Minimizing shrink of cob for chinking & caulking

Hi guys,

Just before winter I found leaks in our log house filled them with a cob mixture. I figured if I did it as dry as possible I would reduce shrinking, however gaps still were formed around the place that let in freezing air. I thought this might happen so brought in an extra wheel barrow of clay before the ground froze so I could do mid-winter repairs.

But I've been thinking this can't be normal. Or is it?

What reduces shrinking in a chinking / caulking mixture? I've seen people adding things like lime, ash etc but I guess this is for added hardness. I would imagine more fibre means less shrinking. Based on a few experiments I did, it seems fibre is more important than how wet it is.

What have you found to work best?

I'm thinking next time around I should try it with a more diverse array of fibres. e.g. not just one like straw.. but a wide range from big to small Straw, hay, goat hair, and then something as small as cattail. Thoughts?

Cheers,
Rob

I have a coating of cob on my rocket mass heater, and it dried without much cracking. What I did was to add some newspaper puree to the mix, and the paper fibers did the trick. Whiz up equal amounts of newspaper and water in a blender and add it to your mix. I think my ratio was two blenderfuls to a 5 gallon bucket of cob mix, but you may need to tinker with the proportions.

John, that is a brilliant idea! Those would be super tiny fibres. Thank you