[Strawbale]Re: Rendering the outside only

[Rene Dalmeijer]

Stewart,

Not rendering the inside surfaces would obviously allow for much higher 
levels of high frequency absorption. I am not sure about low 
frequencies though. I am almost sure these would be lower then when 
plastered with an earth plaster. We have been doing SB sound insulation 
testing here in an acoustic test chamber. a normal SB wall with earth 
plasters achieves roughly 55 dBA. The level of attenuation varies over 
the spectrum Low frequencies are attenuated much less ie roughly 30 dB 
at 63 Hz. For recording purposes this is maybe just enough to prevent 
unwanted background rumble getting into the recording. I expect that 
not plastering the inside surfaces would seriously decrease the 
attenuation at 63Hz maybe to levels below 25dB.

To absorb or attenuate sound at low frequencies you need mass. This 
means the internal plaster is quite an important contribution to the 
amount of low frequency absorption.

Besides the acoustic considerations I always strongly advise to plaster 
all exposed straw bale surfaces for the following reasons:
a) Fire, pest, rot prevention.
b) Structural as the building is load bearing I expect that the walls 
will start to bow outwards due to the uneven load distribution on the 
bales. The plaster on the outer surfaces is much stiffer ie will carry 
most of the loads The inner surface is not as stiff and will deform 
more over time as the bales settle.
c) Due to the almost certain risk of moisture problems developing (due 
to not plastering the inner surfaces) could lead to a health risc as 
spores are free to move into the inner space.

On May 25, 2005, at 07:36, strawbale-request at amper....muni.cz wrote:

> I've been helping a musician build a strawbale recording studio (the 
> 'Strawdio'). It
> is a simple, one storey, load bearing structure - four walls and, 
> soon, a green
> roof.
>
> He has been wondering about the possibility of leaving the inside 
> surface of the
> bales bare. This would be accoustically superior to a rendered or 
> plastered
> surface, and would be ideal for it's intended use. But I wonder if 
> anyone would
> like to comment about the implications of doing this - structurally, 
> thermally, or in
> any other way?
>
Rene