Thermal Conductivity: Un-fired Earth (cob) vs Fired Earth (Fired bricks)

In this article it has been stated that cob has a low thermal conductivity and for this reason it will absorb heat, retain it, and then dissipate it back out slowly; whereas, "fired brick absorbs and release heat very quickly".

I looked up the thermal conductivity of both cob and fired brick and they are listed at 0.6 W/mK.

Can someone please explain in more detailed why fired earth (fired brick) absorbs and releases heat quickly, while un-fired earth (cob) is better at absorbing and releasing heat. I have been searching on the internet but I cannot seem to find a technical explanation

Thank you and all best!

Michelle

This is my guess, Michelle, not an expert opinion, but the far greater mass of a cob wall compared to a fired brick wall would make a huge difference, even if the thermal conductivity is identical.

Maybe cob has more moisture, which must evaporate/condense during the warming/cooling process. This increases the effective heat capacity, while also regulating the humidity of the building interior.

- 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.

Hi Nick,

Welcome to Permies.

Well put, Nick.

A significant factor is how high the clay is fired to make the brick. Old bricks are often soft (can leave a streak like chalk on concrete), and would have conductivity similar to raw dry cob. Newer brick or old very hard brick are fired to higher temperatures and become denser and more conductive, up to the point of being partly vitrified ("made glassy") with particles fused to each other. So there is a considerable range and the best typical permies answer is "it depends".