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 Post subject: Dog + Bear Studio build
PostPosted: Mon May 14, 2012 9:03 pm 
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Hi all,

I think it's time to weigh in with my project and see if I can get some discussion going. I've been looking through the site for quite a while and have based a few construction decisions on advice given here, although most of the external shell build is pretty much set in stone anyway. At the moment it's just the external shell (details below) but i'll be moving on to the 'room within a room' in the next few months. Hopefully there's enough info here but let me know if you need anything else.

Location: Australia, Victoria, Trentham (75mins NW of Melbourne)

Me: been a guitarist/singer/songwriter for many years. Got into sound over the past 8 years. Have done a hell of a lot of ‘normal’ building in my life with my step-father and in my own business. I own Rod’s book (and others) which now has many dog-eared pages and I’ve been trawling this site for about 18 months.

Studio Overview: We mainly work in the folk/jazz/acoustic genre’s and have been recording out of various rental properties for the past six or so years. Word of warning: never rent a house to me as there will be 'discrete' holes drilled in the walls to run cable. (all patched up before moving on though :D )However, now that we’ve bought our own place it’s time to set up a dedicated studio.

It’s quiet up here during the day (averaging around 40db, measured ‘A’ weighting/slow) and quieter at night. So as you can guess it’s not isolation from the outside world that I need, but the other way round. Once the studio is operational I’ll be opening the doors to anyone who wants recording time, so that may take me out of the relatively ‘quiet’ recordings we’ve been doing and into the louder genre’s as well.

I won’t be hunting down work with loud bands, but if they find me I don’t want to have to knock them back as the bank still owns the house!

Our closest neighbor is around 20-25m (65-80ft) from the studio, and that’s the back of their house which is kitchen and laundry, not bedrooms. My neighbours aren’t too picky but I’d rather over isolate and avoid any future problems if they one day decide to sell their house.

How loud are we? As I said it’s mostly acoustic bands that we record and even if they have a full kit the drummer isn’t really pounding it so they’re probably around the 90db range ( that’s a guess as I haven’t actually measured a live kit). But as I say, if I get a pumping band in with a heavy hitter and amplified bass I’m guessing we get into the 100-110db range!

So maybe I’m paranoid about the isolation factor, but I’d rather build it right first time and spend the $’s rather than getting problems with the neighbours down the line.


Building stage: It's a free-standing construction and at the moment I’ve got the slab ( external dimension 9.3m x 6.4m/30.5ftx21.1ft). External walls are brick 110mm (4.3inches) wide with piers so it’s self supporting. Wall height is 2.7m (8.8ft) with a cathedral ceiling height around 3.3 m (10.8ft).
Roof is zincalume (sheet metal) as it’s the only real option for me construction wise.

Budget: I’m lucky in two respects, one-my wife is a professional musician so she’s happy to spend as much coin as we need to be operational. (even before painting the inside of the house do you believe!) and two-I’ve got the skills and tools to construct everything myself except elec, and I’ve got some buddies in the game that can help me there. I’m hoping I can get it constructed for $20-$25K (not inc gear) but I won’t set that as a hard limit as I’d rather have a better sounding room than a cheap one.

My main concern at the moment is about the roof in terms of isolation. I’ve been reading through as many posts about the subject as I can find, but I still feel like I could use some pointing in the right direction.

The finished product will be a fully decoupled 'room within a room' construction which i'll be asking questions about down the line, but for now my question is this:

In a free-standing construction like this, do I need to achieve as much isolation with the external roof as I do with the external walls? Ie: do I need to effectively have as much mass on the exterior roof as I do with the exterior walls? I'm pretty sure I can't achieve that without doing a vaulted brick roof, which ain't gonna' happen, but i'm interested in some more experienced opinions.


Any ideas/comment/criticism welcomed, and thanks to everyone who puts their 2cents-worth into this amazing resource.

steve

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PostPosted: Tue May 15, 2012 2:56 am 
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Hi Steve, and Welcome! :)

Quote:
As I said it’s mostly acoustic bands that we record and even if they have a full kit the drummer isn’t really pounding it so they’re probably around the 90db range ( that’s a guess as I haven’t actually measured a live kit).
I think you are being just a bit optimistic there! Playing drums as quietly as 90 dB is really hard to do: most drummers I know couldn't keep it that quiet, even if their lives depended on it! Maybe soft jazz played with wire brushes, very gently...

Realistically, I'd suspect at least 100 to 110 would be a more accurate level, and 115 dB is common for a drum kit played hard. Or even more...

Quote:
But as I say, if I get a pumping band in with a heavy hitter and amplified bass I’m guessing we get into the 100-110db range!
That's also on the low side: I'd say closer to 120.

Also, I'd suggest that you switch to measuring "C" weighting and slow response to get a more realistic reading. "A" weighting is fine for low levels, and is what most noise regulations specify (which is good, since it is less sensitive to bass frequencies! :) ). but for typical band levels, "C" weighting is far more realistic.

Quote:
So maybe I’m paranoid about the isolation factor,
Being "paranoid" about isolation is a good thing, in my book! :)

OK, so assuming the worse: 115 dB inside, and you need 40 outside. That works out to 75 dB of isolation, which is a pretty tall order. Doable, but not easy: fortunately, you probably only need that "under 40 dB" level by the time the sound reaches the neighbor's house, and fortunately sound decays at 3 to 6 dB per doubling of distance (depending on the surroundings), so assuming that you set a goal of 35 dB at your neighbor's wall, 20m-25m away, and worse-case drop off at only 3 dB per distance doubling, if you can get your level down to around 50 dB at one meter from the wall, then you are in the ball-park (theoretically you'd get down to 35 dB at 32m, but allow a little "fudge factor", and I'm being very conservative with those estimates).

So 50 dB outside, and 115 dB inside is 65 dB TL. Much better. There are several ways of doing that.

Quote:
Building stage: It's a free-standing construction and at the moment I’ve got the slab ( external dimension 9.3m x 6.4m/30.5ftx21.1ft). External walls are brick 110mm (4.3inches) wide with piers so it’s self supporting. Wall height is 2.7m (8.8ft) with a cathedral ceiling height around 3.3 m (10.8ft).
Great! That's an excellent basic shell to start with. Slab on grade with brick outer leaf bodes well for high levels of isolation. That alone should get you into the high 40's in terms of transmission loss.

Quote:
Roof is zincalume (sheet metal)
Darn! OK, scratch the above: that takes you down to maybe low 20's, best case. Especially if it is mostly in drums and heavy bass.

Quote:
... as it’s the only real option for me construction wise.
Does that roof already exist? From the way you phrase that, it sounds like you don't have a roof yet. So why is it the only option? Don't they sell 2x6's, plywood, and asphalt tiles in Australia any more? :)

Quote:
I’m hoping I can get it constructed for $20-$25K (not inc gear) but I won’t set that as a hard limit as I’d rather have a better sounding room than a cheap one.
That is good news, and also bodes well for good isolation, and good acoustics. A realistic budget is the real starting point. We occasionally get folks dropping in here who want to build the next Abbey Road in their cube-shaped garage for US$ 50... (It's hard to explain to them that they are several zeros short on their budget...) But you are on the right track: So just take your best guess, multiply by any random number between 2 and 99, add in the national budget of a few small countries, and you should be about right with a realistic estimate! :)

Quote:
My main concern at the moment is about the roof in terms of isolation. I’ve been reading through as many posts about the subject as I can find, but I still feel like I could use some pointing in the right direction.
Exactly. The roof is the weak link, from the point of view of isolation. As you already know, the only way to stop sound for a reasonable cost is with mass (lots of it), damping (truckloads of it), and decoupling (total, absolute and complete). Thin metal sheeting on a light framework qualifies as "none of the above". I'd really suggest that the roof strategy needs a re-think, if your goal is to achieve high isolation.

You need to build an entire shell that gets you 65 dB of transmission loss in all directions, and for all building elements. That means not just the walls and floor, but also the roof, doors, windows, HVAC system, electrical system, and everything else. In practical terms, that means that every part of your outer leaf needs to match the surface density of your brick walls. Brick has a density of around 1800-2800 kg/m3. Call it 2300, for simplicity. That works out to roughly 250 kg/m2 surface density (your wall is 110 mm thick). So every square meter of your outer shell should weigh 250 kilograms, theoretically. As you can guess, a square meter of tin roof weighs only a very tiny fraction of that. Even worse, it is very flexible, and will resonate mightily, given even half a chance. (Ever head the sound of heavy rain on a tin roof?...)


OK, so that density is exaggerating a bit: You'd only need such high density if you were going to rely purely on mass law. If you go to two-leaf MSM construction, then you can drop that by a factor of ten for each leaf, and get even better isolation. But we are still talking about surface densities of around 25 kg/m2 on each leaf. Your tin roof is still not on the radar!

MDF, drywall and plywood come in somewhere around 600 to 800 kg/m3, so you need two layers of 16mm plywood on your roof to get to that level of surface density.

The other option you could consider, if that tin roof already exists and you don't want to re-build it, is to do a (gasp! :shock: ) triple-leaf roof. In other words, build two leaves below the tin: In that case, you'd have to make the middle leaf three layers of 16mm "something", and the final leaf (your room ceiling) could be two layers. You'd be compensating for the third leaf with the extra mass on the middle leaf, plus larger air gaps. But you'd still need to damp that tin with something.

Quote:
In a free-standing construction like this, do I need to achieve as much isolation with the external roof as I do with the external walls?
Yep! Especially if your concern is neighbors many meters away, combined with lots of bass energy (kick, toms, snare, bass guitar, keyboards, etc.).

Quote:
do I need to effectively have as much mass on the exterior roof as I do with the exterior walls?
Not necessarily as much mass, as I pointed out above: rather, you need as much isolation, which can be achieved in other ways than mass alone.

Of course, if you were planning to have only a single leaf (no "room in a room"), then yes, you would need to have the same mass on the roof. Also the windows (how much does 2 inch thick glass cost?), and the doors (how much do bank vault doors cost?).

But regardless of the construction method, you still need to have the same level of isolation designed in to every square centimeter of your studio. If you design the walls to give you 60 dB if isolation, but your roof only gives you 30, then your entire total isolation is 30, and you wasted a lot of money on building your walls to 60.

Think of it this way (you might have seen this before: I often use the analogy here): Sound is like water, and your room is like an aquarium (only upside-down: the roof of your room is like the floor of the aquarium). If you want your aquarium to hold water, then you cannot achieve that by putting glass on all four walls but only using paper for the floor! Nor cardboard, carpet, and old pillow, or scrunched up newspaper. The only thing that will work, is glass. Anything else will not hold water. Likewise, if you put glass on the aquarium floor, cardboard on one side, a piece of wood on another, paper on a third, and carpet on the forth, it ain't gonna work! Each of those "holds water" to a certain extent, but all of the water is going to gush out through the weakest side: the paper. And once the water is out, well then it is all over the place: splattering everywhere. It doesn't just exit through the paper and carry on traveling that way in a straight line! rather, it splurts out all over.

Same with your room: If you do your walls and roof to different levels, then sound will just take the easy path out: through the roof. And once it is out, then it is out all over, not just straight up. the difference with sound is that different frequencies will "splatter" in different directions: the highs will, indeed, mostly go straight up and only bother the birds. But the lows will wrap around the building, and spread out in all directions with equal intensity, just like water pouring out of the aquarium onto the floor: the puddle expands equally in all directions.

So with a weak roof, your drums and bass will easily be heard over at your neighbor's place.

Add to the above the facts that low frequency sound is the hardest to stop anyway, and travels the longest distance with the least attenuation, and you see the problem.

Have you ever walked past a night club, disco, or hotel ballroom where there's some loud music going on? What do you hear? "Booom-Booom-Booom". You hear the bass, not the singers. You hear the kick and snare, not the crash and ride. You hear the low growl of the electric guitars, not their high shriek. And it doesn't really matter where you are in relation to that club/disco: you can hear it all over, not just outside the front door: it still sounds loud 20 meters down the street.

Low frequency sound is always hard to isolate, and the only way of doing it successfully is with an "all or nothing" fully enclosing shell, that isolates to the same level all around. Including the roof.

So I'd really consider things you could do to improve that roof. Possibly replace it, or alternatively just plan to use other methods to compensate for the deficiency that it represents.

My $0.02.


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PostPosted: Tue May 15, 2012 8:45 am 
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Thanks for the input Stuart, and for giving me some realistic figures to work on for isolation.

I figured the roof was going to be a bit tricky, hence posting up with the build at this point. The zincalume roof is on the shell and I'd prefer to go for the option of beefing it up rather than replacing the entire roof.

having said that, would it be feasible to take the zinc off, screw two layers of 16mm ply to the battens, and then screw the zinc back down to the ply? That would be a PITA job, but if it's the best option then i'd certianly consider it.

the (gasp!) 3 leaf option sounds like the easier of the two but where would i put the evil 3rd leaf? Closer to the Zinc, or further away, or it doesn't matter?

The external pitch of the roof is 26.5 degrees and the internal pitch is 12.5 degrees, so i've got a considerable air gap between the tin and the bottom chord of the trusses. Around 900mm at the ridge and 250 at the walls. I was going to fill the entire roof cavity with insulation (of the right density, I'm still researching that one) so hopefully that would help with the damping of the tin roof.

The easiest way i can see of doing the 3-leaf is to attach the third leaf on the bottom chord of the trusses which gives me the large air gap between the Zinc and the third leaf, leaving me with about 150-200mm between the 3rd leaf and the internal ceiling.

The other 3-leaf option is to attach the 3rd leaf to the bottom of the battens, and between the top chord of the trusses. Fiddly work, but also possible.

Thanks again Stuart :) I'd better get myself to work now so I can pay for all the plywood i'm going to have to buy!!

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PostPosted: Tue May 15, 2012 9:53 am 
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Maybe you could post some photos of that roof, from the inside, so we can see what you are seeing.

Quote:
having said that, would it be feasible to take the zinc off, screw two layers of 16mm ply to the battens, and then screw the zinc back down to the ply?
Well, you COULD do that, but as you say, what a PITA! Plus, if it is not leaking now, it most likely would be afterwards... As they say: "If it ain't broke, don't fix it". That might be an option, though, especially if you could put some type of rubber membrane in between, both for damping the metal and also as waterproofing. Food for thought...

Quote:
where would i put the evil 3rd leaf? Closer to the Zinc, or further away, or it doesn't matter?
Theoretically, if you have to build a 3-leaf then the best way to do it is half of the total mass on the middle leaf, a quarter on each of the other leaves, and equal size gaps on both sides. However, the mass of the metal isn't that high, so you can leave that out of the picture. Plus, that's for optimal isolation with 3 leaves, and also easy calculation with the equations, but you'll still get decent isolation with uneven air gaps, as long as none of them are less than about 15 cm.

Quote:
The external pitch of the roof is 26.5 degrees and the internal pitch is 12.5 degrees, so i've got a considerable air gap between the tin and the bottom chord of the trusses. Around 900mm at the ridge and 250 at the walls.
That sounds like plenty. With that type of distance, the third "tin" leaf is probably not going to be an issue.

Quote:
I was going to fill the entire roof cavity with insulation (of the right density, I'm still researching that one)
Depends on what type of insulation. For MSM damping, look for around 30 kg/m3 if you are using fiberglass insulation, or around 50 kg/m3 of you are using mineral wool.

Quote:
The easiest way i can see of doing the 3-leaf is to attach the third leaf on the bottom chord of the trusses which gives me the large air gap between the Zinc and the third leaf, leaving me with about 150-200mm between the 3rd leaf and the internal ceiling.
That would probably work fine, provided that the inner leaf is totally decoupled from the outer two.

Quote:
I'd better get myself to work now so I can pay for all the plywood i'm going to have to buy!!
Well it doesn't have to be plywood! Maybe you could work a little less if you substitute MDF or OSB! :)

I mentioned plywood in the case of replacing the tin with an "asphalt shingle" type of roof, but for mass inside the roof you could use OSB, MDF or even drywall, if that would let you be a bit lazier... :)


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PostPosted: Tue May 15, 2012 3:10 pm 
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here's an shot of the room at the moment. The brickwork you can see at the end still has to brought up to the bottom chord of the truss.

Attachment:
truss roof.jpg


I'm still thinking about whether to go with the roof re-build or the 3rd leaf! On the plus side for the re-build is that I can wait till the summer to do it when the weather is better, and concentrate on getting the internal rooms built while it's wet here. On the minus side.....i've got to re-build the roof :shock:

Whether I was to do either the re-build or 3rd leaf, I'd consider using yellow tounge flooring to add mass. A sheet 3600 x 900 weighs 45 kg. I'm not sure how that compares to ply or mdf, and it's about $40 a sheet here so it's a relatively cheap option.

I'd also have to take another look at the computations for the trusses and see what load they'll take. One layer of yellow tounge equates to about 1 tonne over the whole roof.


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PostPosted: Sun Jul 01, 2012 9:42 pm 
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okay, sooooo to keep up to date on the build......

I've decided on going with a triple leaf roof, but as Stuart has pointed out, the third 'tin' leaf should have a negligible effect on isolation because of the size of the roof cavity.

I'm going to add two layers of red-tongue sheet to the bottem chord of the trusses, and seal them airtight with Fuller Firesound. Red-tongue is a 22mm flooring sheet and two layers gives me around 30kg/m2 surface density, which if i understand correctly, is a good weight for and external shell.

The internal rooms will be fully decoupled, with a rockwool filled air cavity of minimum 150mm and two layers of 16mm fyrcheck. I'll put up a few more photo's of the existing build and some sketchup floor plans when i figure out how to :?.

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PostPosted: Mon Jul 02, 2012 1:41 pm 
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I'm going to add two layers of red-tongue sheet to the bottem chord of the trusses, and seal them airtight with Fuller Firesound. Red-tongue is a 22mm flooring sheet and two layers gives me around 30kg/m2 surface density, which if i understand correctly, is a good weight for and external shell.
Sounds pretty good. That's the middle leaf, so it should have a lot of mass on it, and 30 kg/m2 is good.

However, check with a structural engineer to make sure your roof trusses can handle that load. It's probably OK, but don't take my word for it: get a qualified engineer to check it out and OK it, in writing.

I'm also wondering if it might be worthwhile hanging that stuff from RC, instead of attaching it directly to the trusses: I think it makes sense to do that, so that your middle leaf is decoupled from both the inner and outer ones, for maximum isolation.

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PostPosted: Tue Jul 03, 2012 3:13 pm 
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Cheers Stuart. Just got off the phone to the company that engineered the trusses and they say it's not a problem, and are sending out a re-detailed comp for the load.

Soundman2020 wrote:
I'm also wondering if it might be worthwhile hanging that stuff from RC, instead of attaching it directly to the trusses: I think it makes sense to do that, so that your middle leaf is decoupled from both the inner and outer ones, for maximum isolation.


That sounds like a good idea to me Stuart, and one that I would never have thought of myself. Thanks :D .

I'm guessing that the isolation value of having the RC between the truss and Red-tongue, outweighs any flanking path that might occur when i'm sealing the Red-Tongue against the top plates and end brick work?

Having said that, would Fuller Firesound caulk even create a flanking path? It's apparently STC 65.

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PostPosted: Tue Jul 03, 2012 5:09 pm 
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I'm guessing that the isolation value of having the RC between the truss and Red-tongue, outweighs any flanking path that might occur when i'm sealing the Red-Tongue against the top plates and end brick work?
Leave a small gap around the entire perimeter of that middle leaf, so that the Red-tongue does not actually touch anything except the RC, and fill that gap with the caulk. In fact, considering how thick it is, I'd suggest using backing rod and caulk, if your local code allows that. And of course, you do this for EACH layer individually. So one you have the first layer of Red-tongue up there, put the backer rod in, then caulk. Then put the second layer up, more backer rod, and caulk again.

Quote:
It's apparently STC 65.
Yeah, so they say, I'm sure... but that all depends on how you use it! It will only get you STC-65 if it is used as part of a system that gets STC-65. Used all by itself, it won't get you anywhere. That's the part that many manufacturers conveniently "forget" to mention: their products only perform to those wonderfully high numbers when used as part of a system that is designed to get there. A piece of cardboard can get you STC-65, if used right... ! :) And a 6" thick reinforced concrete wall might not even get you STC-25, if built wrong.

The isolation doesn't depend on the product: it depends on the entire system.

But anyway, that fire caulk will still cause flanking, regardless of its STC rating, if it creates a solid bridge across the gap. In order to NOT flank, it has to decouple, so it needs to remain flexible and "rubbery" even after it has fully cured. So check with the manufacturer if does remain soft and flexible after it sets.


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PostPosted: Wed Jul 04, 2012 10:07 am 
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I had planned to leave a gap for the caulking but hadn't thought of backer rod. thanks for the tip there Stuart.

As for the caulk itself, I think I might just try to hunt down some SilenSeal in australia, as I don't want to take the chance with a product that says it's 'acoustic caulk'. At the end of the day it might be a bit of a price difference, but there's no way of getting back to the red-tongue once it's in!

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PostPosted: Fri Jul 06, 2012 8:06 pm 
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and.... the other option i'm considering for the middle leaf (red-tongue) is to use Green-Glue between the two layers. Given that the external walls are brick and the red-tongue leaf is the most mass heavy leaf in the roof, i figure it's the best place for GG to help with the roof isolation.

So I guess it's a matter of trying to get the middle leaf, with two layers of 22mm red-tongue (30kg/m2) with GG between, to compare with 110mm brick walls.

I've been in contact with the GG distributor in Melbourne, and GG for roof the adds roughly $1K to the roof system. I figure it's a good investment in isolation if it helps with the bottem end.

I'll also be using GG acoustic caulk as the price difference between that and anything else i can find is negilgble.

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PostPosted: Mon Jul 16, 2012 8:27 pm 
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so, after spending today on the phone discussing products with various suppliers, I've come up with the following plan for the middle leaf of the roof and a reasonably close costing.

Rondo STWC isolation mounts to attach to the bottem chord of the truss, then....

Furring channel for said mounts, then....

1 layer red tongue sheet, with a backer-rod and GG acoustic-sealant perimeter, then...

Green glue layer...then

2nd layer of red-tongue sheet,backer-rod and GG sealant.

This sounds to me like the best option I can come up with (along with a lot of advice along the way from Stuart) for the middle leaf, but if anyone has any other ideas/products then I'm all ears!!! It gives me a 30kg/m, isolated middle leaf.

The other bit of research i've looked at is the insulation to use between the outer and inner leaf. At the moment i've come up with (amongst other options) Fletchers FI32 semi ridged 50mm glasswool. I've seen John refer to it as the equivalent of corning 703 in another post. At 32kg/cubic meter it ticks the right box for fibreglass density, but is the thickness suitable for inner/outer leaf insulation?

Having said that, to increase the thickness from 50 to 100mm changes the $'s figure for inner/outer leaf insulation from around 2.5K to 5K! I'm not sure the budget can take it.

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PostPosted: Wed Jul 18, 2012 3:37 am 
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Quote:
At 32kg/cubic meter it ticks the right box for fibreglass density, but is the thickness suitable for inner/outer leaf insulation? ... to increase the thickness from 50 to 100mm changes the $'s figure for inner/outer leaf insulation from around 2.5K to 5K! I'm not sure the budget can take it.
Put it this way: how much isolation do you need, and especially in low frequencies? Insulation in the cavity has a large effect on isolation. It can improve the total wall isolation by anywhere up to about 16 dB, but more like 5 to 10 dB for an average wall. The more you put in, the better it will be. There may be fire code restrictions on how much you can put in, but if not then filling the entire cavity maximizes isolation. So 50mm is certainly much better than nothing at all, but even more thicker would improve isolation. Have you looked around to see if there is anything alternative in the same density range, but less expensive?

I guess the basic answer is: "put in as much as you can afford".

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PostPosted: Mon Jul 30, 2012 1:14 pm 
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thanks Stuart, I've managed to find an alternative insulation at 24kg p/m and 110mm thick, which ticks the right box in the budget as well :D

For those of you in Australia, it's a new variation of Bradford Soundscreen in fibreglass, not rockwool. I was on the phone to Bradford and apparently they were having a lot of trouble with DIY installers screwing up the rockwool product, so they made the glasswool which is apparently easier to handle. I wouldn't know as I've only used polyester insulation before.

I've also changed the middle leaf to 3 layers of 16mm fyrcheck as opposed to the red-tongue. 3 layers of fyrcheck gets me more density for less $'s. i'm getting a good price from the supplier as i'm buying the mounts,channel,gyprock and insulation from them.

Another quick question about insulation placement with a diagram and all!!
Attachment:
middle leaf insulation detail.jpg

Is there an isolation advantage in having insulation above the middle leaf, and if yes, should it be the same 24kg/m glasswool as i'm using in the cavitys?


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Last edited by stevev on Mon Jul 30, 2012 4:07 pm, edited 2 times in total.

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PostPosted: Mon Jul 30, 2012 3:43 pm 
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Is there an isolation advantage in having insulation above the middle leaf, and if yes, should it be the same 24kg/m glasswool as i'm using in the cavitys?
Yes, and maybe! :) Yes, the insulation helps in a couple of ways, so it is worth doing. And maybe you could use the 24 kg/m3 insulation, or you could use anything similar that isn't too expensive. It's not critical, as long as it is in the ball park density ranges for each type. So if you can get that 24 kg/m3 stuff at a good price, then go for it!

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