vermiculite Floor insulation & Weep holes?

[paul600rr]

Hi another question just poured my hearth slab waiting for it to cure with regards to the floor insulation I’m going for vermiculite & cement mix around 4inch - 5 inch thick is it a 5:1 mix with vermiculite to Cement ratio for the floor ? And what is the drying times for the vermiculite insulation to dry before starting to lay firebrick floor/dome on top and last question regarding drilling some weep holes in floor slab can they be anywhere under the floor of the dome and how many & size needed is 10mm ok ?? do you then just a mesh on top so when i pour the vermiculite that doesn’t fully block the holes up thanks Paul

[david s]

A 5:1 ratio is the correct one for underflooe insulation as it is a good compromise betweem strength and insulating value. It needsto have sufficient strength to support the weight of the oven. A far leaner (more insulating) mix can be used ober the dome. The big advantage of vermiculite or perlite is that they are way cheaper than a manufactured insulating board. The drawback is that they are very hygroscopic so will take up a large amount of water that needs to be mostly eliminated, preferably before building over it. The required water addition can vary depending on the grade of the xermiculite or perlite and how much moisture it already contains from the atmosphere. Generally the fimer the material the more water is required. The easiest way is to mix the cement with the stuff dry, then add wtyer while mixing (in a barrow so you can feel and observe during miximg. When the water begins to pool in the bottom of the barrrow, that is the correct amount. Too much water will wash yje cement off the grains creating an inconsistent mix.
Dryiing is prblematic and mty slab drying experiment atached may be useful.

Vermicrete insulating slab copy.doc.zip

[paul600rr]

A 5:1 ratio is the correct one for underflooe insulation as it is a good compromise betweem strength and insulating value. It needsto have sufficient strength to support the weight of the oven. A far leaner (more insulating) mix can be used ober the dome. The big advantage of vermiculite or perlite is that they are way cheaper than a manufactured insulating board. The drawback is that they are very hygroscopic so will take up a large amount of water that needs to be mostly eliminated, preferably before building over it. The required water addition can vary depending on the grade of the xermiculite or perlite and how much moisture it already contains from the atmosphere. Generally the fimer the material the more water is required. The easiest way is to mix the cement with the stuff dry, then add wtyer while mixing (in a barrow so you can feel and observe during miximg. When the water begins to pool in the bottom of the barrrow, that is the correct amount. Too much water will wash yje cement off the grains creating an inconsistent mix.
Dryiing is prblematic and mty slab drying experiment atached may be useful.

[ATTACH]n458993[/ATTACH]

Thanks for reply David any ideas on weep holes ?//

[Giovanni Rossi]

paul600rr, you may want to take a little time and research this topic on the Forum before you place your insulation layer. There's a lot of info and photos.

Many builders here recommend a layer of mosaic tiles with webbed backing laid upside down if using a v/p-crete insulation layer. The webbing helps support the insulation until it sets. This both protects the weep holes and creates channels under the insulation for moisture to find its way to the weep holes.

[david s]

The webbed backing may not be sufficient to hold the vermicrete away from the gaps in the tiles. This depends on how fine the vermiculite or perlite grade is and also how wide the gaps between the tiles are. Something stiffer like aluminium insect screen might be a better solution.