Please help confirm wall structure design

Posted: Wed Jul 16, 2008 12:26 am

Hi guys, I need some more advice before entering the next phase of construction. Right now I've been debating with my construction guy on what to do about the 2nd layer of walls. Please take a look at the following images:

For this one, the idea is that the original existing brick/concrete wall already counts as a leaf, and all I need to do is leave 2" of air gap, and then erect my stud/fiberglass/gypsum layer as the 2nd leaf. This is the idea I'm supporting.

In this image, the construction guy is saying that we should add a layer of stud/fiberglass to the existing brick/concrete wall to add even more soundproofing. His idea sounds fine on paper, but I can't tell if adding that layer of stud/fiberglass would screw up the two-leaf structure. Since that additional layer of stud/fiberglass does not contain gypsum boards (originally he wanted to put gypsum boards on it and I told him that's a big nono as it'll make it three-leaf instead of two), I would assume it technically does not count as an extra leaf, right? But what does it mean? The extra layer of stud/fiberglass only increases the air gap between the gypsum and the existing brick/concrete wall?

So guys, your suggestions?

Posted: Wed Jul 16, 2008 6:56 am

I would bet the additional layer would give little(to no) improvement. That outer wall is concrete right? Air gap depth doesn't matter much - and 3" of glass adds little. Stops resonances BETWEEN walls and there's not gonna be any resonance in that concrete.
Looks like it's directly coupled throught the floor anyway. A subfloor would be more effective.

Posted: Wed Jul 16, 2008 7:00 am

If you want the thickness, use a third layer of drywall in the inner leaf. NOT soundboard.

Andre

Posted: Wed Jul 16, 2008 7:53 am

avare wrote:

If you want the thickness, use a third layer of drywall in the inner leaf. NOT soundboard.

Andre

Really? That's directly scanned from Jeff Cooper's book. It's a bit long in the tooth since it was published in the 80's. Is that information dated by now?

Posted: Wed Jul 16, 2008 7:58 am

I'll go Andre' one better... and I'm only about 1/4 kidding here.

What about adding sheet lead between the gypsum layers on the exterior walls?

This is just a consumers perspective, and thus the 1/4 kidding... With so much of the US steel and metal production now being done in China, I am guessing that lead (pb) would less of a problem getting there than it is here. Therefore, it could be more cost effective of a solution.

Then again, it might be that it's actually much harder to get because it's all tagged for export.

just my .02

Posted: Wed Jul 16, 2008 8:29 am

Lunatique wrote:

avare wrote:

If you want the thickness, use a third layer of drywall in the inner leaf. NOT soundboard.

Andre

Really? That's directly scanned from Jeff Cooper's book. It's a bit long in the tooth since it was published in the 80's. Is that information dated by now?

The sound board is just about useless - the added mass means a lot.

The added air space can also add some value - but no need to frame an additional wall - I would not add additional insulation directly against a concrete wall - condensation is another issue to consider.

Just pull your inner wall further away from the outer one. BTW - just so we are clear here - the air space is the distance from the wall PLUS the depth of the frame - do in your case with the 2" wall clearance it would be 5 1/2" - not 2".

Which raises another point - the wall frame.

It is a fact that 2x6 @ 24" centers give a marked increase in LF attenuation over 2x4 framing at 16" centers. If I were you I would consider that......... and then that automatically gives you an additional 2" if air space at the same time - with an increase in fluffy insulation of 2" as well.

The air space and insulation have more value that Cathode_ray mentions - I will address that in another post.

Sincerely,

Rod

Posted: Wed Jul 16, 2008 9:03 am

Just out of curiosity... what is the "Vibration Isolation Board" underneath your bottom plate? By the looks of the drawing, it appears to be some type of laminated wood... like a plywood? It may just be something I'm not familiar with.

Rather than some type of "board", I would think a good healthy (matched to the weight/mass of the wall) EPDM strip, the same width of your stud plate, would be a better option. If I'm wrong, I'm wrong.

Also, in general terms, wouldn't you also be better off with the gypsum extending all the way to 1/2" of the finished floor? Then fill the gap with acoustic caulk and backer rod?... Thereby creating a better chance for an airtight seal?

Posted: Wed Jul 16, 2008 11:10 am

Thanks for the help guys!

MadMax - The "Vibration Isolation Board" I think is celotex, which is probably what Jeff also meant by "Sound Deadening Board"? In today's world, we'd probably swap that out with neoprene instead (or any other kind of anti-vibration padding)?

I have no idea if extending the gypsum down is better or worse. Like I said, that's a diagram straight out of Jeff Cooper's book: http://www.amazon.com/Building-Recording-Studio-Jeff-Cooper/dp/0916899004/ref=sr_1_1?ie=UTF8&s=books&qid=1216227954&sr=1-1

Lead would be nice, but I'd have to look into whether that's been outlawed here as well.

Rod - Ok, so based on my interpretation of your suggestion, I shouldn't bother putting studs/fiberglass agains the existing wall, and just leave enough air space between the existing wall and the gypsum (which is the actual 2nd leaf, not where the studs/fiberglass starts). In that case, I'm thinking 4" of 703 or 7.5 fiberglass, and leave 6" of air space (based on your suggestion of 2x6 studs)? So do I stuff the entire depth of the 6" of the studs with fiberglass (which would be 6" thick), or do I just do 4" and leave 2" as air space?

Posted: Wed Jul 16, 2008 11:28 am

Lead has it's pluses, mass... Well mass is about it.

Being a poison with such a bad rap you can't have it in paint. I would doubt you can build with it.

I suggest taking Pb and knocking a few protons and such off, tightening up the lattice work (which will give you more mass!!) and using those more stable elements. Au and Pt.

Plenty of books have been written about how to do this, it's called Alchemy.

In case you miss my point.
Not everything you read is gospel. Take this post for instance.

Posted: Wed Jul 16, 2008 4:42 pm

Greener wrote:

Lead has it's pluses, mass... Well mass is about it.

If I'm not mistaken its considered a limp mass.

Quote:

Being a poison with such a bad rap you can't have it in paint. I would doubt you can build with it.

Uhhhh... you haven't been paying attention to several different builds, have you? Including mine.

I have already put 3/32" of sheet lead on my exterior door. Additionally, I have enough to put it on each of the 6 remaining doors.

Also, every room which generates x-ray (or any other type of general radiation) is required by the FDA to be lead shielded to a minimum height of 8 ft. This includes dentist offices, chiropractors, hospitals, factory's, etc.

Oh, and it's not a poison. Yes, it is poisonous... but not a poison per se'...

from Wiki:
Lead is a poisonous metal that can damage nervous connections (especially in young children) and cause blood and brain disorders. Long term exposure to lead or its salts (especially soluble salts or the strong oxidant PbO2) can cause nephropathy, and colic-like abdominal pains. The concern about lead's role in cognitive deficits in children has brought about widespread reduction in its use (lead exposure has been linked to schizophrenia). Most cases of adult elevated blood lead levels are workplace-related.[14] High blood levels are associated with delayed puberty in girls.[15]

Also from the same article;
* Lead is a major constituent of the lead-acid battery used extensively in car batteries.
* Lead is used as a coloring element in ceramic glazes, notably in the colors red and yellow.
* Lead is used to form glazing bars for stained glass or other multi-lit windows. The practice has become less common, not for danger but for stylistic reasons.
* Lead is used as projectiles for firearms and fishing sinkers because of its density, low cost compared to alternative products and ease of use due to relatively low melting point.[21]
* Lead or "sheet-lead" is used as a sound deadening layer in such areas as wall, floor and ceiling design in sound studios where levels of airborne and mechanically produced sound are targeted for reduction or virtual elimination.
* Lead is used in some candles to treat the wick to ensure a longer, more even burn. Because of the dangers, European and North American manufacturers use more expensive alternatives such as zinc.[22]
* Lead is used as shielding from radiation, e.g. in x-ray rooms.
* Molten lead is used as a coolant, eg. for lead cooled fast reactors.
* Lead glass is composed of 12-28% lead oxide. It changes the optical characteristics of the glass and reduces the transmission of radiation.
* Lead is the traditional base metal of organ pipes, mixed with varying amounts of tin to control the tone of the pipe.
* Lead is used as electrodes in the process of electrolysis.
* Lead is used in solder for electronics, although this usage is being phased out by some countries to reduce the amount of environmentally unfriendly waste.
* Lead is used in high voltage power cables as sheathing material to prevent water diffusion into insulation.
* Lead is used for the ballast keel of sailboats. Its high weight-to-volume ratio allows it to counterbalance the heeling effect of wind on the sails while at the same time occupying a small volume and thus offering the least underwater resistance.
* Lead is added to brass to reduce machine tool wear.
* Lead has many uses in the construction industry, e.g. lead sheets are used as roofing material, cladding, flashings, gutters and gutter joints, and on roof parapets. Detailed lead mouldings are used as decorative motifs used to fix lead sheet.
* Lead is frequently used in scuba diving weight belts to counteract the diver's natural buoyancy and that of his equipment.
* Lead is often used to balance the wheels of a car; this use is being phased out in favor of other materials for environmental reasons.
* Lead is still widely used in statues and sculptures.

Quote:

Not everything you read is gospel. Take this post for instance.

I concur with your last statement.

Posted: Wed Jul 16, 2008 9:19 pm

I wonder how particular a Taylor guitar air gap is to ,say, a Fender acoustic?

I think it matters... What? That is mass/air/mass used to build a flat-top isn't it?

Posted: Wed Jul 16, 2008 10:28 pm

Quote:

I'm thinking 4" of 703 or 7.5 fiberglass,

There is no advantage to 703 type in walls. Use the cheapest porous material you can find.

Andre

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