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Discussion Starter · #1 · (Edited)
World's first straw bale & mud storeroom cinema build

Well, after 4+ years of planing, concrete is being poured on Tuesday. I'm moving my current screen out of the living room and under the house; into a purpose built storeroom. It will be 4.8m x 6.4m, 2.7m high, six-seater 'black hole'. 'Budget' build. Function over form, 'no frills', cheapest way to get the job done. Hence, straw bale / earth plaster construction. 6 weeks off work to smash out the build. Rendering is very labour intensive. But I'm always angry, 'Hulk smash!'

Inspired by this:
http://thelaststraw.org/strawbale-sound-isolation-acoustics/

Construction:

  • Two-string (460mm wide building quality bales laid flat density 120-130kg/m3)
  • Earth/clay straw plaster 25mm and 35mm (intentionally asymmetrical cover)
  • No reinforcing plaster netting or mesh or any form of pinning
  • measured performance: 53dBA (A-weighting)
  • measured 33.5db at 63hz, and 29.6 at 50hz (far better than an STC-65 rated wall that only achieves 23dB at 63hz)
And this recording studio:
http://www.audiotechnology.com.au/wp/index.php/the-final-straw-constructing-a-strawbale-studio/

I'll be using a 35x90 timber stud frame, at 810mm centres (which is the length of the bales). I'll stack up the bales like dominoes and drive plywood stakes into the end of each bale, and then attach the stakes to the studs. Then I'll render earth plaster on the inside, lime plaster on the outside. There will be only the straw, which is flexible (replacing the performance of the typical resilient channel), connecting the two heavy mass surfaces. The plasters will act like the constrained layer damping. I'll lay the bales on their side, giving 350mm thick walls, with 40-50mm thick render each side.

For the ceiling, I'll be attaching plasterboard directly under the floorboards above, with carpet glue in between and acoustic caulking around each board. The carpet glue will hopefully perform in a similar way as green glue does for the constrained layer damping. No, I don't expect it to work 'just as well', or better. I just can't afford the GG, so if it works half as well for half the price, I'll be pleased. If someone wishes to persuade me otherwise, perhaps through bitter disappointment using carpet glue, please tell me! Perhaps I might change my mind...

Below the joists I'll have a suspended ceiling with a double layer of 16mm firestop plasterboard, carpet glue sandwiched in-between. Sitting on top, between ceiling and above floor, will be 75mm of acoustic bulk insulation batts. Ceiling will be on acoustic isolation mounts.

For ingress/egress, I'll have two 2040x820mm solid core doors, with acoustic seals, with MDF boards attached with carpet glue (and screws - otherwise the MDF would slide off) on the inside of each door. So each door will act as a constrained layer damping surface. A door for each side of the bales.

For the projector, I have not decided but I might just build a 900x900x1800 free standing frame behind the second row to house the gear, and avoids penetrations in the ceiling. Sound system is currently Audyssey heights & wides, B&Ws, but in a few years, I'll likely end up with Atmos (update - 28/10/2018 went 7.2.4 ATMOS [Denon avr-x4400h runout]) and some DIY Sound Group gear.

The floor covering will be poured adobe earth floor, 20mm. This is essentially sand, clay, straw/chaff, with some ground psyllium seed husk to give it a little bounce. Linseed oil is poured on to harden the surface.

http://weblife.org/cob/cob_038.html

As for colour, I am currently leaning towards unfinished plasterboard for the ceiling, covered with the cheapest black cotton, chuppah style. Paint reflects too easily. I will probably use cotton fabric for the walls too, as the clay plaster will be naturally a reddy colour. However, unstablised earth plaster is not that strong, so I have not figured out how to attached much to the walls. The ceiling weight is limited too. For the air-con, I'll sandwich some plywood between a couple of the bales, perhaps T-shape, to allow mounting.

After the room is built, I'll probably decide I need some ventilation, so will build two 'silenced' boxes; intake/exhaust - out of cob (sand, straw, and clay).

If I manage to finish the build, I hope to get a local university interested in testing the insulation performance - for free, naturally! You may be surprised, but I am somewhat skeptical as to how much subwoofers can be attenuated, as the way I understand it, all sound proofing does is push the resonances down to lower frequencies. With 2x18" subs, if they hit 20hz my neighbours 3m away will still hate me. Still, I am hopeful that sound quality inside the storeroom will be much improved by the walls acting as base traps, thus increase the perception of louder base?

Sorry for my English - it's English.

Uploading pictures now...
 
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Discussion Starter · #2 ·
Here's the area I'm working with.
 

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Discussion Starter · #3 · (Edited)
And here's some design ideas. Any questions and/or suggestions welcome!


28/10/2018 - purchased 7.2.4 ATMOS avr and so now will not be installing the front wides, just adding additional amp and running four ceiling ATMOS speakers
 

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Very interesting project. Please post pictures along the way as we certainly haven't seen any straw bale construction methods documented on this forum as long as I have been a member. Just some food for thought. I see some support poles and I can't quite figure out where they are in the final plan. I did a project where I was able to add 16 inches of room width by keeping the poles inside the theater and then hiding them with speaker columns. I was lucky where they were positioned.
 

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Discussion Starter · #5 ·
Thanks for your interest - and advice. Well. Yes. After eating your food (for thought) I have come to the conclusion; 'there's probably numerous good reasons why no one else has attempted this...'.

So, it will be nice to document this for the historical record. Worse case scenario is that it will become a headline and most widely read sticky in the, 'what I would have done differently' section. I don't know why Conrad's Heart of Darkness is coming to mind...

About the support poles, they will be inside the bale walls. One pole is in red at the top of the Atmos speaker layout plan. The room will be in the 'undercroft' area on the below plan. I had intended to keep the garage area. But if anyone can persuade me to go bigger, then please do! But a garage does have its uses.

I have one question about the junction between the ceiling and walls. I intend to run the render right up to the underside of the existing floor and then butt the suspended ceiling, which will be on isolation mounts, up against it. Is this a good idea? Conversely, I could run the ceiling into the side of the straw bale, and then the render up to the ceiling. Note, the gap between the ceiling will have acoustic batts bulk insulation.
 

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Discussion Starter · #6 ·
More photos. The screen will be a free standing framed arrangement adapted from it's existing site.



Question? The 22-30degrees prescribed for the front left and right speakers falls just to the sides of the screen. Currently I have all three speakers behind the screen. I should move the left and right out past the screen, yes?
 

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My personal opinion is that speakers to the side of the screen is just fine and is often optimal for audio and optimal for video if considered separately.
 

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The 22-30degrees prescribed for the front left and right speakers falls just to the sides of the screen. Currently I have all three speakers behind the screen. I should move the left and right out past the screen, yes?
Yes, you may need to toe the speakers in. Or get a bigger screen
 

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Discussion Starter · #9 ·
Cheers Bigus.

Another question - I really wish the room was wider.

I could use a different construction method; light straw (slip straw). This method uses clay coated straw between timber framing - downside is that it is a lot more labour intensive. But I could use that instead of the entire bale, but with the same thickness render. It would reduce the thickness of the walls by 200mm - so given me an extra 400mm width (and length). And reduce thermal insulation substantially, but sound reduction shouldn't be compromised too much as the render is really doing the important work.

Or I could just go wider and sacrifice another foot of the garage area (might reduce resale value as space become awkward)? Or I could extend even wider, say another 1.5 metres, if I created an L-shape for the car (the screen would be along the top wall adjacent to the existing laundry area).

If it were your place, what would you do?
 
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Discussion Starter · #10 ·
Cheers, BigMouth.



I'd love a bigger screen, but I'm struggling for lumins already with HDR content. Perhaps in the rebuild 5-10 years from now.
 
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I have one question about the junction between the ceiling and walls. I intend to run the render right up to the underside of the existing floor and then butt the suspended ceiling, which will be on isolation mounts, up against it. Is this a good idea? Conversely, I could run the ceiling into the side of the straw bale, and then the render up to the ceiling. Note, the gap between the ceiling will have acoustic batts bulk insulation.
I'm not familiar with the physical characteristic of an earth render layer. After looking at the diagram it is not clear of the advantages of one approach over the other.
 

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Discussion Starter · #12 · (Edited)
Edit - (sorry, Bigmouth, you did indeed answer my question - see next post) Actually, I meant if people thought going wider would be worth it. But I acknowledge that without knowing the trade-off for possible detriment to the sound insulation, it's difficult to answer.

As for the benefit, I'm just thinking from sound performance (not insulation) point-of-view, as I don't want people to be too near the side speakers. Currently the faces of the side surrounds are 1.2m from the listeners' ears in the side seats.

But looking at the site just now, creating an L-shape won't help the side surround width, so that's out (see diagram, larger green box to the top).

But the different construction method gains me 200mm each side (green long boxes top and bottom) - so would give 1.4m each side for the side listeners which I recall is near the optimum 1.5m for Atmos? But who cares about them? I'll be the one sitting in the best spot. Everyone else are just tag-a-longs. And likely won't appreciate the better sound anyway (ie. will be less discerning.). Right?
 

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Love the "Heart of Darkness" in reference to breaking new ground! :D
 

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Discussion Starter · #14 ·
Bigmouth, what I meant was, if you were doing a room within a room construction, whereby the inner wall meets the ceiling, would you extend the plasterboards (x2, CLD) past the ceiling, or just end the wallboard at the ceiling, lapped butt joint (if that's the right description)?
 

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Discussion Starter · #15 ·
Quick check - I am not planning to isolate the floor. That's okay, right?

Thinking about it now, I suppose I could have made the room much smaller and poured a slab in complete isolation to the rest of the house, and build on that. I have until tomorrow morning to change my mind...
 

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Wider might be worth it, if your room finish plans include acoustically transparent fabric. They would be perceived as finished walls but
hiding speakers, acoustical treatments and even a wiring chase around the room, are potential pluses to this approach.
 

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Discussion Starter · #17 ·
Thanks Tedd. That's a great idea I have not considered, even if I don't go wider - cheers!
 

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Discussion Starter · #18 ·
Digger used to remove top fill and remove the silty sand (according to the official soil test) underneath. The plan is to use that material for the rendering. The concreters described it as brickie loam. About 2m3?
 

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Discussion Starter · #19 ·
Steel work inspected by the engineer before the pour. Needed a bit more overlap. Luckily we had some extra reio' on hand, that I've been meaning to get rid of for the past 3 years. Lesson? Never throw anything away if you can help it - there may be a home cinema builder out there somewhere in desperate need!
 

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Discussion Starter · #20 · (Edited)
Pest control man came to install a termite protection at the side of the slab and around all the steel posts.


Then the pour. The pour! A sizable chunk of change (in my 'budget' build), due to the side walls. Oh. Now I'm poor.

The high walls are needed to keep the straw bales out of the rain. They give me 200mm above ground level on the exterior. Normally, a thick a 220mm slab would have been poured over the entire top, but I wanted to keep the ceiling height to max. It will be 2700.

I had the engineer design the slab to go 150mm lower than what we ended up doing, but I chickened out. Everything I read about building generally and straw bale building specifically, recommends going up, not down! Potentially, if the concrete cracks I'll be sitting in a swimming pool. And I have really bad drainage on that external wall. If I have problems I may have to install a drain pit and pump outside. The concreter said the sandy soil should encourage the water to drain away under the slab. They were terrific guys too, working so hard to get the job done in just a couple of days.
 

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