Rockwool and multi layer Gyproc questions

[Paul881]

So I have nearly finished the outside construction of my oak feather edge clad self contained studio and am now starting to plan the inside.

I would like some comments/suggestions on how I will construct the inside layers:

The roof has 125mm joists and 100mm stud walls.

I will use 75mm Rockwool RWA45 for the roof which is 40 degrees double pitched leaving an air gap of 50mm to the outside roof layer. The inside ceiling is approx 4.1m long and each pitch is about 2.4M deep including an external overhang; the actual pitch span to the wall plate from the ridge board is 1.9m.

I intend to use a vapour barrier to hold the RWA45 in place; should I also use a mesh top stop the RWA settling or will the VB be sufficient to do that? I am installing a combo A/C unit for heat/cooling which also provides fresh air ventilation.

For the ceilings I will use a resilient bar at 250mm spacing.

In the walls, I will use 100mm RWA45; vapour barrier and resilient bar (RC1)spaced 400mm.

The walls and roof will all have 19mm Gyproc plank; 12.5mm Acoustic board and a final layer of 15mm acoustic board. I intend to use Green Glue in between each layer and screw each layer to the RC1 taking care not to short circuit the Gyproc layers to the stud walling.

At the corners and Gyproc joins I will use acoustic mastic.

Have I missed anything?

[Soundman2020]

The walls and roof will all have 19mm Gyproc plank; 12.5mm Acoustic board and a final layer of 15mm acoustic board.

Why? What is the reason for using three different thicknesses of three different materials? The supposed gain you get from differing coincidence dips and refraction angles is offset by the lost mass, and is way up there in the area of the spectrum anyway, where you already have oodles of isolation. And that layer of thin, flimsy, flexible, low-mass 12.5mm stuff is not helping at all! Much better to go with two layers of that 19mm stuff, or even three of plain old 15mm drywall.

At the corners and Gyproc joins I will use acoustic mastic.
Have I missed anything?

Backer rod?



- Stuart -

[Paul881]

I understood that this gave the biggest bang for the buck in terms of overall thickness of the plasterboards, sound attenuation and cost.

Whats backing rod? Resilient bar?

Okay, just googled it and saw what it is. Great idea! Many thanks.

[Paul881]

A couple of other things:

You can see from my post that I have 125mm roof joists. The advice I have been given is not to fill it with a 75mm and 50mm layers of Rockwool but only 75mm and keep a 50mm air gap. This seems counter intuitive to me - surely increasing the mass would be more advantageous?

And if not, why then is the advice to fill the 100mm stud walls with 100mm Rockwool?

Will the Rockwool in the 40 degree apex roof be kept in place by the vapour barrier or do I also need a mesh?

[Soundman2020]

I understood that this gave the biggest bang for the buck in terms of overall thickness of the plasterboards, sound attenuation and cost.

The thickness of the mass on a leaf doesn't matter for sound isolation. It isn't even part of the equations for calculating isolation. What matters is mass. In other words, weight. The more weight you have, the better. The only two variables in the equations for calculating the MSM resonance of a wall are the mass of the two leaves, and the distance between them. Increase either and you increase isolation. Thickness simply isn't a factor there (OK, it does play an indirect role for the very low end of the spectrum, below MSM resonance, but if you built your wall so badly that those frequencies are in the audio spectrum, then you have much bigger problems than worrying about leaf thickness!).

Cost isn't a factor either: Sound waves don't care how much you pay for your mass!

In other words, get the cheapest mass you can. Look for the highest density drywall you can find, which is usually the fire-rated stuff. The reason why you want thicker drywall is not because the thickness itself helps, but because thicker drywall has more mass, and mass is what you want. You could even make a very thin leaf if you wanted, by using extremely dense materials such as steel plate or even lead sheeting, and that would make a very effective wall that would also be very thin: just a few millimeters would do it. But it would also be very expensive...

The advice I have been given is not to fill it with a 75mm and 50mm layers of Rockwool but only 75mm and keep a 50mm air gap. This seems counter intuitive to me -

You are absolutely right! Not only is it counter-intuitive, it is plain wrong. You can ignore that source of advice from now on! To start with, the air gap is the entire distance between leaves, not just the part that isn't filled with insulation! After all, insulation is mostly air anyway...

Let me explain...

surely increasing the mass would be more advantageous?

It's not the increased mass that helps here, but the increased path of gas flow resistance. Let's take a few steps back...

The reason that your wall will isolates is because it is a tuned filter, kind of like a parametric equalizer set up as a bandpass filter. You tune it to a specific frequency that is below the range of sound that you want to isolate. At its resonant frequency, it is transparent to sound, and lets it through as though it wasn't even there. At 1.4 times that frequency ("RF"), it starts to isolate. At twice the RF it is isolating reasonably, and and 3 times RF it isolates well, getting better and better the higher up you go. So if you tune the wall to a frequency of 10 Hz, then at 20Hz and all higher frequencies it is isolating well.

Technically, your wall is a damped tuned MSM system. Basically, a weight hanging from a spring with a shock absorber on it. "MSM" stands for "Mass-Spring-Mass". The two "M"s are your two leaves (which is why you want mass on them!), and the "S" is the air plus insulation in between. (Yes, air is a really good spring, especially at the speed of sound.) The insulation in the air gap is the "shock absorber", more technically a "damper", since it absorbs the motion of the spring to a certain extent, thus converting some of the sound energy into heat.

That's the basic concept. It's not the mass of the mineral wool that matters, but the air flow resistance. It is trying to hinder the movement of the sound waves as they bounce around inside the wall, and that is a GOOD thing. You WANT to do that.

It is a common misconception that "air gap" refers to the space not occupied by insulation: not true. The entire space between the leaves is the air gap, regardless of whether or not there is any insulation in there. Adding insulation to the air gap changes one of the constants in the equation for MSM resonance, and ALWAYS decreases the resonant frequency, which is good, as it increases isolation.

If you don't put insulation in the wall cavity, then you have no damper on your spring! So you are reducing isolation considerably (as much as 16 dB, in some cases). You want as much insulation in the air gap as you can get. If your local building code allows it, then fill the entire air gap with insulation, to maximize isolation. Or at least fill as much as your building code allows.

The advice you got to not put in deep insulation is the same as someone telling you to take out the shock absorbers from your car, in order to improve the ride... Get the picture? Bad advice.

And if not, why then is the advice to fill the 100mm stud walls with 100mm Rockwool?

Because that's good advice! Even better advice is to fill the entire cavity, if you can.


- Stuart -

[Paul881]

Wow, what a fantastic response! Many thanks Stuart, I deeply appreciate your words of advice and the time and trouble you took to explain it in words I could (eventually!) understand.

Following your post I am looking at the plasterboard options afresh and find that the heaviest board is a Gyproc impact resistant board at 13.9Kg/M2. Soundbloc is next lighter at 12.6 Kg/M2 then Fireline is 11.7Kg/M2 and ordinary board is 9.8Kg/M2. So it seems like either two (or three) layers of impact board with Green Glue would be the optimum?

Another question - how do I fix the plasterboard layers? The first one is screwed to the resilient bar taking care not to short circuit the spring by screwing into the stud wall but how does the second board layer get fixed?

And is there somewhere that has a table of Resilient bar spacings for a given weight/M2?

Final question (for tonight!) is what size of backer rod do you recommend for the corner joints?

[Soundman2020]

So it seems like either two (or three) layers of impact board with Green Glue would be the optimum?

Yup! Sounds like that's the best option, for sure. That is 5/8" (15mm) drywall, correct?

Another question - how do I fix the plasterboard layers? The first one is screwed to the resilient bar taking care not to short circuit the spring by screwing into the stud wall but how does the second board layer get fixed?

The same way, with longer screws: Also into the RC, through the first layer. Lay out your screw schedule carefully so you screws from one layer don't run into screws on a lower layer.

And is there somewhere that has a table of Resilient bar spacings for a given weight/M2?

Ask the manufacturer!

A comment: It would be better to forget the RC and just build a simple stud frame about an inch away from the existing studs. You'll improve your isolation even more like that. If space is at a premium, you can even build that second leaf "inside out"...

- Stuart -

[Paul881]

So it seems like either two (or three) layers of impact board with Green Glue would be the optimum?

Yup! Sounds like that's the best option, for sure. That is 5/8" (15mm) drywall, correct?

Correct

A comment: It would be better to forget the RC and just build a simple stud frame about an inch away from the existing studs. You'll improve your isolation even more like that. If space is at a premium, you can even build that second leaf "inside out"... - Stuart -

Unfortunately, I don't have room for a "room within a room," even using frames - I would lose too much space. I had considered the "inside out" strategy but one day the studio will have to easily revert to a "normal" garden/hobby/home office room so I discounted it.

I'm primarily an acoustic guitar musician; I don't record drums and so I don't need to be in pursuit of the maximum isolation; just a good level of attenuation. I currently have a room in the house and even when I do play electric guitar my neigbours are unaware and thats in a totally untreated room.

[Soundman2020]

OK, I hadn't seen that info in your original post, so wasn't aware that your levels aren't that high.

If that's the case, then you may be doing a bit of overkill by going for three layers of drywall and Green Glue! You can probably work fine with just two layers.


- Stuart -

[Paul881]

Thanks Stuart, thats saved me a bunch of cash!

[Ro]

...lateralus rocks

[Paul881]

After nine years away, I am now finally back in the UK, retired and ready to complete the garden studio that I left in Feb 2012. Its a 4M x 2.8M timber frame construction with a central 3.5M pitched roof which was designed with much input from this forum. It has a concrete floor. Its only intended as an acoustic song-writing space so no loud electric guitars or drums. Therefore, isolation doesnt need to be so extreme but it does need a reasonable amount of isolation.

Where it was left off back in 2012 was with RC installed waiting for two layers of 15mm Gyproc; one layer of Soundbloc and one layer of standard board. I'm now ready to finish it!

I have some questions; I assume that the correct order would be to install the Soundbloc first to the RC and then the standard board with GG in between the layers? Screwing through to the RC on both layers, with no coincident joints. Backing Rod and Calk in the corners and floor gaps.

As Soundbloc only comes with tapered edges, I understand these have to be fully caulked and filled with mud to eliminate and cracks and airgaps with the next layer - is this correct? For the top layer of gyproc, should I get tapered or square edged product?

I'm sure I will have other questions as I get into this but answers on these will get me to the next stage. Thanks!

[gullfo]

welcome back!

i'd suggest using a layer of plywood or OSB on the RC to screw into. this gives you a margin of error on the position of the fasteners, and then you scan readily overlap the spacing for each layer (e.g. if you have Rod's book - he has some guidance on the fastener spacing). use appropriate fastener length on each layer to just penetrate the OSB without risking contact underneath.

[Paul881]

One other question, would I be much better off acoustically having two layers of Soundbloc 15mm instead of one layer of SB and one standard board?

[Paul881]

Thanks Glenn, but I have discounted that as I have limited space plus I would rather have discipline in making the fixings in the right place than have another layer to worry about. I am fortunate enough to have access to a laser line marker which we used to fix the RC so thats what we will do to line up the fixings for the boards.