summing up - wall cavity insulation

[andymel]

I'm about to choose the mineral wool for my walls. I've searched the forum and want to post my state of knowledge before I make my decision on this, maybe anyone wants to clarify something and if not..maybe it helps others.

I should narrow down my price comparisons to products with ~20k-25k rayls/m (20-25 kPa*s/m)?!

OC 703 has a resistance of about 23600 mks rayls / m,
OC 705 has a flow resistance of about 30000 mks rayls / m

...and in terms of density

either mineral wool at around 48 kg/m3 density, or fiberglass at around 32 kg/m3

First question: density - whats better for a high TL in the lower frequency?
Lighter/lower density like in

Lighter is better for bass, in general. Heavier is better for mids and highs.

and...

going lighter than 2.5 improves bass TL at a cost to high frequencies

or the opposite like in...

Heavier than 3pcf improves bass loss slightly, at the cost of higher freq's getting through.

Second question: filling
100%

that it must fill the entire cavity but without being forced into position

50%

half the air cavity taken up with insulation seems to be optimum

Two of my 4 walls are 12cm and two are 14cm deep (studs). The space behind is varying between 2 and ~20cm. Hence, the cavity between my two leafs is between ~14cm and 34cm.
In principle...do you tend to use half of a more expensive/(denser) insulation or use a cheaper product twice as thick?!

Thanks 4 reading, request 4 writing

[Soundman2020]

First question: density - whats better for a high TL in the lower frequency?

Lower density, to a certain extent, but we are not talking major difference here. As long as you don't go overboard (like 100 kg/m3 or something extreme like that)! That would be rather detrimental.

There isn't going to be a huge difference between, say 40 kg/m3 and 50 kg/m3. You can get far more improvement in the lows by adding an extra layer of drywall, or increase the cavity depth. Or both. Take a look at the Wyle report for more details, and I think even IR-761 has some data on this. At best, we are talking maybe a couple of points difference in TL.

Second question: filling
100%
50%

All of it, if you can. Partial filling gives partial damping. Think of it this way: in the equation for MSM resonance, there's a constant that changes, depending on whether you put insulation in your wall or not. The effect of changing that constant is roughly the same as increasing the depth of the wall cavity by a factor of 1.4. In other words, putting insulation in the cavity increases mean path length by 140%. Similar to making your wall 1.4 times thicker. That's a big change. But if you only fill half of the depth, then only the part that you fill "appears" to be 1.4 times thicker: the part you didn't fill is still just the same as it was.

Theoretically, in a wall with an 8" air gap filled with only 2" of insulation, you get an improvement of about 5 dB (considering two layers of 5/8" on each side) as compared to no insulation. With 4" you get an improvement of 9 dB, as compared to no insulation, and filling the entire 8" gives you an improvement of 16 dB as compared to no insulation. All of those are major improvements, but obviously 16 dB is huge, and the 7dB improvement from 50% fill to 100% is also huge.

Obviously, usual disclaimers apply: theory only, your mileage my vary, not responsible for damages, void where prohibited by law. etc.

HOWEVER! Caveat! As Rod once pointed out here, you might not be allowed to fill your entire cavity, due to fire code. Your local code may not allow 100%, so you might be forced to go with less. As always, check your local code.

Another caveat! Do not overfill! Don't jam it in tight, especially if it is mineral wool. You want it just barely filling the width, or even leave a small gap on one side (eg 1/2"). Jamming it in can create flanking paths, which basically shorts out the cavity and seriously degrades the performance of the wall.

In principle...do you tend to use half of a more expensive/(denser) insulation or use a cheaper product twice as thick?!

Personally, I like "cheap and thick"! Percentage fill seems to have a much greater effect than density. And "cheap" has a highly beneficial effect on the resonant frequency of your wallet...


- Stuart -

[andymel]

Wow, I am sure I have wirtten an answer to this post. Maybe I have forgotten to press send at the end

Belated, but many thanks indeed to stuart!!!

I came back to this post to add an interesting pic I found. The relationship between gas flow resistance and density of fiberglass and mineral wool in a product description by Isover.