# soffit bass trapping



## fitzwaddle (Aug 25, 2010)

Hi all, I'm putting insulation (faced pink fluffy) in my soffits, and planning on covering the bottom with fire treated fabric. One layer of faced R-19 in an 8" high soffit.










A few questions:


code: will fire treated fabric over kraft paper faced insulation (facing toward the room) pass inspection?
will one layer of breathable fabric be sufficient to keep stray fibers from escaping?
is pink fluffy alone sufficient for purposes of bass trapping, or would I be better off adding a layer of OC703 at the bottom next to the fabric? And if so, should I keep the facing on the fiberglass under (over) it?

Thanks in advance!


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## bpape (Sep 14, 2006)

You should have no problem doing that. No need for 703 unless you just want something firmer. That would also then allow high frequency absorption that the paper face on the fluffy will minimize. Not a bad thing at all if the rest of the room is properly controlled.

The paper facing itself will form an additional barrier for fibers. Honestly though, as long as it's just up there and you're not beating on it or poking it, the fibers aren't going to come loose anyway.

Bryan


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## fitzwaddle (Aug 25, 2010)

Thanks very much Bryan, good to know I'm not too far off course.

Any comments on the fire safety / building code issue? I live in fear of my building inspector - especially when I see this:










(note sure a layer of fabric would constitute an "approved building material" :scratch


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## bpape (Sep 14, 2006)

I can't really answer that, sorry. My guess would be no - he would not consider cloth, even though fire rated, to be an approved material per that warning. That said, if the fire is high enough to get up to that paper, it's already out of control 

Bryan


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## fitzwaddle (Aug 25, 2010)

Hmm - Moggie on "another forum" indicated he used OC703 w/ FSK facing over pink insulation. I guess I could tear off the kraft paper, and replace with a separate layer of FSK (which has a "flame retardant adhesive" between the aluminum layer and paper layer). Or maybe since the back side of the FSK is kraft paper too, just lay it over the top without removing the kraft paper on the insulation.

Unless anyone has any better ideas?


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## bpape (Sep 14, 2006)

Well, you were considering doing some 703 as a bottom layer anyway. That would certainly solve the problem with the inspector. No need to really even remove the paper. Just tack it up as normal and then use screws with fender washers to mount the 703 with FSK to the underside of the joists.

Bryan


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## SierraMikeBravo (Jul 1, 2007)

I have a question. Why leave the insulation "exposed" at all?


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## fitzwaddle (Aug 25, 2010)

SierraMikeBravo said:


> I have a question. Why leave the insulation "exposed" at all?


I didn't mean to imply completely exposed if that's what you mean, it will be covered with fire retardant fabric - but no "construction material" per se (drywall, mdf, plywood).


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## SierraMikeBravo (Jul 1, 2007)

fitzwaddle said:


> I didn't mean to imply completely exposed if that's what you mean, it will be covered with fire retardant fabric - but no "construction material" per se (drywall, mdf, plywood).


Hi! 

I guess my point is, why leave it without plywood or whatever material you are using? What's the benefit?

Thanks


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## bpape (Sep 14, 2006)

The benefit is that it acts as a broadband bass absorber with restricted high frequency absorption due to the paper or FSK facing. It's a great way to get a good amount of bass control in the room from a decay time perspective if there isn't wall/corner space available and/or you already have those covered but the decay time curve still isn't correct.

Bryan


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## SierraMikeBravo (Jul 1, 2007)

Hi Bryan,

Well, I am sure it could be. My point is that I would not leave this area open. If you wish to use it as a bass trap, it will be more effective by enclosing this space. Low frequencies will be attenuated further by having to pass through the material once, then once again as the lower frequencies exit the soffit. Making the material additionally flexible, such as spacing the lookouts appropriately, will offer even more absorption of the lower freqs. It just seems to me that the potential gains of leaving it exposed and covering it with material are outweighed by the additonal gains of covering it entirely with building materials. Further, if you wish to attenuate high frequencies under the soffit, cover the underside with 1 inch rigid material. If you don't wish to attenuate high freqs, just cover the soffit underside with drywall or cover in fabric. You'll meet code and likely accomplish the intended task more efficiently. This My two cents. :bigsmile:


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## bpape (Sep 14, 2006)

It's just a different way to do bass control. What you're describing will be more efficient, though effective over a narrower band of the bass as it's technically acting as a tuned membrane absorber rather than a broadband absorber. Both have their place - just a matter of what's appropriate for a given room, what else is in there, etc.

Bryan


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## SierraMikeBravo (Jul 1, 2007)

Hi Bryan,

Yep, you are right, but if you vented it with bar diffusors, it wouldn't be. Then, you get best of both worlds AND still meet code.  Or, you could use black mat boards from Johns Manville to line the soffit. Just some additional suggestions. onder:


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