Just one last inside out wall design question

JohnGardner · Post by **JohnGardner** » Mon Aug 16, 2004 1:18 pm

I had planned to put R1.8 75mm pink wall insulation on the back of my inside out wall in the cavity between that and the existing walls. (see attached red circles).

Is it really necessary?, I can save a few bucks by not doing this but I,m guessing John put it there for a reason when he designed the room.

I will be tracking acoustic drums in this room, nearest neighbours 20ft away.

Thanks

JohnG

knightfly · Post by **knightfly** » Mon Aug 16, 2004 2:13 pm

Can you add ALL the elements between you and your neighbors to that sketch, including dimensions? Inside out walls by their design have a narrower air gap which can really weaken isolation; I need to know exactly what you will have between you and them because it doesn't sound to me like you're gonna like the results, especially for drums... Steve

JohnGardner · Post by **JohnGardner** » Tue Aug 17, 2004 9:33 am

Here it is Steve.
Please bear in mind that this wall and ceiling structure WILL be built on a floating floor of 90 x 45mm on its side, 1 x 20mm flooring, neoprene pads and insulation in the cavity, so basically this will be a room within a room

knightfly · Post by **knightfly** » Tue Aug 17, 2004 9:54 am

John, here's why I'm concerned - since adding a leaf in between two leaves of a sound wall worsens performance in the low end and slightly enhances performance in the midrange, it stands to reason that if we calculate your wall performance with the assumption that the middle leaf (your existing inner wallboard) doesn't exist, that the REAL wall, with that middle leaf INCLUDED, will be WORSE at low frequencies - agreed so far? (The reason for this convoluted approach is that the calculator I use doesn't allow more than two leaves - I'm improvising here, using known rules of isolation construction.)

That being said, I ran your construction (minus the middle leaf) through the calculator and came up with the attached graph - note that the low frequency isolation isn't too great even without the center leaf - now, adding a center leaf (your inside wallboard on the drawing) will worsen the performance at the bottom end, where it's already marginal.

Adding another heavy layer of wallboard on the inside new wall would help things by probably 5 or 6 dB, but that mid layer is KILLING you on the low end. I can't make it what it's NOT, I just don't want you to find out AFTER the fact that you're not at all pleased with the isolation you have... Steve

JohnGardner · Post by **JohnGardner** » Tue Aug 17, 2004 11:48 am

Thanks Steve,

So, the million dollar question is - what do you recomend I do bearing in mind that I

(1)can't touch the existing wall (except maybe adding another layer of wall board to the inside of it)

(2)I don't have the money or space to build a totally new double wall inside this room

(3) my cuurent list of materials i can afford is in the drawing above.

Is there a better way to tackle this small drum room with my current materials. I thought the inside out wall was the best option to save space and get maximum STC?
Can anyone come up with an alternative with my current materials/budget thats not "tear down this" or buy "more of that

Thanks All
JohnG

JohnGardner · Post by **JohnGardner** » Wed Aug 18, 2004 7:22 am

Hey Steve,
Here are the photos of the building I am going to convert into my studio. Two shots outside (two walls are weather board, two are sheetrock) - and two shots inside of where the live room will be going - not the windows at one end, these will be covered and caulked before the new room is built.
Attached also are my room plans again.

JohnGardner · Post by **JohnGardner** » Wed Aug 18, 2004 7:41 am

her it is again
JohnG

knightfly · Post by **knightfly** » Wed Aug 18, 2004 3:47 pm

With those conditions, you would actually get 2-3 dB better low frequency isolation by just putting those two new layers of wallboard directly on the existing wall (screwed, not glued) and then fastening your wall treatment to that. This would avoid the triple leaf thing, and although it would lower the STC it would improve low end performance by several dB over a triple leaf wall. YOu could extend the ceiling as you've drawn it to intersect with the existing wall. Not perfect, but likely the best of any options you've got... Steve

JohnGardner · Post by **JohnGardner** » Fri Aug 20, 2004 7:13 am

Hey All

OK, how does this look.

My plan is to:

(1) Screw two new layers of wallboard to existing walls and seal existing windows with a MDF panel backed by insulation.

(2)Build a floating floor on neopreane with insulation in the cavity and seal where it meets new wall board.

(3)Attach a stud around the top of the new walls to hold up ceiling joists.

(4)Attach two new layers of wallboard to these ceiling joists and fill cavity with insulation. Seal ceiling where it meets wall.

(5)Insulation on interior of new room perhaps inside some light framing to hold it up and to run some timber slats across.

Firstly, does this look better?

Secondly, Could you work out approximate STC and lower frequency transmission loss for me?

Thirdly, thanks so much!!

JohnG

knightfly · Post by **knightfly** » Sat Aug 21, 2004 12:53 am

First, if you want this to last beyond 10 years you need to find a source for EPDM rubber, NOT neoprene - it's more expensive, but will last about 25 years instead of 10.

Second, make sure your pads are directly over existing framing members - if you can't see the fasteners for these, buy or rent a stud finder. this is VERY important.

Third, do you have span tables available or do you need me to calculate ceiling joists for you?

Finally, here's a rough idea of what you can get for isolation, PROVIDED you don't goof anywhere... Steve

JohnGardner · Post by **JohnGardner** » Mon Aug 23, 2004 7:14 am

Thanks Steve,

(1)Understand what you are saying about the rubber pads. My builder has a stud finder.

(2) I am OK with the joist span - I can use 45x90mm as it's only 3.0m at the most the ceiling joist spans

(3) So what you are basically saying is that the wall with the STC of 48 is better than an STC of 61 for acoustic drums because the TL at the lower frequecies is better?. I would prefer to build the STC 48 wall. It seems easier to get right than the "inside out wall" and there is less materials so it will be alot cheaper.

Thanks

JohnG

knightfly · Post by **knightfly** » Mon Aug 23, 2004 11:22 am

2 - are your ceiling joists actually 45mm thick? If so, I've no tables that cover that - they're all North American, which are actually 37mm x 90mm for a "2x4" - and as such, they won't span 3 meters with a double layer of wallboard hanging on them. We may need to discuss this part more...

3 - no, I'm saying that you can't even BUILD the first wall I calculated, because you can't remove the inner wallboard before building and leaving it there would cause a triple leaf wall - that example was what you'd get if you COULD remove your inner wallboard and extend your air gap. With a middle leaf and no resilient mounting anywhere, the wall would look a lot worse than the last one, which you can actually BUILD.

If you could have your choice of the two methods including the removal of inner wallboard in the first example, it would probably be a toss-up between the two for overall sound attenuation (because of the point you questioned) - as it is, the last method seems to be about the only choice left, and if done tightly should perform pretty close to the graph I posted.

If possible, I would recommend NOT finishing the inside til you get a chance to check your isolation - then, when you can afford it, adding another layer of 19mm gypsum might be necessary. With un-tested wall building, usually that's the ONLY way to be sure... Steve

JohnGardner · Post by **JohnGardner** » Tue Aug 24, 2004 8:14 am

Hey Steve,

I,ll be fine with the joists. I am only spanning 8.8ft with 45mmx90mm. It will be OK-I have checked

I have been thinking more about these walls and also got an email from John in regard to a studio he built/designed that had exactlly the same external wall to mine. (wallboard - stud - weatherboard)

John pointed out that because the outside weatherboard/sheetrock is only really "weather sealed" not airtight then I could discount it in the wall system I am building.

This being the case my inside out wall would be two leaf system rather than three leaf system which should help things a bit, especially at the lower frequencies:

If I disregard this "exterior cladding" in my design the inside out wall would be:

100mm insulation inside stud
10mm wallboard
13mm wallboard
50mm insulation in 75mm airgap
10mm wall board

As you say I could always add another layer of wall board to the inside of the room if the isolation is less than I required

Thanks

JohnG

knightfly · Post by **knightfly** » Tue Aug 24, 2004 10:57 am

From what I've gleaned from serious acousticians, you can't COMPLETELY ignore that outer layer - it's close enough to still present an acoustic impedance, and still have some finite amount of "air spring" happening between it and the inner wallboard -

Even if you could discount the outer sheath totally, your choice of only 10 and 13mm wallboard, using the 75mm air space, considering this as a separate framed wall, would only give about STC 49, with lower freq's clear down to only about 17-19 dB - not too good for a drum room.

Is there any kind of vapor barrier material anywhere in your wall that you know of? it's sounding to me like you should put ALL your layers of wallboard right on the existing studs (added to the first layer) and caulk well, then see how your wall works - then, if you need more iso, you can add a stud frame and more wallboard - if you do the whole wall and only use 10mm wallboard on the inside, you've got no recourse when you find out the mass doesn't cut it.

I know it isn't my money being spent, but if at all possible you shouldn't think of anything less than 13mm for your wallboard, and 15 or even 19mm would be better - drums are not easy to stop... Steve