[Strawbale]Measured heat flux through a real wall/ceiling
jhollan at amper....muni.cz
Tue Mar 8 19:44:42 CET 2005
(I prefer this subject to the similarly long, but not so informative
Re: [Strawbale]Re: Strawbale digest, Vol 1 #474 - 7 msgs )
I got a simple idea why there is a difference between laboratory tests
with some 10 cm thick layers and real bales.
With pure conduction, the specific heat resistance is proportional to the
thickness of the layer.
However, with some convection involved, this is not the case.
Remember a simple case of two parallel glass panes in a window. Not much
is gained by increasing the gap from 1.5 cm to 2.5 cm and even less by
increasing it to 25 cm. (There is further reason for windows: there is a
distance-independent flow by radiation).
It's better with bales, as the convection is slowed down not just by the
friction of neighbouring air layers having zero velocity at panes and in
the middle and margins of convection cells, but also within the volume,
due to straw. Still, I guess that heat transfer by convection is not
inversely proportional to the bale width, as the cross-section of the flow
within the convection cells rises.
This is something what could somehow be handled with by hydrodynamic
models. It concerns all non-foam insulation materials. Their properties
are limited by convection, and the relevant parameter are pascals per
metre per m/s (pressure drop within the air flow, proportional to velocity
for laminar flow). It should be as high as possible for insulation
There could then exist a formula to convert lambdas to U's for non-foam
materials of various thicknesses and at various temperature differences
(this is another feature, convection flux rises not simply linearly with
temperature difference), using a parameter of their resistance to air
In fact, it's queer that such a formula is not quite common yet.
Some time ago, I tried to find anything on convection in insulation
materials and found a remark that it plays some role for the lightest
mineral wool walls and strongest frosts outside, but that it can be
neglected in most other mineral wool cases.
(I guess that hey could perform better than straw in this respect, having
finer fibres. An uneasy task is to protect it from animals forever, as
it's more tasteful for them than straw.)
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