Build outer leaf with Hempcrete?

[jimmyedson]

First post - but long term lurker. I have bought and read Mr Gervais' book and read many, many posts here and on Gearslutz forums.

I want to design a stand alone small studio on a concrete slab from scratch approx 7.2m(L) x 3.8m(L) x 2.7m (H). It is based on the design supplied by John Sayers called the "Small Studio" but about a metre longer for the live room. i.e. http://www.johnlsayers.com/phpBB2/viewtopic.php?t=5457

I am a musician working with an acoustic duo and a loud blues band. Ideally I would like to record and rehearse both as well as teach in the studio. I am investigating all possible construction methods and am intrigued by the possibilities of Hempcrete - 30cm thick walls - no gaps around walls and ceiling as a contiguous outer leaf. See http://www.hempcrete.com.au/

I would still build a room within a room construction but I like aspects of hemcrete vs standard timber frame - esp environmental factors.

Can anyone comment (no hemp smoking jokes please) about the likelihood of this working for an outer leaf - in particular is it any good for sound isolation?

I cannot get hard data about the acoustic properties apart from this email I received today...

"So far hempcrete wall systems has have little sound testing and research.
What is established is that the minimal requirements for sound insulation of building products is met at large.
Hempcrete has very good acoustic components due to it's open structure, especially when kept un rendered. It could be considered to not render the top part of the room for that purpose.
Hempcrete has a low propagation rate, a figure of ~900Vson m/s against steel with ~6110 Vson m/s.
Hemp concrete is characterised by its high porosity that combines the microscopic pores of its binder and its vegetable particles,
and bigger pores between the particles. Thus, its sound absorption is intermediate between the excellent absorption of hemp shiv, and the reflexion of the binder.
We have not translated this figure back to the Australian
• According to AS 3.8.6 & AS/NZS 1276.1 (ISO 717.1)
• Ctr = low frequency sound correction factor
• Rw + Ctr = sound reduction index min = 50
• STC = sound transmission class min = 45
• IIC = impact insulation class = TBA
But we are working to get those figures.
"

Thanks for any information...

[Deesubsee]

It's worth a close look. I only briefly looked at the link and they use "airtight" as a selling point, which is good from an isolation point of view. I did not find a spec for the weight percent of the product that is made up from the hemp component, but cellulose-like materials are decent materials for isolation. I did not try to find a cost and that should factor into your comparison. See if you can find a density spec and see how the strength compares to concrete structures, stick frame, etc.

[jimmyedson]

It's worth a close look. I only briefly looked at the link and they use "airtight" as a selling point, which is good from an isolation point of view. I did not find a spec for the weight percent of the product that is made up from the hemp component, but cellulose-like materials are decent materials for isolation. I did not try to find a cost and that should factor into your comparison. See if you can find a density spec and see how the strength compares to concrete structures, stick frame, etc.

Thanks for your reply Deesubsee,

I have now received the following info about the density of hempcrete:

"1m3 of hempcrete for walls is made up of approx 100kg hemp and 250kg lime binder
1m3 of hempcrete for ceiling insulation is made up of 100kg hemp 125kg binder

the wall mix is 400-420 kg/m3 after 90 days curing.

the structural strength is indeed supplied by the timber frame, the hempcrete functions as external/internal cladding and insulation. "

Overall it seems worth getting a costing given the great insulation properties using 30cm thick hempcrete walls - I will keep you posted when I get that info.

Jim

[Deesubsee]

That's interesting in that it's not all that dense, compared to something like cement. A 30cm wall thickness would add up to significant mass, however.

[Soundman2020]

the wall mix is 400-420 kg/m3 after 90 days curing.

That's much lighter than drywall, so it will have to be pretty thick to get decent levels of isolation. But I'm assuming that it normally is done pretty thick?

Assuming 30cm thickness, by itself that would give you about 40 dB of isolation (TL), which is a bit better than a standard house wall (drywall on both sides of a 2x4 stud frame), but not a lot. As part of a proper MSM system it would be better, of course.

But the big question here is: how much isolation do you need?

- Stuart -

[jimmyedson]

Thank you for your valuable input Stuart - much appreciated.

Your estimate of about 40 dB of isolation (TL) is very helpful and fine for my needs initially and it could be slightly better as I will be adding a 20 - 30mm render coat to the exterior. I am interested in the other benefits of hempcrete using a 30cm thick wall including very good insulation saving heating/cooling costs, effectively a sealed room with continuous hemcrete walls and ceiling creating a cocoon with no air gaps apart from the door and HVAC system. Adelaide gets some days in summer around 40 degrees celcius and it is not often below 8 degrees.

As I will be building from scratch my idea is to build the outer leaf in hempcrete with one window - (2 panes thick laminated glass window) and a heavy door then measure the ouside levels with my blues band playing. We have a loud drummer - and it will be around 120db approx.

If the outer leaf works well I might get away with it as a single leaf structure- neighbours are around 30 metres away. I won't be doing professional recordings just recording demos and practices.

If it goes well I have the option in building a room within a room if the need arises.

The studio will be more a multimedia studio than a full on recording space - composing, teaching, video production, workshop - multi-use.
I will be receiving a costing from the hempcrete builder next week.

I am looking at the Daikin Ururu Sarara air con system mentioned by John Sayers as the ideal solution to bring in fresh air as well as cooling/heating.

Regards
Jim

[jimmyedson]

The quote came in quite bit higher than I anticipated so I am now looking into the options of being an owner builder and undertaking a hempcrete building 3 day course.
This would save enough to make the project feasible but it is a bit daunting knowing how much I have to learn.

I will also explore more conventional building approaches.

Will keep you posted.

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