Natural convection in the (Belgian) double wall

logo_smallIt is a habit to use a double wall in a sea climate. The outer wall becomes wet due to rain and the inner wall remains dry. The increase of the energy price forced people to use that cavity for insulation.

Jan Lecompte from the KU Leuven building department measured the effect of eventual natural convection in such a cavity and reported this to the society. He found that gaps around and in the thermal insulation induce a large thermal loss, up to 200% more than what should be expected from the declared thermal insulation value of the used materials.


In case permeable thermal insulation is used (glass wool or stone wool), natural convection through the thermal insulation becomes important and higher densities are needed. In case of low densities, another 150% extra heat loss becomes possible.


The solution is that the path around the impermeable thermal insulation should be 100% interrupted. This is hard to fulfill with polymer foams, which are continuously deforming due temperature and humidity. On the other hand, in case of mineral wool, rather high densities have to be used to reduce the permeability.

In a previous post, we have already shown that intrinsic natural convection in low density mineral wool is possible. Now, we learn that natural convection around the thermal insulation is also possible.

It is clear that well installed GLAPOR cellular glass will do the job perfectly and at a price comparable with high density mineral wool. It is also obvious that accurate thermal conductivity measurements in laboratories are not telling everything. The race for the best thermal conductivity at the expense of a high cost is not helping our environment.


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