R-13 vs Mineral Wool in a vaulted ceiling...

[olectric]

I am starting to drywall my 5/12 pitch garage ceiling. We are opening the ceiling up to the roof deck. My question is: with 2x4 trusses, is 8-lb 3" mineral wool going to be better for sound isolation than standard R-13? I will be hanging 2 layers of 5/8" drywall. I have read it both ways on the forum. Any help would be much appreciated!

--Justin

[jhbrandt]

Justin,

Filled with fluffy is best. Use the rock wool for acoustic panels.

Cheers,
John

[gn87berner]

Justin, Are you sure your 2x4 trusses will support the existing roof plus your added materials? Are you leaving the trusses whole or dismantling them? Decoupling some way will yield more soundproofing. It will make little difference what absorption you use in your 2x4 cavity regarding sound proofing. (Plus, I'm sorry to say I don't believe you will get very good isolation.) Use the cheapest you can find, the lighter and fluffier is best for lower frequencies. Mass,decoupling first 2 principles. Hope this helps. The Five Principles of Soundproofing / Sound Isolation

[olectric]

Thanks for the answers, guys.

We actually cut out most of the cross beams on the trusses...All they were doing was adding more weight hanging from the rafters. We left one large truss in the control room, which is sturdy as can be. Where the others were cut out, we added rafter ties. Extra gussets were added on either side to all of the rafters. These rafters were made out of what is now 60-year-old slow-growth lumber...it's much heavier and probably twice as strong as new lumber. It was twice as hard to cut through, anyway...

I might get someone else's opinion (like a local engineer), but I am really not worried about hanging two layers of drywall on this ceiling.

[PaulP]

Originally Posted by olectric

We actually cut out most of the cross beams on the trusses...All they were doing was adding more weight hanging from the rafters.

Did someone with building experience ok this ? You don't usually want to
carve up trusses like that, they're built the way they are for a reason.

Paul P

[olectric]

Originally Posted by PaulP

Did someone with building experience ok this ? You don't usually want to
carve up trusses like that, they're built the way they are for a reason.

Paul P

My brother in law, who used to frame houses for a living and now is an estimator for a commercial construction company, did the work. I trust his judgement. There was a bit of work to be done building up and strengthening the roof system. It seems that the roof of this garage was built as fast as possible with most of the cross beams being added as an afterthought. The one truss we left intact in the control room is the one doing most of the work. The others were reinforced with gussets and rafter ties. It's a solid job.

Got another question for y'all:

I have a double wall between my tracking and control rooms...there is a 1" space between the two walls. I was planning on filling each wall with 8lb rock wool...but y'all are saying that it doesn't really matter what's inside my walls. I am wondering if this applies to my double wall? It seems to me that having more mass in each 2x4 wall would be better since the walls are decoupled from each other. I had bought enough rock wool to completely surround my tracking room...since every wall is either 1" from an exterior cinder block wall or 1" from another wall, it seems to be sufficiently decoupled...but, if it would be better for sound absorption to use the pink stuff, I'll do that instead...This stuff is fascinating!

[jhbrandt]

Pink stuff in the walls. Use the rock wool for acoustic panels, etc...

Cheers,
John

[olectric]

Haha...ok, ok I get it. Sorry to make you repeat yourself!

[Bassmankr]

That does not look very structurally sound to my eyes. I hope you don't live in a cold climate where there will be any snow load on that roof. A 2" x 4" roof joist with a 2' spacing given the spans in the pic is not going to hold much of a load. I see you did add small collar beams at the ridge but they should be lower so you form a decent size triange. The purpose of the low hanging tie beam is to keep your walls from collasping outward and form LARGE triangles which support the roof load (that's why they are called roof trusses and not just joists). If you want to keep that space stripped and open then just expose the wood trusses or replace structure with wire cable. Looks like you made the structure unsound and not to code. I see plenty of local news reports about building colapses because of remodeling work failing to properly brace or taking out structural elements.

[LaLaFaV]

Looks great, Justin. It's cool to see that you went with the higher vaulted ceiling. thumbsup

[olectric]

Originally Posted by Bassmankr

That does not look very structurally sound to my eyes. I hope you don't live in a cold climate where there will be any snow load on that roof. A 2" x 4" roof joist with a 2' spacing given the spans in the pic is not going to hold much of a load. I see you did add small collar beams at the ridge but they should be lower so you form a decent size triange. The purpose of the low hanging tie beam is to keep your walls from collasping outward and form LARGE triangles which support the roof load (that's why they are called roof trusses and not just joists). If you want to keep that space stripped and open then just expose the wood trusses or replace structure with wire cable. Looks like you made the structure unsound and not to code. I see plenty of local news reports about building colapses because of remodeling work failing to properly brace or taking out structural elements.

We get snow here in TN, but it's very little, and it hardly ever sticks. The only added load that I anticipate is the two layers of 5/8" drywall that we will hang. Notice the rear wall framing that extends to the roof 8" in from the rear gable...That should help a lot. We only removed 3 crossbeams on one side of the truss that is pictured, and we removed a 4th on the other side. The rest of the trusses are intact, as the half of the garage not shown in that pic has a dropped ceiling. The only spot that could maybe be a concern is the 72" span where the 3 crossbeams were cut out. If you think I should redo the crossbeams, I'm open to that. We just thought we would make drywalling easier on ourselves...

[PaulP]

Originally Posted by olectric

We get snow here in TN, but it's very little, and it hardly ever sticks.

Well if Dallas can get 10" of snow things may be changing. Might be best
to err on the safe side.

Paul P

[DanDan]

Just in case you are feeling generous, don't stuff the fluffy stuff tightly in the cavities. Just loosely touching both sides, contact damping them, would be optimum IMHO. I have read the other situation both ways also. I have seen figures for 705 and the like doing better than the light stuff. Can't find them now though, plus I am not sure I really believe it. I believe the light contact touch is more important.
Happy St. Patrick's Day,
DD

[mrcomber]

This is good info. I'm about to start construction on a mid size post production facility using the double wall buildout. I had priced out 8 lb mineral wool to fill all the cavities and it came out to $6K. So the consensus is that R13 (or higher?) should provide a comparable or better transmission loss? That would sure save me some dough. I'm going to use Johns Manville 814 (3lb) for the acoustic treatment.

[avare]

Originally Posted by mrcomber

So the consensus is that R13 (or higher?) should provide a comparable or better transmission loss?

There is no consensus. It is acoustic fact.

Andre

[jhbrandt]

Originally Posted by avare

There is no consensus. It is acoustic fact.

Andre

YES. -- Thank you!

[DanDan]

Just some reinforcement on what has been said here.
From a quick glance through, they seem quite clear that a full fill is required, not a half fill of batts or a hanging 'blanket'. This sounds like a +1 to the light fluffy nicely filled. However do remember that packing it tightly will cause the two leaves to start coupling a little at LF, so not too enthusiastic I suggest.
Furthermore they also state that density doesn't seem to matter.
Perhaps the most interesting is the superiority of the metal studs over wood.
Party Walls & Solutions

Newsflash- Apparently Saint Patrick has had to resign over the scandals......

DD

[mrcomber]

very good, thank you all for your insight.

[Bassmankr]

I'd at least add some lower 2" x 6" collar beams and one or two steel cables as tie beams with turnbuckles (if the side wall has already bowed out then you can tighten the turnbuckle to bring it back to plumb). That will still keep that space realitively open. Just don't have any big guys walk on the roof in that area, like I said a two by four at that span won't hold much load, you can experiment on the ground with a two by four up off the ground on blocks. That way whe it breaks as someone steps on it they will only drop afew inches LOL.