Double Wall, Staggered Studd, or Whisper Clips...confused. :) - AVS Forum
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post #1 of 17 Old 04-24-2012, 10:28 PM - Thread Starter
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Alright acoustic experts, I need some help.

So I am in the planning stages of my dedicated Home Theater addition (18'Wx22'Lx10'H). I was working with the builder on plans today and had pretty much figured on going with 2x4 staggered studs on 2x6 plates. He tossed out an issue I did not expect in that Oregon requires R-21 insulation in walls now and he did not believe there would be room to hit that R-Value without going with a Spray Foam (don't want)...might be able to get there with blown cellulose, but he doubted it. He is going to check.

He suggested we go with double 2x4 walls and use Whisper Clips to mount drywall to the ceiling since we will do a single ceiling...I will be doing 2 layers of 5/8" drywall with Green Glue in between on walls and ceiling with house wrap, 3/4 Plywood and Hardiplank on the exterior. Room is on a 6" concrete slab.

I am concerned about losing too much space, so I wondered about the gap I need to leave between the walls. To get the space I need, I would only be able to leave about a 1" gap. Is this enough? Does resonance negate the benefits of decoupling here?

Would I be better to go with 2x4 staggered walls on a 2x8 plate instead?

I can do either of those options much cheaper than whisper clips throughout and, according to Soundproofingcompany.com, Double wall is best.

FYI, I am much less concerned with sound getting into the house than I am with sound getting out of the house to the neighbors, although I will be using dedicated HVAC in the theater and will seal the double entry doors as well as possible to minimize transmission to the rest of the house.

Thoughts?

As always, thanks in advance!

Shan
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post #2 of 17 Old 04-25-2012, 11:00 AM
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Congrats on the coming new space!

I believe double wall is the preferred method so long as the inner wall is decoupled from the rest of the framing. If the framer nails the top plate for the inner wall to the bottom of the ceiling joists, the inner wall will not be decoupled and won't perform as well. I think you can avoid this problem by using clips to attach the top plate of the inner wall to the ceiling joists.

If I understand all of this correctly, room-within-a-room construction performs best; then double wall, clips/hat channel, and staggered stud (in decending order).

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To get the space I need, I would only be able to leave about a 1" gap. Is this enough? Does resonance negate the benefits of decoupling here?

I think a 1" gap will be fine. You will fill the entire space with insulation, so you shouldn't have to worry about resonances.

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post #3 of 17 Old 04-25-2012, 11:06 AM
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Shan,

You're not considering double drywall for the ceiling? You mentioned "...a single ceiling."

The walls as described would not normally need clips, as the staggered framing technique decouples the wall. The problem is the insulation. The route you're going sounds like you'll somewhat couple the leaves of this wall, resulting in poor decoupling and insufficient cavity absorption.

Can you consider double studs? An exterior framed 2x4 or 2x6 wall with an interior 2x4 wall? This would accommodate more insulation and offers the option to mix foam and fiber, resulting in good absorption.

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post #4 of 17 Old 04-25-2012, 12:56 PM - Thread Starter
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Thanks for the feedack. Sorry for the confusion about the ceiling. I will be Doing double drywall with GG between. Planning to decouple it with whisper clips unless you think it would be ok to screw directly into the joists. 10-12 inches of cellulose insulation above.

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post #5 of 17 Old 04-25-2012, 12:58 PM
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Don't screw the drywall to he joists. Careful that the celluose is not packed at all.

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post #6 of 17 Old 04-25-2012, 12:59 PM - Thread Starter
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Ya Ted, What I want to do on the walls is 2 2x4 walls with a 1 inch gap between. If that will be good I will give the builder the go ahead. Just worries the 1 inch gap is not enough.

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post #7 of 17 Old 04-25-2012, 01:02 PM
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The gap is enough IF the two sets of studs aren't connected in any way

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post #8 of 17 Old 04-25-2012, 02:01 PM
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You might want to check Ecobatt High Density insulation as you could get R21 out of 5.5" thick batts with their R21HD batts. See http://www.ecobatt.us/files/Ecobatt_Data_Sheet.pdf and http://www.ecobatt.us/index.html. You can buy the product from a local Insulation reseller. Prices are competitive with the pink type of insulation for a possibly better product. (BTW, I am not affiliated with Knauf, just think their stuff is good!)
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post #9 of 17 Old 04-25-2012, 02:04 PM
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Ted,

You wrote "The gap is enough IF the two sets of studs aren't connected in any way". What would be the minimum gap?

Thank you!
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post #10 of 17 Old 04-25-2012, 02:10 PM
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Fiberglass is as good as it gets for our sound isolation needs, however.

A 1" gap is generally fine, as any warping, etc from wood studs would never deflect that much.

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post #11 of 17 Old 04-25-2012, 02:28 PM - Thread Starter
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Willswine, thanks for the pointer. The problem with this type of insulation, according to my builder, is that compressing the insulation behind the extra studs hurts the R-Value and he did not believe the insulation inspection would pass as a result...they are pretty picky out here.

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post #12 of 17 Old 04-25-2012, 03:17 PM
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Ted: so you would not recommend using say a 3/4" or half inch gap?

Shan: I see though I was pointing to ecobatt HDs as they could also be used in a double stud wall to achieve even higher thermal (and possibly sound?) insulation. (Their PDF brochure has some STC measurements.)

Cheers!
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post #13 of 17 Old 04-25-2012, 05:50 PM - Thread Starter
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Ahh, OK, I misunderstood your post. I'll take a look at it as an alternative to the pink stuff. Thanks!!
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post #14 of 17 Old 04-26-2012, 12:33 AM
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Ted, ecobatts are fiberglass insulation, just anothef brand than pink.

Shan,

If you use R21HD ecobatts in staggered wall you can achieve R21 in the voids between the 2x4s. Where there is a 2x4, you do not compress the R21HD: you cut it to fit the gap. That might pass inspection. Check with your inspector. Report if you can.

If you go double 2x4 stud wall with the 1" air gap (total thickness 8" not including covering Sheetrock), you can get R30 just by using R15HD ecobatts to fill the void between the 2x4s. Add to that the R value of the 1" air gap and of the Sheetrock (?) and you should get both superior thermal and sound insulation though at a possibly higher price.

I know the recommendation usually is to go with the pink stuff but I am not sure it comes in High Density format, which Knauf rates at higher R values than non HD. Another advantage of ecobatts IMHO is that they are a more sustainable choice and a possible way to achieve higher air quality (no formaldehyde). This might matter to you as the double door entry to the theater recommended with double stud walls will provide an air gap too where the air quality may depend on that between the double studs in the wall. That is assuming there is no flexible seal between the two door frames isolating them from the 1" thick air envelope sandwiched in the double stud walls. I hope to be clear but realize I may not be.

On that topic I would appreciate if e.g. Ted could clarify what is recommended to seal the gap between the two door frames that prevents noise transmission: GOM, vinyl, any other flexible material? Hope I am not over thinking it
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post #15 of 17 Old 04-27-2012, 08:34 AM
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You do not want the higher density insulation. Let's be clear on this. Higher density "fluffy" insulations do NOT perform better in a wall or ceiling. The lower density fiberglass performs slightly better in the low frequencies than the higher density insulations.

The difference is small, and not worth too much dialog and consternation, and we generally recommend using whatever is cheaper and don't compact.

You would NOT want to spend extra funds on an insulation that markets its acoustic superiority. If you like the eco aspect of a fiber such as cotton or polyester, then that's a good reason to use it, but please don't spend the coin looking for an acoustic improvement.

Seal the 1/4" gap between two door jambs with a quality acoustical sealant.

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post #16 of 17 Old 04-27-2012, 11:05 AM - Thread Starter
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The cheapest pink it is then. Thanks Ted.

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post #17 of 17 Old 04-27-2012, 11:38 AM
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Quote:
Originally Posted by williswine View Post

Ted: so you would not recommend using say a 3/4" or half inch gap?

The size of the gap does not affect the decoupling - any gap is sufficient. But you want to leave enough to make sure any warping or sagging in the future doesn't bridge that gap.
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