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I cannot find anything that distinguishes damp (or exterior) walls from dry internal walls.According to some Google sources, a 4" concrete wall should attenuate a 2.4GHz signal by at least 16dB.
I suspect it is down to moisture content, and the blocks being aerated (~1/3rd the density of solid concrete).The nearest figure I have seen browsing the internet is 11dB attenuation for an 8-inch concrete block with hollow cavities inside at 2GHz. I would expect more than this for 8-inches of block with no internal cavity at 2.4GHz. So I'm still puzzled.
Internal dividing walls between semi-detached houses in the UK such as you show in the plan are often only single-brick thickness - one block.
So something like equivalent to 33mm of very dry concrete?