sides of the studs w/o isolation it still transmits.
I'm not saying it doesn't have some benefit and it depends greatly on how much but a typical exterior wall cavity w/ the air volume and uncompressed batt won't make a tremendous difference. If they did put in extra and took some extra steps it can...
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We insulated our half bath walls with fiberglass. It doesn't help much with noise. It might keep a full bath warmer after a shower though.
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I agree, you only get minimal noise reduction unless you build a wall with 2x6 top and bottom plates and 2x4 staggered studs so you have 2 isolated drywall panels on each side. This is the proper way to build a sound-reducing wall with normal building materials:
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Top down view of staggered 2x4 studs on 2x6 plates, then you insulate for maximum quietness. This is a great method for home theatre rooms, furnace rooms, etc.
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Well I can see my ascii art work failed because the text wrapped.
But the idea is to build 2 isolated 16 inch OC stud walls on 2x6 plates with 2x4's, offsetting the studs by 8 inches. Sound will not transmit easily because one sheet cannot vibrate the opposite sheet because there are no common studs that hang both sheets.
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Well, my previous home had ALL of the interior walls insulated, and it was MUCH quieter than my current home that doesn't have ANY interior walls insulated.
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Ron wrote:

Again, it depends on what was actually done. As someone else noted, it's mass and isolation that do sound deadening effectively, and a simple fiberglass batt doesn't accomplish much of either.
That's not to say one can't do effective sound isolation, but it has to be more than simply adding 3" pink glass in a tubafor wall to be of much benefit. So, your builder apparently knew what were doing and took some steps.
As noted earlier, that's what I presumed (perhaps erroneously, he's not come back w/ a real clarification when asked that I've seen so far) OP had as that's what I ran into most frequently.
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I bought the home from a guy that was a contractor, so he knew what he was doing. VERY nice custom built home that I stupidly let my ex-wife have!
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I had remembered insulation was fairly effective, but apparently not.
http://www.saecollege.de/reference_material/pages/STC%20Chart.htm has sound transmission class numbers for different wall constructions. Insulation adds, but not a lot (like you said).
http://en.wikipedia.org/wiki/Sound_transmission_class has more information on STC and says it is "roughly the decibel reduction in noise a partition can provide".
A local university hospital used 5/8 rock on 3.5" metal studs with insulation between patient rooms (might have been double 5/8 rock each side). But they were real strict on penetrations. Electrical boxes had to be in different stud partitions. Doors had seals on all 4 sides. Cracks are a major flaw in isolation.
===================== I agree with whoever said you want to fish between the insulation and drywall, not thorough the insulation.
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wrote:

Fibreglass insulation is medeocre at best for sound proofing, but rock wool is quite effective (also known as mineral wool or spun slag)

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probably for noise reduction
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Any ideas how to get the insulation out without removing the sheetrock?
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jim evans wrote:

just use a fish tape to pull the wires through.
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Use the fishsnake several times with a hook on it, that will clear out most of the insulation.
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wrote:

I'm a little uncertain about that. I tried to push a flat plumbers snake (which I've used to fish wires before) up the wall and couldn't get it to go. It was like pushing into a pillow. And there's a fairly large bundle of cables that have to go through the wall. Here's a photo of about 2/3rds the bundle that has to be pushed through this mush -- http://img395.imageshack.us/my.php?image blebundlece4.jpg
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jim evans wrote: ...

Start at upper point and go down instead of up and use something stiffer and smaller diameter (like baling wire size/stiffness) initially.
What is the actual insulation and how compressed is it? If they really packed the cavity full (was assuming just a R-9 batt in a 2x4 wall which would leave a lot of air space) may be a trick. How far you gotta' try to go?
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