insulation thickness for walls and ceiling

[aschatzb]

Hello,

I'm trying to decide the best bet to insulate my walls and ceiling with Rockwool in the live room. The thicknesses of rockwool available in my area are 2",3",4". Some of the walls are 2x6 timber, some 2x4 timber. The 2x6 walls have a 1" air gap between them and the exterior walls of the building.

Is 4" rockwool thick enough for 2x6 walls or should I double up (a layer of 2" and a later of 3", or some other combination?)

The 2x4 walls have about a 3" air gap between them and the exterior walls. I'm a little worried about the 3" insulation falling back in the 3" cavity between the walls... Is plastic netting that is sometimes used to prevent from happening ok in studio use? Or should I use something else and perhaps overstuff the 2x4 walls with 4' insulation.

As for the ceiling joist, we used 2x8s. I'm trying to get the max isolation from my neighbor up the hill. What's my best bet here?

Also, as a side question. The exterior walls of the building, which are unfinished inside, are stuffed with owen cornings unbacked fiberglass, and the walls have been covered in a thin plastic sheeting, I think to pass code. Is this exceptable, or will it create some kind of issues for isolation.

thank you:)

andrew

[sharward]

Double up. You want full with slight pressure on your leaves. Don't overfill though, because that will cause them to short out.

[aschatzb]

Any tips on my other concerns?

thanks:)

[AVare]

You want full with slight pressure on your leaves. .

?

Andre

[sharward]

Was in a rush... You want your wall cavities filled, just slightly overfilled, so that there's light pressure on each leaf. Right? (At least that's what I thought I'd learned...)

[drfrankencopter]

You want to make sure that the insulation touches the outter layers of drywall. If you have to overstuff to do so, then thats okay...if you can get away with not overstuffing thats good too. You would really have to overstuff the cavity to get a conductive path straight through the wall (i.e. shorting it out).

The idea behind the insulation is that it will damp out the vibrations of the drywall panels, so it needs to touch them. Overstuffing is probably the easiest way to ensure this...

Cheers,

Kris

[aschatzb]

thanks guys:)

do you have any suggestions about my question concerning the plastic covering the insulation in the exterior walls?

also, what about making sure the insulation doesn't fall back in the air gap?

does anyone have experience with what thickness insulation works well for doubling up in a 2x6 wall?

thanks:)
andrew

[sharward]

Sorry I missed the "pastic netting" question.

If I understand the question correctly, you're considering using something attached to the outer edge of the studs to hold the insulation against the wallboard, rather than falling away from the wallboard towards the surface that is 3" away from the studs. (Right?)

If so, I would recommend not doing that. Instead, just get more insulation and stuff the cavity. I would prefer to see them filled with Thermafiber Sound Attenuation Fire Blankets (SAFB) - Nominal Density (2.5 pcf) or Roxul Acoustical Fire Batt (AFB) - ≥2" (2.5 pcf)? This stuff will probably satisfy your local building department's requirements for a firebreak in the void between the walls without having to permanently couple them together with lumber. Overstuff slightly so that there's light to moderate pressure on the panels so that they're dampened adequately.

I've seen video of this stuff in fire testing, and it's awesome. Fear the fire.

[AVare]

Overstuff slightly so that there's light to moderate pressure on the panels so that they're dampened adequately.

Where do get this from? From all that I know if anything a slight gap should be left on one side of the cavity. Clarification please.

Andre

edit: 05.08.25 added the word "one" to make the last sentence clearer.

[sharward]

Hey Andre,

I welcome the challenge -- if I'm wrong, then definitely, let's set the record straight for everyone I've confused.

I don't have much time to search for all the relevant threads, but I did find this post from Steve in January of this year that I think illustrates my point pretty well.

That's just for the acoustics side of things. In terms of fire protection, I know there's a Canadian IRC document somewhere that legitimizes the use of insulation as a fireblock -- something code calls for anytime you have a vertical cavity adjacent to a horizontal cavity (i.e., a room-within-a-room with an inner leaf ceiling attached to the inner leaf walls). If all the cavities are full of insulation -- particularly of the type that I linked to above -- then there's less risk of fire spreading from the vertical cavity to the horizontal cavity. I received a verbal OK to do this in my project from the senior inspector and plan checker at my city -- we'll see if that holds true over the next week or two as my plans are being scrutinized by those authorities...

--Keith

[AVare]

Thanks Keith. I am thinking acoustically. I read the post you referenced. I think the best to wait for Steve to jup in this thread and clear us up. It makes no difference which one is right or wrong. It isimportant to find out the truth.

Andre

[sharward]

Cool. For the record, though (no pun intended ), in the event of a dispute between good acoustics and being safe/legal, I think safe and legal should win.

[AVare]

Keith:

Is this from the NRC document you referred to earlier in this thread


CONSTRUCTION INNOVATION Volume 3, Number 3, Spring 1998

These findings indicate that, from an acoustics perspective, the best way of satisfying the code requirement for fire stopping is to use a double-stud wall with sufficient insulation so that the width of the air space in the wall is 25 mm or less. This approach does not reduce the effectiveness of the sound isolation of the wall assembly, and can actually improve it if the wall cavity is less than full to begin with

Andre

[sharward]

You rule! Yes, I believe that is what what I saw, way back when I thought firestopping was going to be my biggest obstacle. (Ha!)