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GSBN:To Matts, Re Thermal Properties of Straw Bale Walls

Hi Matts,
Don't mind about the ¯, I don't know either how to type it if I don't have a Danish keyboard.
The tests regarding heat insulation in our study were conducted at the Danish Technological Institute, which has the apperatus and expertice to do the meaurements, both standard measurements of the conductivity (lambda-value) and guarded hot box tests. 
We used mainly small bale for the study but also som bigger, which had a higher density. Therefore, the lambda-values were measured for different densities. 
Afterwards, the person in charge at the Technological. Lars Olsen, did some more tests on the influence of the density. I cannot find the right now, but it was clear that the density did not influence the lambda-value very much. A curve showing the lambda-value vs. density will therefore not have a very marked extreme. Thats what I mean by "flat" optimum. The best one for the straw in question was, I think, about 80 kg/m3. 
The reason is the it is NOT the air that insulates. The insulation originates from the resistance when the heat pass from one material to another. This means that the ideal insulation material has very small cavities separated by very thin walls. But the walls will also transmit heat, so when the wall thickness is given there is an optimal size of the cavities. 
Since there are these two counteracting effects the actural density is not so important. It also yields for mineral wool. But you will often see that for a certain manufacturer the best insulating product is more dense than the normal product, because smaller cavities requires more material to separate the cavities.
As you might have guessed I do not belive that gabs in SB-walls should be filled with straw-clay mix as the clay reduces the insulation ability. As long as some straw are filled into all gabs it doesn't matter that much how dense it is. Infiltration should be prevented by ensuring an air-tight layer on both the inside and the outside.  
Sorry for the long explantion.


Fra: GSBN p? vegne af MattsMyhrman@...
Sendt: s¿ 20-05-2007 02:11
Til: GSBN@...
Emne: Re: SV: GSBN:Re Thermal Properties of Straw Bale Walls


Forgive this computer Luddite the lack of the "/" through the "o" in your
name.   I keep promising myself that I'll learn how to do that stuff but life
keep intervening.   That said, I'd appreciate some clarification of what you mean
by "The optimum [density} is very flat for most materials, and it surely is
flat for straw."

Matts (he of the maximum cranial density) Myhrman

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