Spent mushroom grow blocks? Good R value, and they’re even fire retardant. I just don’t know if you can get them in quantity down there.
Discussion
I do have concerns about fungal and spore load. Could be that gassing it with chlorine dioxide would denature the spores.
You will always have fungus because nature abhors a vacuum. Nearly all the grow blocks are lions mane or oyster, so if they’re dormant there, they have primacy. They’re safe mushrooms. In theory, you would have less bad fungus because of the good.
The block itself is exclusively the spent grains grow media and mycelium, the “roots” if you will. The spores live exclusively in the fruiting body. If the block is dry, there is no fruiting body.
Here’s some I was going to compost, but they got rained on. Ignore the algae. You’d be getting dry blocks, and they wouldn’t be ugly like this. The white is the roots of the mushroom. Just really corky fiber. 
