Insulator density and thermal conductivity

[yulija]

How does insulator's density affect its thermal conductivity?
For example, considering such insulators as rock wool, It seems that the thermal conductivity should increase with an increase in density (due to elimination of 'air pockets'). However, experimental results, in many cases, look more like the one on the picture.http://www.microthermgroup.com/CNZH/assets/img_density.jpg

 

[Bystander]

"Real" vs. "ideal" materials? There're all sorts of electrical properties to thermal conductivity.

 

[Chestermiller]

If the density is very low, the large void fraction permits convection currents in the air which enhances the thermal conductivity. So increasing the density of the insulator at first has the effect of lowering the "apparent" thermal conductivity. But then, as you say, eliminating air pockets raises the thermal conductivity at higher insulation densities.

Chet