It is always interesting to see how other look to the market you like to get. Lorna Gibson, famous MIT professor, gives a lot of courses about cellular solids. The course notes are public domain and are nicely organized in an MIT-website.
In this post, we discuss the notes about thermal insulation. The paper gives a list of thermal conductivities with solid materials like polystyrene, polyurethane, glass, with gases and also the resulting foams. It shows that glass foams have a higher thermal conductivity because glass itself has a higher thermal conductivity.
The paper discusses the different heat transport mechanisms and shows that foams with 1mm cell size have no contribution to convective heat transfer. The paper explains also why at too low densities the thermal conductivity increases. It seems that the contribution of radiation becomes too important.
As a consequence, it seems that not only the compressive strength but also the thermal conductivity induces an under limit on the density. It seems that a relative density of 0.04 or 100 kg/m³ density is an under limit.