Drying out vermicrete hearth insulation slab

[david s]

Yes, I'd be placing the first layer of vermicrete now. After a few days the outer surface of the vermicrete will turn a very light grey and look quite dry. Do not be misled because almost certainly it will still contain moisture deeper in. A week of drying in ideal conditions will eliminate much of this moisture though. I'm presuming you are not using ceramic fibre blanket but rather vermicrete insulation only. If you purchase a cheap garden moisture meter it can be useful to tell you how dry your insulation is. When mixing the vermicrete I use a 10;1 :3 vermicrete, cement, water proportion (fine grade vermiculite requires more water). 10:1 is about as lean as you can go whilst still maintaining a workable mix, while a richer brew will have less insulation value (see attached table). A handful of powdered clay for every litre of cement also helps making it workable. Start from the bottom building a wall against the dome with a flat top to take the next layer. Proceed in this manner until you get all the way to the top. As it leans in it becomes easier. Do your drying fires before sealing in all that moisture with an outer rendered shell.
Attached is an experiment i did with drying a vermicrete slab that you may find useful.
Vermicrete insulating slab copy.doc.zip