Well, which material you choose as an insulator depends on what you
want your insulator to do. Sawdust isn't a better insulator for many
kinds of jobs.
Styrofoam is generally lighter than most sawdusts because it has a
higher fraction of air in it. Air cannot flow through styrofoam as
easily as through sawdust, and so sawdust may have a greater convection
component to its thermal conductivity than styrofoam. Sawdust comes in
many different kinds -- the particles may be fine or coarse, affecting
its thermal insulation properties.
One advantage of styrofoam is that it can be made into rigid
containers. While the strength isn't great, it is enough to make a
coffee cup or a cooler out of. With sawdust, you'd need some kind of
rigid container to hold it, or you'd have to glue it together, which
would increase the thermal conductivity.
Styrofoam may be cheaper to buy, but there is a lot of sawdust
made anyway and you might be able to get all you want for free from
someone just waiting to get rid of it. Some kinds of styrofoam, when
they degrade, release halons into the environment which have been
implicated in ozone destruction high in the atmosphere, so sawdust may
be more environmentally friendly.
Styrofoam melts at temperatures which are readily attainable in
the kitchen. Don't put a styrofoam cup on a hotplate or in the oven --
it will melt. Sawdust, on the other hand, can catch fire.
Your question just said "insulator" and I answered so far only
about thermal insulation. You can imagine other kinds of insulation --
electrical and acoustic, for example. Due to the possibility of
connected paths for air to flow through sawdust, and the fact that air
will allow electricity to flow through when the voltage is high enough
(it will "break down" and allow a spark to go through), I'd imagine
that sawdust is a poor choice for an electrical insulator. Also the
catching-fire part doesn't help in this case either. Styrofoam isn't a
good choice for electrical insulation either. As for acoustic
insulation, the sawdust may actually work better because the dust
particles are able to move and slide against each other, dissipating
sound energy better than a piece of styrofoam, which may just reflect
the sound or vibrate coherently.
(published on 10/22/2007)