Gd Q Ed. If it was organic material, then I cd visualise the little sacs within the fibre structure greedily absorbing water vapour (but how then converting to liquid within themselves - and why willing to release the water under drying conditions?). Anyway, a wool jumper can absorb masses of sweat and still feel dry - like my fabulous Icebreaker ultra-fine merino wool thermal top, which doesn't even get smelly after days of use!

But unfired clay (cob), which is almost unique in its similar hygroscopicity, amongst non-organic materials, must do the trick by other means - the labyrinth of capillary pASSAGES between the particles. Condensation within such 'solids' doesn't happen bang at dewpoint, but is present to some extent all the time.

First individual water vapour molecules become attached to the pore walls; at higher RH these start to clump together into a liquid film; at still higher they may bridge right across the pore's width, at which point capillary transport by surface tension will begin. So there's quite a lot of 'liquid' within a porous solid way below 100% RH. At 95% RH, cumulative effect makes the condensation curve suddenly goes steep - but even at 100% it's not all over because solids can become super-saturated.

Maybe, maybe not, such hygroscopic materials, whether organic or unfired clay, don't allow liquid condensation to form on their surface because water molecules, whether vapour or liquid, get whisked away into the interior as they arrive.

Organic materials must have the capillary labyrinth thing happening, as well as their active sacs mechanism.

Either mechanism does convert vapour to liquid, so can store masses volumetricaLLY - JUST NOT SURE WHETHER CONDENSATION AT THE SURFACE IS PART OF THE MECHANISM, WHETHER IT wd promote or impede absorbtion.

FT, yes, I think we have pretty much the same understanding of what's going on. All I'd say that your original version I quoted above is a bit misleading/confusing as condensation is happening at “the surface” at well below the dew point unless your meaning of “the surface” is restricted to the flat bits of the outer surface - which is what I now think you meant.

ST, trouble is dew points only apply over flat surfaces of pure water whereas, as FT describes, we're not talking about flat surfaces here. I think you're right, though, that the phase change energy is an important consideration as it's the relatively high conductivity and thermal mass of the cob which allows it to dry out again quickly when the RH drops - it can supply heat to evaporate the water promptly.