- Air pockets are, in general, the primary means of insulation... hence naturally-evolved animal fur and feathers that trap air in between their layers, sleeping bags, jackets, and double-glazing.
- Fired clay has much of its (outermost) silty / clayey material converted to glass during the firing process.
- Glass conducts heat relatively well, consider how quickly you lose heat when placing your wrist on a glass table... this is also the reason windows must be double glazed.
- Cob will have no vitreous (glassy) material doing heat conduction, and instead has a far higher ratio of air pockets in between the particles of rammed earth, thus does heat insulation quite well.
- Cob heat conductivity will be further reduced by (natural) fibres.
Bottom line: Energy is used in the firing process to convert the brick's internal structure from high entropy (sand and silt) to low entropy (well-structured solid glass).
While this makes the typical fired brick stronger and cleaner (and easier to clean), it also means it conducts heat very well, which can be considered either a pro or a con depending on your needs.
