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In order to explore the relationship between effective thermal conductivity of an evacuated powder and the bulk thermal conductivity of the same material, the effective thermal diffusivities of particulate NaCl and Dianin's inclusion compound with ethanol guests (abbrev. ED) with effective porosities ≈ 0.5 were measured and used to determine their effective thermal conductivities below 300 K. Calculations showed that contact heat conduction is the predominant mechanism, i.e., heat transfer by radiation and by conduction through the gas phase are negligible in the measurement conditions. The effective thermal conductivity of particulate as-synthesised ED powder was found to be proportional to the bulk thermal conductivity for three different samples. On the other hand, the effective thermal conductivity of NaCl powder was found to have a softer temperature dependence than the bulk thermal conductivities reported for measurements of NaCl single crystals. This was related to increased concentration of structural defects formed during mechanical grinding of the NaCl sample.